/*
 *    PsychToolbox3/Source/Linux/Screen/PsychWindowGlue.c
 *
 *    PLATFORMS:
 *
 *        This is the Linux/X11 version only.
 *
 *    AUTHORS:
 *
 *        Mario Kleiner       mk      mario.kleiner.de@gmail.com
 *
 *    HISTORY:
 *
 *        2/20/06             mk      Created - Derived from Windows version.
 *
 *    DESCRIPTION:
 *
 *        Functions in this file comprise an abstraction layer for probing and controlling window state, except for window content.
 *
 *        Each C function which implements a particular Screen subcommand should be platform neutral.  For example, the source to SCREENPixelSizes()
 *        should be platform-neutral, despite that the calls in OS X and Linux to detect available pixel sizes are different.  The platform
 *        specificity is abstracted out in C files which end it "Glue", for example PsychScreenGlue, PsychWindowGlue, PsychWindowTextClue.
 *
 *    NOTES:
 *
 *    TO DO:
 *
 */

#include "Screen.h"

// If non-zero entries, then we are or have been running on Mesa
// and mesaversion encodes major.minor.patchlevel. Gets zero-initialized
// in X11 screen glue at Screen() load time, then assigned proper values
// during PsychOSOpenOnscreenWindow() if running under Mesa:
int mesaversion[3];

/* Following code is shared between the classic X11/GLX backend and the new Waffle backend: */

/* These are needed for realtime scheduling control: */
#include <sched.h>
#include <errno.h>

// utsname for uname() so we can find out on which kernel we're running:
#include <sys/utsname.h>

// Needed for our custom PRIME outputSink timestamping via UDP connection
// to the hacked up modesetting ddx:
#include <sys/socket.h>
#include <netinet/in.h>

typedef struct _buf {
    uint64_t frame;
    uint64_t usec;
    int scrnIndex;
    unsigned char flags;
} _buf;

static int prime_sockfd[kPsychMaxPossibleDisplays] = { 0 };
static int prime_sockfd2[kPsychMaxPossibleDisplays] = { 0 };

/** PsychRealtimePriority: Temporarily boost priority to highest available priority on Linux.
 *    PsychRealtimePriority(true) enables realtime-scheduling (like Priority(>0) would do in Matlab).
 *    PsychRealtimePriority(false) restores scheduling to the state before last invocation of PsychRealtimePriority(true),
 *    it undos whatever the previous switch did.
 *
 *    We switch to RT scheduling during PsychGetMonitorRefreshInterval() and a few other timing tests in
 *    PsychOpenWindow() to reduce measurement jitter caused by possible interference of other tasks.
 */
psych_bool PsychRealtimePriority(psych_bool enable_realtime)
{
    static psych_bool old_enable_realtime = FALSE;
    static int   oldPriority = SCHED_OTHER;
    const  int   realtime_class = SCHED_FIFO;
    struct sched_param param;
    static struct sched_param oldparam;

    if (old_enable_realtime == enable_realtime) {
        // No transition with respect to previous state -> Nothing to do.
        return(true);
    }

    // Transition requested:
    if (enable_realtime) {
        // Transition to realtime requested:

        // Get current scheduling policy and back it up for later restore:
        pthread_getschedparam(pthread_self(), &oldPriority, &oldparam);

        // Check if realtime scheduling isn't already active.
        // If we are already in RT mode (e.g., Priority(2) call in Matlab), we skip the switch...
        if (oldPriority != realtime_class) {
            // RT scheduling not yet active -> Switch to it.
            // We use the smallest realtime priority that's available for realtime_class.
            // This way, other processes like watchdogs can preempt us, if needed.
            param.sched_priority = sched_get_priority_min(realtime_class);
            if (pthread_setschedparam(pthread_self(), realtime_class, &param)) {
                // Failed!
                if(!PsychPrefStateGet_SuppressAllWarnings()) {
                    printf("PTB-INFO: Failed to enable realtime-scheduling [%s]!\n", strerror(errno));
                    if (errno==EPERM) {
                        printf("PTB-INFO: You need to run Matlab or Octave with root-privileges, or run the script PsychLinuxConfiguration once for this to work.\n");
                        printf("PTB-INFO: See /usr/share/doc/psychtoolbox-3-common/README.Debian to make this work.\n");
                    }
                }
                errno=0;
            }
        }
    }
    else {
        // Transition from RT to whatever-it-was-before scheduling requested: We just reestablish the backed-up old
        // policy: If the old policy wasn't Non-RT, then we don't switch back...
        if (oldPriority != realtime_class) oldparam.sched_priority = 0;

        if (pthread_setschedparam(pthread_self(), oldPriority, &oldparam)) {
            // Failed!
            if(!PsychPrefStateGet_SuppressAllWarnings()) {
                printf("PTB-INFO: Failed to disable realtime-scheduling [%s]!\n", strerror(errno));
                if (errno==EPERM) {
                    printf("PTB-INFO: You need to run Matlab or Octave with root-privileges or run the script PsychLinuxConfiguration once for this to work.\n");
                    printf("PTB-INFO: See /usr/share/doc/psychtoolbox-3-common/README.Debian to make this work.\n");
                }
            }
            errno=0;
        }
    }

    //printf("PTB-INFO: Realtime scheduling %sabled\n", enable_realtime ? "en" : "dis");

    // Success.
    old_enable_realtime = enable_realtime;
    return(TRUE);
}

/* The following code is only used for implementation of the classic X11/GLX backend: */
#ifndef PTB_USE_WAYLAND

// Perform OS specific processing of Window events:
void PsychOSProcessEvents(PsychWindowRecordType *windowRecord, int flags)
{
    Window rootRet;
    unsigned int depth_return, border_width_return, w, h;
    int x, y;

    (void) flags;

    // Trigger event queue dispatch processing for GUI windows:
    if (windowRecord == NULL) {
        // No op, so far...
        return;
    }

    // No-Op if we are not running on a X11 based display backend:
    if (!windowRecord->targetSpecific.privDpy || !windowRecord->targetSpecific.xwindowHandle) return;

    // GUI windows need to behave GUIyee:
    if ((windowRecord->specialflags & kPsychGUIWindow) && PsychIsOnscreenWindow(windowRecord)) {
        // Update windows rect and globalrect, based on current size and location:
        PsychLockDisplay();
        XGetGeometry(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.xwindowHandle, &rootRet, &x, &y,
                     &w, &h, &border_width_return, &depth_return);
        XTranslateCoordinates(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.xwindowHandle, rootRet,
                              0,0, &x, &y, &rootRet);
        PsychUnlockDisplay();

        PsychMakeRect(windowRecord->globalrect, x, y, x + (int) w, y + (int) h);
        PsychNormalizeRect(windowRecord->globalrect, windowRecord->rect);
        PsychSetupClientRect(windowRecord);
        PsychSetupView(windowRecord, FALSE);
    }
}

#ifndef PTB_USE_WAFFLE

/* XAtom support for setup of transparent windows: */
#include <X11/Xatom.h>

// For detection of DRI3/Present support:
#include <X11/Xlib-xcb.h>
#include <xcb/xcb.h>
#include <xcb/dri3.h>

// For DPMS control:
#include <X11/extensions/dpms.h>

// Use dedicated x-display handles for each onscreen window?
static psych_bool usePerWindowXConnections = FALSE;

// Use GLX version 1.3 setup code? Enabled INTEL_SWAP_EVENTS and other goodies...
static psych_bool useGLX13 = FALSE;

// Event base for GLX extension:
static int glx_error_base, glx_event_base;

// Number of currently open onscreen windows:
static int x11_windowcount = 0;

#ifndef GLX_BUFFER_SWAP_COMPLETE_INTEL_MASK
#define GLX_BUFFER_SWAP_COMPLETE_INTEL_MASK	0x04000000
#endif

#ifndef GLX_BufferSwapComplete
#define GLX_BufferSwapComplete	1
#endif

typedef struct GLXBufferSwapComplete {
    int type;
    unsigned long serial;	/* # of last request processed by server */
    Bool send_event;		/* true if this came from a SendEvent request */
    Display *display;		/* Display the event was read from */
    GLXDrawable drawable;	/* drawable on which event was requested in event mask */
    int event_type;
    int64_t ust;
    int64_t msc;
    int64_t sbc;
} GLXBufferSwapComplete;

/* Detect DRI3/Present support. Must be called under PsychLockDisplay() protection. */
static psych_bool IsDRI3Supported(PsychWindowRecordType *windowRecord)
{
    const xcb_query_extension_reply_t *dri3extension;
    xcb_dri3_query_version_cookie_t cookie;
    xcb_dri3_query_version_reply_t *reply;
    xcb_dri3_open_cookie_t open_cookie;
    xcb_dri3_open_reply_t *open_reply;
    xcb_generic_error_t *error;
    int major, minor;
    Display *dpy = windowRecord->targetSpecific.deviceContext;

    // Try to connect to DRI3 extension:
    dri3extension = xcb_get_extension_data(XGetXCBConnection(dpy), &xcb_dri3_id);
    if ((dri3extension == NULL) || !dri3extension->present) {
        return(FALSE);
    }

    cookie = xcb_dri3_query_version(XGetXCBConnection(dpy), XCB_DRI3_MAJOR_VERSION, XCB_DRI3_MINOR_VERSION);
    reply = xcb_dri3_query_version_reply(XGetXCBConnection(dpy), cookie, &error);
    free(error);
    if (reply == NULL) return(FALSE);

    major = reply->major_version;
    minor = reply->minor_version;
    free(reply);
    if (major < 0) return(FALSE);

    open_cookie = xcb_dri3_open(XGetXCBConnection(dpy), RootWindow(dpy, DefaultScreen(dpy)), None);
    open_reply = xcb_dri3_open_reply(XGetXCBConnection(dpy), open_cookie, NULL);

    if (!open_reply) return(FALSE);
    if (open_reply->nfd != 1) return(FALSE);
    close(xcb_dri3_open_reply_fds(XGetXCBConnection(dpy), open_reply)[0]);
    free(open_reply);

    // Seems we are running on a DRI3 capable driver. A modern Mesa
    // version would use DRI3/Present on such a driver. Is DRI3/Present
    // disabled by env variable?
    if (getenv("LIBGL_DRI3_DISABLE")) return(FALSE);

    // No. Check if we are on Mesa version 10.0.0 or later, as earlier Mesa versions
    // don't support DRI3/Present at all:
    const char* verstring = strstr((const char*) glGetString(GL_VERSION), "Mesa");
    if (!verstring || (sscanf(verstring, "Mesa %i.%i", &major, &minor) != 2) || (major < 10)) return(FALSE);

    // Ok, this is a DRI3 enabled driver, and we are on a DRI3/Present capable Mesa,
    // so in all likelyhood DRI3/Present will be used for rendering and presentation.
    // Is the X-Server recent enough? Servers older than 1.16.3 are seriously buggy
    // wrt. DRI3/Present, so a word of warning would be due:
    if (XVendorRelease(windowRecord->targetSpecific.deviceContext) < 11603000) {
        if (PsychPrefStateGet_Verbosity() > 1) {
            printf("\nPTB-WARNING: XServer version older than 1.16.3 with defective DRI3/Present implementation detected!\n");
            printf("PTB-WARNING: Stimulus presentation will not work correctly. Either upgrade your XServer, or disable\n");
            printf("PTB-WARNING: DRI3/Present. One way to disable DRI3 is to set the environment variable LIBGL_DRI3_DISABLE\n");
            printf("PTB-WARNING: to a non-zero value, then restart Octave or Matlab. Another way to disable DRI3 is via\n");
            printf("PTB-WARNING: some xorg.conf settings. Cfe. the man pages of your display driver via 'man intel',\n");
            printf("PTB-WARNING: 'man radeon' or 'man nouveau', depending on the graphics card you use.\n");
            printf("PTB-WARNING: Will continue, but expect hangs, sync failure or visually corrupted stimuli until this is fixed.\n\n");
        }
    }

    // DRI3/Present enabled and in use:
    windowRecord->specialflags |= kPsychIsDRI3Window;

    return(TRUE);
}

/*
 *    PsychOSOpenOnscreenWindow()
 *
 *    Creates the pixel format and the context objects and then instantiates the context onto the screen.
 *
 *    -The pixel format and the context are stored in the target specific field of the window recored.  Close
 *    should clean up by destroying both the pixel format and the context.
 *
 *    -We mantain the context because it must be be made the current context by drawing functions to draw into
 *    the specified window.
 *
 *    -We maintain the pixel format object because there seems to be now way to retrieve that from the context.
 *
 *    -To tell the caller to clean up PsychOSOpenOnscreenWindow returns FALSE if we fail to open the window. It
 *    would be better to just issue an PsychErrorExit() and have that clean up everything allocated outside of
 *    PsychOpenOnscreenWindow().
 */
psych_bool PsychOSOpenOnscreenWindow(PsychScreenSettingsType *screenSettings, PsychWindowRecordType *windowRecord, int numBuffers, int stereomode, int conserveVRAM)
{
    char windowTitle[32];
    PsychRectType screenrect;
    CGDirectDisplayID dpy;
    int scrnum;
    XSetWindowAttributes attr;
    unsigned long mask;
    Window root;
    Window win;
    GLXContext ctx;
    GLXFBConfig *fbconfig = NULL;
    GLXWindow glxwindow = (XID) 0;
    XVisualInfo *visinfo = NULL;
    int i, x, y, width, height, nrconfigs, buffdepth;
    GLenum glerr;
    int attrib[41];
    int attribcount=0;
    int stereoenableattrib=0;
    int depth, bpc;
    int windowLevel;
    int major, minor;
    int xfixes_event_base1, xfixes_event_base2;
    psych_bool xfixes_available = FALSE;
    psych_bool newstyle_setup = FALSE;
    int gpuMaintype = 0;
    const char* mesaver = NULL;
    psych_bool mesamapi_strdupbug = FALSE;
    int saved_default_screen = 0;

    // Include onscreen window index in title:
    sprintf(windowTitle, "PTB Onscreen Window [%i]:", windowRecord->windowIndex);

    // First opened onscreen window? If so, we try to map GPU MMIO registers
    // to enable beamposition based timestamping and other special goodies:
    if (x11_windowcount == 0) PsychScreenMapRadeonCntlMemory();

    // Retrieve windowLevel, an indicator of where non-fullscreen windows should
    // be located wrt. to other windows. 0 = Behind everything else, occluded by
    // everything else. 1 - 999 = At layer windowLevel -> Occludes stuff on layers "below" it.
    // 1000 - 1999 = At highest level, but partially translucent / alpha channel allows to make
    // regions transparent. 2000 or higher: Above everything, fully opaque, occludes everything.
    // 2000 is the default.
    windowLevel = PsychPrefStateGet_WindowShieldingLevel();

    // Set windowing system backend type to zero, aka "classic" aka X11/GLX: Not strictly needed, as this is init-default.
    windowRecord->winsysType = 0;

    // Set OpenGL api type to classic desktop OpenGL: Not strictly needed, as this is init-default.
    windowRecord->glApiType = 0;

    // Init userspace GL context to safe default:
    windowRecord->targetSpecific.glusercontextObject = NULL;
    windowRecord->targetSpecific.glswapcontextObject = NULL;

    // Which display depth is requested?
    depth = PsychGetValueFromDepthStruct(0, &(screenSettings->depth));

    // Map the logical screen number to the corresponding X11 display connection handle
    // for the corresponding X-Server connection.
    PsychGetCGDisplayIDFromScreenNumber(&dpy, screenSettings->screenNumber);
    scrnum = PsychGetXScreenIdForScreen(screenSettings->screenNumber);

    // Default to use of one shared x-display connection "dpy" for all onscreen windows
    // on a given x-display and x-screen:
    windowRecord->targetSpecific.privDpy = dpy;

    // Override with per-window x-display connection if requested by environment variable:
    usePerWindowXConnections = (getenv("PTB_USEPERWINDOWXCONNECTIONS")) ? TRUE : FALSE;
    if (usePerWindowXConnections) {
        // Open a dedicated X-Display connection for this onscreen window. This is meant to
        // avoid parallel ops on multiple onscreen windows, e.g., async swaps via flip-threads,
        // from blocking on a single shared x-display connection.
        // The dedicated handle is a clone of the x-display handle/connection for
        // the parent-screen associated with this onscreen window:
        //
        // NOTICE: As of September 2011 and X-Server 1.9.x and 1.10.x, this doesn't work
        //         well at all! It is entirely useless in its current form, just left for
        //         documentation, and in case we find some better - and actually working -
        //         use case for per-window x-display connections in the future.
        //
        // Problems with the current approach:
        //
        // * If we create each onscreen window (== GLXDrawable) and associated OpenGL
        //   contexts on a separate x-display connection, the OpenGL contexts apparently
        //   can't share resources like texture objects, FBO's, PBO's, VBO's, display lists
        //   or shaders. This is an absolute no-go for PTB's rendering architecture!
        //
        // * If we use the master dpy connection for OpenGL ops and only dedicated connections
        //   for non-OpenGL GLX ops, e.g., the OML_sync_control functions, then those functions
        //   fail with BadContext errors and i couldn't find any way or hack around it.
        //
        // * Other than those cases, there isn't any real use for dedicated x-display connections
        //   at the moment.
        //
        // It is unclear if these problems are due to bugs in the X-Server / DRI2 infrastructure,
        // really weird and subtile bugs in our code, or if we simply tried doing something that
        // is unsupported and forbidden by design of the X-Window system / X11-Protocol.
        //
        // Anyway leave the setup code here, it is disabled by default anyway, so no harm done,
        // and maybe useful for testing if it is our bug, their bug or unsupported behaviour...
        PsychLockDisplay();
        windowRecord->targetSpecific.privDpy = XOpenDisplay(DisplayString(dpy));
        PsychUnlockDisplay();
        if (NULL == windowRecord->targetSpecific.privDpy) {
            // Failed! We are sooo done :-(
            printf("\nPTB-ERROR[XOpenDisplay() failed]: Couldn't get a dedicated x-display connection for this window to X-Server.\n\n");
            return(FALSE);
        }
    }

    PsychLockDisplay();

    // XFixes extension version 2.0 or later available and initialized?
    if (XFixesQueryExtension(dpy, &xfixes_event_base1, &xfixes_event_base2) &&
        XFixesQueryVersion(dpy, &major, &minor) && (major >= 2)) xfixes_available = TRUE;
    major = minor = 0;

    // Init GLX extension, get its version, determine if at least V1.3 supported:
    useGLX13 = (glXQueryExtension(dpy, &glx_error_base, &glx_event_base) &&
    glXQueryVersion(dpy, &major, &minor) && ((major > 1) || ((major == 1) && (minor >= 3))));

    // Initialize GLX-1.3 protocol support. Use if possible:
    glXChooseFBConfig = (PFNGLXCHOOSEFBCONFIGPROC) glXGetProcAddressARB((const GLubyte *) "glXChooseFBConfig");
    glXGetFBConfigAttrib = (PFNGLXGETFBCONFIGATTRIBPROC) glXGetProcAddressARB((const GLubyte *) "glXGetFBConfigAttrib");
    glXGetVisualFromFBConfig = (PFNGLXGETVISUALFROMFBCONFIGPROC) glXGetProcAddressARB((const GLubyte *) "glXGetVisualFromFBConfig");
    glXCreateWindow = (PFNGLXCREATEWINDOWPROC) glXGetProcAddressARB((const GLubyte *) "glXCreateWindow");
    glXCreateNewContext = (PFNGLXCREATENEWCONTEXTPROC) glXGetProcAddressARB((const GLubyte *) "glXCreateNewContext");
    glXDestroyWindow = (PFNGLXDESTROYWINDOWPROC) glXGetProcAddressARB((const GLubyte *) "glXDestroyWindow");
    glXSelectEvent = (PFNGLXSELECTEVENTPROC) glXGetProcAddressARB((const GLubyte *) "glXSelectEvent");
    glXGetSelectedEvent = (PFNGLXGETSELECTEDEVENTPROC) glXGetProcAddressARB((const GLubyte *) "glXGetSelectedEvent");

    PsychUnlockDisplay();

    // Check if everything we need from GLX-1.3 is supported:
    if (!useGLX13 || !glXChooseFBConfig || !glXGetVisualFromFBConfig || !glXCreateWindow || !glXCreateNewContext ||
        !glXDestroyWindow || !glXSelectEvent || !glXGetSelectedEvent || !glXGetFBConfigAttrib) {
        useGLX13 = FALSE;
        printf("PTB-INFO: Not using GLX-1.3 extension. Unsupported? Some features may be disabled.\n");
    } else {
        useGLX13 = TRUE;
    }

    // Check if this should be a fullscreen window, and if not, what its dimensions
    // should be:
    PsychGetScreenRect(screenSettings->screenNumber, screenrect);
    if (PsychMatchRect(screenrect, windowRecord->rect)) {
        // This is supposed to be a fullscreen window with the dimensions of
        // the current display/desktop:
        x=0;
        y=0;
        width=PsychGetWidthFromRect(screenrect);
        height=PsychGetHeightFromRect(screenrect);

        // Mark this window as fullscreen window:
        windowRecord->specialflags |= kPsychIsFullscreenWindow;

        // Copy absolute screen location and area of window to 'globalrect',
        // so functions like Screen('GlobalRect') can still query the real
        // bounding gox of a window onscreen:
        PsychGetGlobalScreenRect(screenSettings->screenNumber, windowRecord->globalrect);
    }
    else {
        // Window size different from current screen size:
        // A regular desktop window with borders and control icons is requested, e.g., for debugging:
        // Extract settings:
        x=windowRecord->rect[kPsychLeft];
        y=windowRecord->rect[kPsychTop];
        width=PsychGetWidthFromRect(windowRecord->rect);
        height=PsychGetHeightFromRect(windowRecord->rect);

        // Copy absolute screen location and area of window to 'globalrect',
        // so functions like Screen('GlobalRect') can still query the real
        // bounding gox of a window onscreen:
        PsychCopyRect(windowRecord->globalrect, windowRecord->rect);
    }

    // Select requested depth per color component 'bpc' for each channel:
    bpc = 8; // We default to 8 bpc == RGBA8
    if (windowRecord->depth == 30)  { bpc = 10; printf("PTB-INFO: Trying to enable at least 10 bpc fixed point framebuffer.\n"); }
    if (windowRecord->depth == 33)  { bpc = 11; printf("PTB-INFO: Trying to enable at least 11 bpc fixed point framebuffer.\n"); }
    if (windowRecord->depth == 48)  { bpc = 16; printf("PTB-INFO: Trying to enable at least 16 bpc fixed point framebuffer.\n"); }
    if (windowRecord->depth == 64)  { bpc = 16; printf("PTB-INFO: Trying to enable 16 bpc fixed point framebuffer.\n"); }
    if (windowRecord->depth == 128) { bpc = 32; printf("PTB-INFO: Trying to enable 32 bpc fixed point framebuffer.\n"); }

    // Setup pixelformat descriptor for selection of GLX visual:
    if (useGLX13) {
        attrib[attribcount++]= GLX_RENDER_TYPE; // Use RGBA true-color visual.
        attrib[attribcount++]= GLX_RGBA_BIT;    // Use RGBA true-color visual.
    } else {
        attrib[attribcount++]= GLX_RGBA;        // Use RGBA true-color visual.
    }

    attrib[attribcount++]= GLX_RED_SIZE;    // Setup requested minimum depth of each color channel:
    attrib[attribcount++]= (depth > 16) ? bpc : 1;
    attrib[attribcount++]= GLX_GREEN_SIZE;
    attrib[attribcount++]= (depth > 16) ? bpc : 1;
    attrib[attribcount++]= GLX_BLUE_SIZE;
    attrib[attribcount++]= (depth > 16) ? bpc : 1;
    attrib[attribcount++]= GLX_ALPHA_SIZE;
    // Alpha channel needs special treatment:
    if ((bpc != 10) && (bpc != 11)) {
        // Non 10/11 bpc drawable: Request a 'bpc' alpha channel if the underlying framebuffer
        // is in true-color mode ( >= 24 cpp format). If framebuffer is in 16 bpp mode, we
        // don't have/request an alpha channel at all:
        attrib[attribcount++]= (depth > 16) ? bpc : 0; // In 16 bit mode, we don't request an alpha-channel.
    }
    else if (bpc == 10) {
        // 10 bpc drawable: We have a 32 bpp pixel format with R10G10B10 10 bpc per color channel.
        // There are at most 2 bits left for the alpha channel, so we request an alpha channel with
        // minimum size 1 bit --> Will likely translate into a 2 bit alpha channel:
        attrib[attribcount++]= 1;
    }
    else {
        // 11 bpc drawable - or more likely a 32 bpp drawable with R11G11B10, ie., all 32 bpp
        // used up by RGB color info and no space for alpha bits. Therefore do not request an
        // alpha channel:
        attrib[attribcount++]= 0;
    }

    // Stereo display support: If stereo display output is requested with OpenGL native stereo,
    // we request a stereo-enabled rendering context.
    if (stereomode==kPsychOpenGLStereo) {
        attrib[attribcount++]= GLX_STEREO;
        stereoenableattrib = attribcount;
        attrib[attribcount++]= True;
    }

    // Multisampling support:
    if (windowRecord->multiSample > 0) {
        // Request a multisample buffer:
        attrib[attribcount++]= GLX_SAMPLE_BUFFERS_ARB;
        attrib[attribcount++]= 1;
        // Request at least multiSample samples per pixel:
        attrib[attribcount++]= GLX_SAMPLES_ARB;
        attrib[attribcount++]= windowRecord->multiSample;
    }

    // Support for OpenGL 3D rendering requested?
    if (PsychPrefStateGet_3DGfx()) {
        // Yes. Allocate and attach a 24 bit depth buffer and 8 bit stencil buffer:
        attrib[attribcount++]= GLX_DEPTH_SIZE;
        attrib[attribcount++]= 24;
        attrib[attribcount++]= GLX_STENCIL_SIZE;
        attrib[attribcount++]= 8;

        // Alloc an accumulation buffer as well?
        if (PsychPrefStateGet_3DGfx() & 2) {
            // Yes: Alloc accum buffer, request 64 bpp, aka 16 bits integer per color component if possible:
            attrib[attribcount++] = GLX_ACCUM_RED_SIZE;
            attrib[attribcount++] = 16;
            attrib[attribcount++] = GLX_ACCUM_GREEN_SIZE;
            attrib[attribcount++] = 16;
            attrib[attribcount++] = GLX_ACCUM_BLUE_SIZE;
            attrib[attribcount++] = 16;
            attrib[attribcount++] = GLX_ACCUM_ALPHA_SIZE;
            attrib[attribcount++] = 16;
        }
    }

    // Double buffering requested?
    if(numBuffers>=2) {
        // Enable double-buffering:
        attrib[attribcount++]= GLX_DOUBLEBUFFER;
        attrib[attribcount++]= True;

        // AUX buffers for Flip-Operations needed?
        if ((conserveVRAM & kPsychDisableAUXBuffers) == 0) {
            // Allocate one or two (mono vs. stereo) AUX buffers for new "don't clear" mode of Screen('Flip'):
            // Not clearing the framebuffer after "Flip" is implemented by storing a backup-copy of
            // the backbuffer to AUXs before flip and restoring the content from AUXs after flip.
            attrib[attribcount++]= GLX_AUX_BUFFERS;
            attrib[attribcount++]=(stereomode==kPsychOpenGLStereo || stereomode==kPsychCompressedTLBRStereo || stereomode==kPsychCompressedTRBLStereo) ? 2 : 1;
        }
    }

    // It's important that GLX_AUX_BUFFERS is the last entry in the attrib array, see code for glXChooseVisual below...

    // Finalize attric array:
    attrib[attribcount++]= None;

    PsychLockDisplay();

    root = RootWindow( dpy, scrnum );

    // Select matching visual for our pixelformat:
    if (useGLX13) {
        fbconfig = glXChooseFBConfig(dpy, scrnum, attrib, &nrconfigs);
    } else {
        visinfo = glXChooseVisual(dpy, scrnum, attrib );
    }

    if (!visinfo && !fbconfig && (stereoenableattrib > 0)) {
        // Failed to find matching visual and OpenGL native quad-buffered frame-sequential
        // stereo requested. Probably the GPU does not support it. Disable it as we have a
        // fallback implementation for this case.
        attrib[stereoenableattrib] = False;

        // Retry:
        if (useGLX13) {
            fbconfig = glXChooseFBConfig(dpy, scrnum, attrib, &nrconfigs);
        } else {
            visinfo = glXChooseVisual(dpy, scrnum, attrib );
        }
    }

    if (!visinfo && !fbconfig) {
        // Failed to find matching visual: Could it be related to request for unsupported native 10/11/16 bpc framebuffer?
        if (((windowRecord->depth == 30) && (bpc == 10)) || ((windowRecord->depth == 33) && (bpc == 11)) || ((windowRecord->depth == 48) && (bpc == 16))) {
            // 10/11/16 bpc framebuffer requested: Let's see if we can get a visual by lowering our demand to 8 bpc:
            for (i=0; i<attribcount && attrib[i]!=GLX_RED_SIZE; i++);
            attrib[i+1] = 8;
            for (i=0; i<attribcount && attrib[i]!=GLX_GREEN_SIZE; i++);
            attrib[i+1] = 8;
            for (i=0; i<attribcount && attrib[i]!=GLX_BLUE_SIZE; i++);
            attrib[i+1] = 8;
            for (i=0; i<attribcount && attrib[i]!=GLX_ALPHA_SIZE; i++);
            attrib[i+1] = 1;

            // Retry:
            if (useGLX13) {
                fbconfig = glXChooseFBConfig(dpy, scrnum, attrib, &nrconfigs);
            } else {
                visinfo = glXChooseVisual(dpy, scrnum, attrib );
            }
        }
    }

    if (!visinfo && !fbconfig) {
        // Failed to find matching visual: Could it be related to multisampling?
        if (windowRecord->multiSample > 0) {
            // Multisampling requested: Let's see if we can get a visual by
            // lowering our demand:
            for (i=0; i<attribcount && attrib[i]!=GLX_SAMPLES_ARB; i++);
            while(!visinfo && !fbconfig && windowRecord->multiSample > 0) {
                attrib[i+1]--;
                windowRecord->multiSample--;

                if (useGLX13) {
                    fbconfig = glXChooseFBConfig(dpy, scrnum, attrib, &nrconfigs);
                } else {
                    visinfo = glXChooseVisual(dpy, scrnum, attrib );
                }
            }

            // Either we have a valid visual at this point or we still fail despite
            // requesting zero samples.
            if (!visinfo && !fbconfig) {
                // We still fail. Disable multisampling by requesting zero multisample buffers:
                for (i=0; i<attribcount && attrib[i]!=GLX_SAMPLE_BUFFERS_ARB; i++);
                windowRecord->multiSample = 0;
                attrib[i+1]=0;

                if (useGLX13) {
                    fbconfig = glXChooseFBConfig(dpy, scrnum, attrib, &nrconfigs);
                } else {
                    visinfo = glXChooseVisual(dpy, scrnum, attrib );
                }
            }
        }

        // Break out of this if we finally got one...
        if (!visinfo && !fbconfig) {
            // Failed to find matching visual: This can happen if we request AUX buffers on a system
            // that doesn't support AUX-buffers. In that case we retry without requesting AUX buffers
            // and output a proper warning instead of failing. For 99% of all applications one can
            // do without AUX buffers anyway...
            //printf("PTB-WARNING: Couldn't enable AUX buffers on onscreen window due to limitations of your gfx-hardware or driver. Some features may be disabled or limited...\n");
            //fflush(NULL);

            // Terminate attrib array where the GLX_AUX_BUFFERS entry used to be...
            attrib[attribcount-3] = None;

            // Retry...
            if (useGLX13) {
                fbconfig = glXChooseFBConfig(dpy, scrnum, attrib, &nrconfigs);
            } else {
                visinfo = glXChooseVisual(dpy, scrnum, attrib );
            }

            if (!visinfo && !fbconfig && PsychPrefStateGet_3DGfx()) {
                // Ok, retry with a 16 bit depth buffer...
                for (i=0; i<attribcount && attrib[i]!=GLX_DEPTH_SIZE; i++);
                if (attrib[i]==GLX_DEPTH_SIZE && i<attribcount) attrib[i+1]=16;
                printf("PTB-WARNING: Have to use 16 bit depth buffer instead of 24 bit buffer due to limitations of your gfx-hardware or driver. Accuracy of 3D-Gfx may be limited...\n");
                fflush(NULL);

                if (useGLX13) {
                    fbconfig = glXChooseFBConfig(dpy, scrnum, attrib, &nrconfigs);
                } else {
                    visinfo = glXChooseVisual(dpy, scrnum, attrib );
                }
                if (!visinfo && !fbconfig) {
                    // Failed again. Retry with disabled stencil buffer:
                    printf("PTB-WARNING: Have to disable stencil buffer due to limitations of your gfx-hardware or driver. Some 3D Gfx algorithms may fail...\n");
                    fflush(NULL);
                    for (i=0; i<attribcount && attrib[i]!=GLX_STENCIL_SIZE; i++);
                    if (attrib[i]==GLX_STENCIL_SIZE && i<attribcount) attrib[i+1]=0;
                    if (useGLX13) {
                        fbconfig = glXChooseFBConfig(dpy, scrnum, attrib, &nrconfigs);
                    } else {
                        visinfo = glXChooseVisual(dpy, scrnum, attrib );
                    }
                }
            }
        }
    }

    if (!visinfo && !fbconfig) {
        // Another possible problem: No alpha channel available, e.g., on the XMing X-Server for MS-Windows hosts:
        printf("PTB-WARNING: Have to disable alpha channel due to limitations of your gfx-hardware or driver. Some 3D Gfx algorithms may fail, Screen('Blendfunction') will be ineffective.\n");
        for (i = 0; i < attribcount && attrib[i] != GLX_ALPHA_SIZE; i++);
        attrib[i+1] = 0;

        // Retry:
        if (useGLX13) {
            fbconfig = glXChooseFBConfig(dpy, scrnum, attrib, &nrconfigs);
        } else {
            visinfo = glXChooseVisual(dpy, scrnum, attrib );
        }
    }

    PsychUnlockDisplay();

    if (!visinfo && !fbconfig) {
        printf("\nPTB-ERROR[glXChooseVisual() failed]: Couldn't get any suitable visual from X-Server.\n\n");
        return(FALSE);
    }

    PsychLockDisplay();

    if (fbconfig && (windowLevel >=1000 && windowLevel < 2000)) {
        // Transparent window requested and fbconfig's found. Iterate over them
        // and try to find one with 32 bit color depths:
        for (i = 0; i < nrconfigs; i++) {
            buffdepth = 0;
            if ((Success == glXGetFBConfigAttrib(dpy, fbconfig[i], GLX_BUFFER_SIZE, &buffdepth)) && (buffdepth >= 32) &&
                (visinfo = glXGetVisualFromFBConfig(dpy, fbconfig[i])) && (visinfo->depth >= 32)) {
                fbconfig[0] = fbconfig[i];
            if (PsychPrefStateGet_Verbosity() > 3) printf("PTB-INFO: Choosing GLX framebuffer config %i for transparent window.\n", i);
            break;
                }
                else if (PsychPrefStateGet_Verbosity() > 4) {
                    printf("PTB-INFO: Trying GLX framebuffer config %i for transparent window: Depths %i bpp.\n", i, buffdepth);
                }
        }
    }

    // If this setup is fbconfig based, get associated visual:
    if (fbconfig) visinfo = glXGetVisualFromFBConfig(dpy, fbconfig[0]);

    // Setup basic window attributes:
    attr.background_pixel = 0;  // Background color defaults to black.
    attr.border_pixel = 0;      // Border color as well.
    attr.colormap = XCreateColormap(dpy, root, visinfo->visual, AllocNone);  // Dummy colormap assignment.
    attr.event_mask = KeyPressMask | StructureNotifyMask;                    // We're only interested in keypress events for GetChar() and StructureNotify to wait for Windows to be mapped.

    // Mask of everything we define(d):
    mask = CWOverrideRedirect | CWBackPixel | CWBorderPixel | CWColormap | CWEventMask;

    // Old style of override_redirect handling requested? This was used until beginning 2013
    // and worked well for us, but it prevents the windowmanager from seeing properties on
    // our windows which allow us to control desktop composition, e.g., on KDE/KWIN and GNOME-3/Mutter,
    // as well as on other wm's compliant with latest ICCCM spec:
    // Ok, for now we only use the new-style path if we are running under KDE/KWin and user
    // doesn't explicitely override/forbid that choice. Otherwise we use the old path, as
    // that seems to perform better, at least on tested Unity/compiz, GNOME3-Shell and LXDE/OpenBox.
    //
    // UPDATE June-2014: Do not even use new-style on KDE, unless forced by setenv("PSYCH_NEW_OVERRIDEREDIRECT", "1")
    // Turns out the new-style override redirect doesn't play well with KDE multi-display setups. It causes KDE
    // to cut off all parts of the fullscreen window except for the first video output, making this unworkable on
    // anything but single display setups. We may rework this code at some later point, but for now just disable.
    //
    // UPDATE September-2014: Unless we are using KDE on an Intel gpu, where god-knows-why we need new-style
    // override redirect handling, because KDE doesn't recognize our fullscreen windows as such and wouldn't
    // unredirect them without the new override redirect setup and signalling. Strangely other desktop environments
    // do have no problem detecting our fullscreen windows on an Intel gpu, e.g., Unity, GNOME-3/GNOME-2, ...
    //
    // UPDATE November-2014: KDE doesn't unredirect fullscreen windows by default on Intel gpus deliberately,
    // to work around bugs in some versions of the Intel-ddx, that's why the Intel trouble. So we must use
    // new-style for KDE to unredirect fullscreen windows. However, this doesn't work for multi-display
    // spanning windows due to what i think is a KWin bug. Therefore we enforce old-style setup again if there
    // are at least two video outputs connected to our target x-screen. This means the user has to manually
    // disable desktop composition if he needs dual/multi-display stimulation on a Intel gpu under KDE/KWin,
    // as the manual "user says so" override is the only method that worked. (KWin window rules were proven
    // ineffective as well):
    //
    // UPDATE April-2015: Use newstyle_setup if user wants it, or if this is a KDE single display setup,
    // where it helps. On KDE multi-display we can't use it due to the KWin problems mentioned above, on
    // other desktop environments we don't need it. This is like before, just we also use this on KDE +
    // non-Intel gpu's, to save the user the extra setup step for "unredirect_fullscreen_windows" in the KDE
    // GUI, as this is a bit more convenient.
    //
    // UPDATE June-2015: Use old style setup also on KDE with multiple X-Screens, not only multiple outputs
    // on one X-Screen. Otherwise at least the future KDE 5.3 Plasma desktop will do stupid things.
    PsychGetGPUSpecs(screenSettings->screenNumber, &gpuMaintype, NULL, NULL, NULL);
    if (!getenv("PSYCH_NEW_OVERRIDEREDIRECT") &&
        ((PsychPrefStateGet_ConserveVRAM() & kPsychOldStyleOverrideRedirect) ||
        !getenv("KDE_FULL_SESSION") || (PsychScreenToHead(screenSettings->screenNumber, 1) >= 0) ||
        (PsychGetNumDisplays() > 1))) {
        // Old style: Always override_redirect to lock out window manager, except when a real "GUI-Window"
        // is requested, which needs to behave and be treated like any other desktop app window:
        attr.override_redirect = (windowRecord->specialflags & kPsychGUIWindow) ? 0 : 1;
    }
    else {
        // New style: override_redirect by default:
        newstyle_setup = TRUE;

        attr.override_redirect = 1;

        // Don't override if it is a GUI window, for some reasons as in classic path:
        if (windowRecord->specialflags & kPsychGUIWindow) attr.override_redirect = 0;

        // Don't override if it is a fullscreen window. The NETM_FULLSCREEN state should
        // take care of fullscreen windows nicely without need to override. Although we
        // could override for transparent fullscreen windows if we were extra paranoid,
        // we *must not* override for opaque (standard) fullscreen windows, because that
        // would prevent us from disabling desktop composition on our window by use of
        // special properties -- the window manager wouldn't notice those properties/requests
        // on a override_redirected window.
        // Now here's a catch: While the WM picks up our window properties this way, and at
        // least KWIN honors them by disabling composition, the WM also takes this opportunity
        // to "misplace" our onscreen window on multi-display setups. It follows various rules on
        // where a fullscreen window should be placed on a multi-display setup, but while these
        // rules are pretty sensible for regular desktop users, they are absolutely not what we
        // want. How to solve? We let the WM have its way during window creation, so it picks up
        // our window props. Later on (see below) after the Window is mapped and the WM satisified,
        // we set the override_redirect flag to lock out the WM, then move the window to its proper
        // location, no that the WM can't interfere anymore.
        if (windowRecord->specialflags & kPsychIsFullscreenWindow) attr.override_redirect = 0;
    }

    if (PsychPrefStateGet_Verbosity() > 3) printf("PTB-INFO: Using %s-style override-redirect setup path for onscreen window creation.\n", (newstyle_setup) ? "new" : "old");

    // Create our onscreen window:
    win = XCreateWindow( dpy, root, x, y, width, height,
                            0, visinfo->depth, InputOutput,
                            visinfo->visual, mask, &attr );

    if (PsychPrefStateGet_Verbosity() > 4) printf("PTB-INFO: GLX Visual info depths is %i bits\n", visinfo->depth);

    // Set hints and properties:
    {
        XSizeHints sizehints;
        sizehints.x = x;
        sizehints.y = y;
        sizehints.width  = width;
        sizehints.height = height;
        // Let window manager control window position if kPsychGUIWindowWMPositioned is set:
        sizehints.flags = USSize | (windowRecord->specialflags & kPsychGUIWindowWMPositioned) ? 0 : USPosition;
        XSetNormalHints(dpy, win, &sizehints);
        XSetStandardProperties(dpy, win, windowTitle, "PTB Onscreen window",
                                None, (char **)NULL, 0, &sizehints);
    }

    // Setup window transparency for user input (keyboard and mouse events):
    if (xfixes_available && (windowLevel >= 1000) && (windowLevel < 1500)) {
        // Define region as an empty input region:
        XserverRegion region = XFixesCreateRegion(dpy, NULL, 0);

        // Assign as region in which window receives input events, thereby
        // setting the input region to empty, so the window is transparent
        // to any input events like key presses or mouse clicks:
        XFixesSetWindowShapeRegion(dpy, win, ShapeInput, 0, 0, region);

        // Destroy region after assignment:
        XFixesDestroyRegion(dpy, region);
    }

    // Create corresponding glx window:
    if (fbconfig) {
        glxwindow = glXCreateWindow(dpy, fbconfig[0], win, NULL);
    }

    PsychUnlockDisplay();

    // Make sure a potential slaveWindow of us resides on the same X-Screen == has same screenNumber as us,
    // otherwise trying to perform OpenGL context resource sharing would end badly:
    if ((windowRecord->slaveWindow) && (windowRecord->slaveWindow->screenNumber != screenSettings->screenNumber)) {
        // Ohoh! Let's abort with some more helpful error message than a simple hard application crash:
        printf("\nPTB-ERROR:[glXCreateContext() resource sharing] Our peer window resides on a different X-Screen, which is forbidden. Aborting.\n\n");
        return(FALSE);
    }

    PsychLockDisplay();

    // Create associated GLX OpenGL rendering context: We use ressource
    // sharing of textures, display lists, FBO's and shaders if 'slaveWindow'
    // is assigned for that purpose as master-window. We request a direct
    // rendering context (True) if possible:
    if (fbconfig) {
        // Workaround for bug introduced into Mesa 12.0, potentially backported to some 11.2 releases
        // by commit cf804b4455fac9e585b3600a8318caaced9c23de. glXCreateNewContext() does not perform
        // some validation of fbconfig correctly against the X-Screen stored in the fbconfig for our
        // target X-Screen, but instead always validates against X-Screen 0 -- the DefaultScreen(dpy).
        // This ends badly whenever we are not trying to create a context on X-Screen 0. Work around
        // this by temporarily assigning dpy->default_screen as our target screen 'scrnum' while
        // glXCreateNewContext is called. Note: A bug fix for this bug was submitted upstream to resolve
        // this properly, but that doesn't help us if we need to run on affected Mesa 12.0.x releases,
        // as shipping with Ubuntu 16.10 and presumably future Ubuntu 16.04.2-LTS:
        saved_default_screen = ((_XPrivDisplay) dpy)->default_screen;
        ((_XPrivDisplay) dpy)->default_screen = scrnum;

        ctx = glXCreateNewContext(dpy, fbconfig[0], GLX_RGBA_TYPE, ((windowRecord->slaveWindow) ? windowRecord->slaveWindow->targetSpecific.contextObject : NULL), True);
    } else {
        ctx = glXCreateContext(dpy, visinfo, ((windowRecord->slaveWindow) ? windowRecord->slaveWindow->targetSpecific.contextObject : NULL), True );
    }

    PsychUnlockDisplay();

    if (!ctx) {
        printf("\nPTB-ERROR:[glXCreateContext() failed] OpenGL context creation failed!\n\n");
        return(FALSE);
    }

    // Store the handles:

    // windowHandle is a GLXWindow. Fallback path assigns a Window. Both are typedef'd
    // to XID, so this cast and storage is safe to do:
    windowRecord->targetSpecific.windowHandle = (fbconfig) ? glxwindow : (GLXWindow) win;

    // xwindowHandle stores the underlying X-Window:
    windowRecord->targetSpecific.xwindowHandle = win;

    windowRecord->targetSpecific.deviceContext = dpy;
    windowRecord->targetSpecific.contextObject = ctx;

    // Set flags to mark this window as a classic X11/GLX window:
    windowRecord->specialflags |= (kPsychIsX11Window | kPsychIsGLXWindow);

    PsychLockDisplay();

    // Create rendering context for async flips with identical visual and display as main context, share all heavyweight ressources with it:
    if (fbconfig) {
        windowRecord->targetSpecific.glswapcontextObject = glXCreateNewContext(dpy, fbconfig[0], GLX_RGBA_TYPE, windowRecord->targetSpecific.contextObject, True);
    } else {
        windowRecord->targetSpecific.glswapcontextObject = glXCreateContext(dpy, visinfo, windowRecord->targetSpecific.contextObject, True);
    }

    PsychUnlockDisplay();

    if (windowRecord->targetSpecific.glswapcontextObject == NULL) {
        printf("\nPTB-ERROR[SwapContextCreation failed]: Creating a private OpenGL context for async flips failed for unknown reasons.\n\n");
        return(FALSE);
    }

    // External 3D graphics support enabled?
    if (PsychPrefStateGet_3DGfx()) {
        // Yes. We need to create an extra OpenGL rendering context for the external
        // OpenGL code to provide optimal state-isolation. The context shares all
        // heavyweight ressources likes textures, FBOs, VBOs, PBOs, display lists and
        // starts off as an identical copy of PTB's context as of here.
        PsychLockDisplay();

        // Create rendering context with identical visual and display as main context, share all heavyweight ressources with it:
        if (fbconfig) {
            windowRecord->targetSpecific.glusercontextObject = glXCreateNewContext(dpy, fbconfig[0], GLX_RGBA_TYPE, windowRecord->targetSpecific.contextObject, True);
        } else {
            windowRecord->targetSpecific.glusercontextObject = glXCreateContext(dpy, visinfo, windowRecord->targetSpecific.contextObject, True);
        }

        PsychUnlockDisplay();

        if (windowRecord->targetSpecific.glusercontextObject == NULL) {
            printf("\nPTB-ERROR[UserContextCreation failed]: Creating a private OpenGL context for Matlab OpenGL failed for unknown reasons.\n\n");
            return(FALSE);
        }
    }

    // Release visual info:
    XFree(visinfo);

    // Release fbconfig array, if any:
    if (fbconfig) {
        // Restore true default screen for dpy:
        ((_XPrivDisplay) dpy)->default_screen = saved_default_screen;
        XFree(fbconfig);
    }

    PsychLockDisplay();

    // Setup window transparency:
    if ((windowLevel >= 1000) && (windowLevel < 2000)) {
        // For windowLevels between 1000 and 1999, make the window background transparent, so standard GUI
        // would be visible, wherever nothing is drawn, i.e., where alpha channel is zero:

        // Levels 1000 - 1499 and 1500 to 1999 map to a master opacity level of 0.0 - 1.0:
        unsigned int opacity = (unsigned int) (0xffffffff * (((float) (windowLevel % 500)) / 499.0));

        // Get handle on opacity property of X11:
        Atom atom_window_opacity = XInternAtom(dpy, "_NET_WM_WINDOW_OPACITY", False);

        // Assign new value for property:
        XChangeProperty(dpy, win, atom_window_opacity, XA_CARDINAL, 32, PropModeReplace, (unsigned char *) &opacity, 1);
    }

    // Is this a non-transparent (fully opaque), non-GUI, fullscreen onscreen window?
    if (!((windowLevel >= 1000) && (windowLevel < 2000)) && !(windowRecord->specialflags & kPsychGUIWindow) &&
        (windowRecord->specialflags & kPsychIsFullscreenWindow)) {
        // Yes. This is a standard stimulus presentation window which should get best
        // timing precision and performance for stimulus presentation. We don't want
        // any desktop composition to interfere with it, so it is eligible for direct
        // page-flipping (unredirected). If we are running under KDE's KWin desktop
        // manager, then we can explicitely ask KWin to disable compositing while
        // our onscreen window is open, by setting a special NETWM property on the window.
        // This approach has just become a NETWM standard that should work with other
        // compositors in the future, e.g., Mutter/GNOME-3 as of 18th December 2012.
        //
        // On other compositors, e.g., compiz / unity et al. this problem is solved by
        // asking them to unredirect_fullscreen_windows, as done by PsychGPUControl.m during
        // installation of PTB.
        //
        // Btw. for other properties that KDE supports/understands see function create_netwm_atoms()
        // in file netwm.cpp, e.g., at http://code.woboq.org/kde/kdelibs/kdeui/windowmanagement/netwm.cpp.html
        //

        // Set KDE-4 specific property: This is supported since around KWin 4.6, since July 2011:
        unsigned int dontcomposite = 1;
        Atom atom_window_dontcomposite = XInternAtom(dpy, "_KDE_NET_WM_BLOCK_COMPOSITING", False);

        // Assign new value for property:
        XChangeProperty(dpy, win, atom_window_dontcomposite, XA_CARDINAL, 32, PropModeReplace, (unsigned char *) &dontcomposite, 1);

        // Set the standardized NETWM property. This is supported in Mutter (== GNOME-3) since
        // 18. December 2012 (see last comment/patch in https://bugzilla.gnome.org/show_bug.cgi?id=683020 ),
        // and will supposedly get supported by other compositing window managers in the future as well,
        // e.g., future KWin/KDE releases or possibly Unity/Compiz:
        dontcomposite = 1;
        Atom atom_window_dontcomposite2 = XInternAtom(dpy, "_NET_WM_BYPASS_COMPOSITOR", False);

        // Assign new value for property:
        XChangeProperty(dpy, win, atom_window_dontcomposite2, XA_CARDINAL, 32, PropModeReplace, (unsigned char *) &dontcomposite, 1);
    }

    // Is this a non-GUI fullscreen window? If so, set the fullscreen NETWM property:
    if (!(windowRecord->specialflags & kPsychGUIWindow) && (windowRecord->specialflags & kPsychIsFullscreenWindow)) {
        // Yes. Set the fullscreen state hint. Any well behaved window manager should understand this as
        // a request to turn the window into a completely decorationless fullscreen window, something
        // similar to what you'd get with override_redirect = 1, just in a less dirty way, that still
        // allows us to communicate our wishes, e.g., wrt. desktop composition, to the window manager.
        Atom stateFullscreen = XInternAtom(dpy, "_NET_WM_STATE_FULLSCREEN", False);
        XChangeProperty(dpy, win, XInternAtom(dpy, "_NET_WM_STATE", False), XA_ATOM, 32, PropModeReplace, (unsigned char *) &stateFullscreen, 1);
    }

    // For some reason we need to use unsigned long and long here instead of
    // int32_t etc., despite the fact that on a 64-Bit build, a long is 64-Bit
    // and on a 32-Bit build, a long is 32-Bit, whereas the XChangeProperty()
    // request says a single unit is 32-Bits? Anyway, it works correctly on a
    // 64-Bit build, so this seems to be magically ok.
    struct MwmHints {
        unsigned long flags;
        unsigned long functions;
        unsigned long decorations;
        long          input_mode;
        unsigned long status;
    };

    enum {
        MWM_HINTS_FUNCTIONS = (1L << 0),
        MWM_HINTS_DECORATIONS =  (1L << 1),

        MWM_FUNC_ALL = (1L << 0),
        MWM_FUNC_RESIZE = (1L << 1),
        MWM_FUNC_MOVE = (1L << 2),
        MWM_FUNC_MINIMIZE = (1L << 3),
        MWM_FUNC_MAXIMIZE = (1L << 4),
        MWM_FUNC_CLOSE = (1L << 5)
    };

    Atom mwmHintsProperty = XInternAtom(dpy, "_MOTIF_WM_HINTS", False);

    struct MwmHints hints;
    memset(&hints, 0, sizeof(hints));
    hints.flags = MWM_HINTS_DECORATIONS | MWM_HINTS_FUNCTIONS;

    // If this window is a GUI window then enable all window decorations and
    // manipulations, except for the window close button, which would wreak havoc:
    if (windowRecord->specialflags & kPsychGUIWindow) {
        hints.decorations = MWM_FUNC_ALL;
        hints.functions   = MWM_FUNC_RESIZE | MWM_FUNC_MOVE | MWM_FUNC_MINIMIZE | MWM_FUNC_MAXIMIZE;
    }
    else {
        // No GUI window: Prevent the user from interacting with / manipulating the window by defining an empty list of allowed actions:
        XChangeProperty(dpy, win, XInternAtom(dpy, "_NET_WM_ALLOWED_ACTIONS", False), XA_ATOM, 32, PropModeReplace, (unsigned char *) NULL, 0);

        // No decorations:
        hints.decorations = 0;

        // No interaction via any controls (buttons etc.):
        hints.functions   = 0;
    }

    // Apply hints:
    XChangeProperty(dpy, win, mwmHintsProperty, mwmHintsProperty, 32, PropModeReplace, (unsigned char *) &hints, sizeof(hints) / sizeof(long));

    // For windowLevels of at least 500, tell window manager to try to keep
    // our window above most other windows, by setting the state to WM_STATE_ABOVE:
    // Note: Can't use it for level 2000. On level 2000 it will be automatically above others, so this
    // isn't needed, but setting it would *undo* some required settings for fullscreen opaque windows.
    if ((windowLevel >= 500) && (windowLevel < 2000)) {
        Atom stateAbove = XInternAtom(dpy, "_NET_WM_STATE_ABOVE", False);
        XChangeProperty(dpy, win, XInternAtom(dpy, "_NET_WM_STATE", False), XA_ATOM, 32, PropModeReplace, (unsigned char *) &stateAbove, 1);
    }

    // Show our new window: Also raise it to the top for
    // non-zero window levels:
    if (windowLevel > 0) {
        XMapRaised(dpy, win);
    } else if (windowLevel != -1) {
        XMapWindow(dpy, win);
    }

    PsychUnlockDisplay();

    // Spin-Wait for it to be really mapped:
    while (windowLevel != -1) {
        XEvent ev;
        PsychLockDisplay();
        XNextEvent(dpy, &ev);
        PsychUnlockDisplay();
        if (ev.type == MapNotify)
            break;
    }

    PsychLockDisplay();

    // If windowLevel is zero, lower it to the bottom of the stack of windows:
    if (windowLevel == 0) XLowerWindow(dpy, win);

    // Setup window transparency for user input (keyboard and mouse events):
    if ((windowLevel < 1500) && (windowLevel >= 1000)) {
        // Need to try to be transparent for keyboard events and mouse clicks:
        XSetInputFocus(dpy, PointerRoot, RevertToPointerRoot, CurrentTime);
    }

    // Is this a non-GUI fullscreen window? In the new-style path?
    if (!(windowRecord->specialflags & kPsychGUIWindow) && (windowRecord->specialflags & kPsychIsFullscreenWindow) && newstyle_setup) {
        // Yes. As we didn't override_redirect it during creation and mapping, so the
        // WM could pick up our special window properties, the WM very likely misplaced
        // it on the screen at a very ergonomic location for desktop users which doesn't
        // suit our needs. Let's fix this:

        // First we override_redirect it to lock out the WM from further manipulations:
        attr.override_redirect = 1;
        XChangeWindowAttributes(dpy, win, mask, &attr);

        // Wait for override to complete...
        XSync(dpy, False);

        // Then we move it to its proper location, now hopefully untampered by the WM:
        XMoveWindow(dpy, win, x, y);

        // Make sure it reaches its target position:
        XSync(dpy, False);
    }

    PsychUnlockDisplay();

    // Ok, the onscreen window is ready on the screen. Time for OpenGL setup...

    // Activate the associated rendering context:
    PsychOSSetGLContext(windowRecord);

    PsychLockDisplay();

    // Try to detect if we are running on top of Mesa OpenGL, and which version:
    mesaver = strstr((const char*) glGetString(GL_VERSION), "Mesa");
    if (mesaver && (3 == sscanf(mesaver, "Mesa %i.%i.%i", &mesaversion[0], &mesaversion[1], &mesaversion[2]))) {
        if (PsychPrefStateGet_Verbosity() > 3) printf("PTB-INFO: Running on Mesa version %i.%i.%i\n", mesaversion[0], mesaversion[1], mesaversion[2]);
    }
    else {
        mesaversion[0] = mesaversion[1] = mesaversion[2] = 0;
        if (PsychPrefStateGet_Verbosity() > 3) printf("PTB-INFO: Not running on Mesa graphics library.\n");
    }

    if (mesaversion[0] > 0) {
        // Running on top of Mesa. Does it have the Mesa mapi bug?
        // Versions older than 10.5.2 have this bug, later versions have a proper bug fix:
        if ((mesaversion[0] < 10) || ((mesaversion[0] == 10) && ((mesaversion[1] < 5) || ((mesaversion[1] == 5) && (mesaversion[2] < 2))))) {
            mesamapi_strdupbug = TRUE;
            if (PsychPrefStateGet_Verbosity() > 3) printf("PTB-INFO: Mesa version < 10.5.2 does not have the mapi strdup() bug fix. Needs the mex file locking workaround.\n");
        }
        else {
            // Mesa 10.5.2 or later. This means it has sane DRI3/Present support. Let's check if the X-Server
            // has sane DRI3/Present support as well, ie., is it at least XOrg 1.16.3?
            if (XVendorRelease(windowRecord->targetSpecific.deviceContext) >= 11603000) {
                // Yes. This combo is considered safe for use with DRI3/Present, so mark
                // our windowRecord as safe for this mode of display:
                windowRecord->specialflags |= kPsychSafeForDRI3;
                if (PsychPrefStateGet_Verbosity() > 3) printf("PTB-INFO: This combo of X-Server and Mesa is considered safe for use under DRI3/Present.\n");
            }
        }
    }

    // Ok, the OpenGL rendering context is up and running.
    // Running on top of a FOSS Mesa graphics driver?
    if ((x11_windowcount == 0) && mesamapi_strdupbug && !getenv("PSYCH_DONT_LOCK_MOGLCORE")) {
        // Yes. At least as of Mesa 10.1 as shipped in Ubuntu 14.04-LTS, Mesa
        // will become seriously crashy if our Screen() mex files is flushed
        // from memory due to a clear all/mex/Screen and afterwards reloaded.
        // This because Mesa maintains pointers back into our library image,
        // which will turn into dangling pointers if we get unloaded/reloaded
        // into a new location. To prevent Mesa crashes on clear Screen -> reload,
        // prevent this mex file against clearing from Octave/Matlab address space.
        // An ugly solution which renders "clear Screen" useless, but the best i can
        // come up with at the moment :(
        if (PsychRuntimeEvaluateString("moglcore('LockModule');") > 0) {
            if (PsychPrefStateGet_Verbosity() > 1) {
                printf("PTB-WARNING: Failed to enable moglcore locking workaround for Mesa OpenGL bug. Trying alternative workaround.\n");
                printf("PTB-WARNING: Calling 'clear all', 'clear mex', 'clear java', 'clear moglcore' is now unsafe and may crash if you try.\n");
                printf("PTB-WARNING: You may add setenv('PSYCH_DONT_LOCK_MOGLCORE','1'); to your Octave/Matlab startup script to work around this issue in future sessions.\n");
            }
            setenv("PSYCH_DONT_LOCK_MOGLCORE", "1", 0);
        }
        else {
            if (PsychPrefStateGet_Verbosity() > 3) printf("PTB-INFO: Workaround: Disabled ability to 'clear moglcore', as a workaround for a Mesa OpenGL bug. Sorry for the inconvenience.\n");
        }
    }

    // Auto-detect and bind all available OpenGL extensions via GLEW:
    glerr = glewInit();
    if (GLEW_OK != glerr) {
        /* Problem: glewInit failed, something is seriously wrong. */
        printf("\nPTB-ERROR[GLEW init failed: %s]: Please report this to the forum. Will try to continue, but may crash soon!\n\n", glewGetErrorString(glerr));
    }
    else {
        if (PsychPrefStateGet_Verbosity() > 3) printf("PTB-INFO: Using GLEW version %s for automatic detection of OpenGL extensions...\n", glewGetString(GLEW_VERSION));
    }

    if ((x11_windowcount == 0) && mesamapi_strdupbug && getenv("PSYCH_DONT_LOCK_MOGLCORE") && !getenv("PSYCH_DONT_LOCK_SCREEN")) {
        // Alternative approach to Mesa bug induced crash: Prevent Screen() from unloading, instead of moglcore:
        mexLock();
        if (PsychPrefStateGet_Verbosity() > 2) printf("PTB-INFO: Workaround: Disabled ability to 'clear Screen', as a workaround for a Mesa OpenGL bug. Sorry for the inconvenience.\n");
    }

    // Check for DRI3/Present operation and assign proper special flag to windowRecord if so:
    if (IsDRI3Supported(windowRecord) && (PsychPrefStateGet_Verbosity() > 3)) printf("PTB-INFO: Window uses DRI3/Present for visual stimulus presentation.\n");

    // Increase our own open window counter:
    x11_windowcount++;

    // Disable X-Windows screensavers:
    if (x11_windowcount==1) {
        int dummy;

        // First window. Disable future use of screensaver:
        XSetScreenSaver(dpy, 0, 0, DefaultBlanking, DefaultExposures);
        // If the screensaver is currently running, forcefully shut it down:
        XForceScreenSaver(dpy, ScreenSaverReset);

        // And just for safety, do it via DPMS disable as well:
        if (DPMSQueryExtension(dpy, &dummy, &dummy)) DPMSDisable(dpy);
    }

    // Some info for the user regarding non-fullscreen mode and sync problems:
    if (!(windowRecord->specialflags & kPsychIsFullscreenWindow) && (PsychPrefStateGet_Verbosity() > 2)) {
        printf("PTB-INFO: Many graphics cards do not support proper timing and timestamping of visual stimulus onset\n");
        printf("PTB-INFO: when running in windowed mode (non-fullscreen). If PTB aborts with 'Synchronization failure'\n");
        printf("PTB-INFO: you can disable the sync test via call to Screen('Preference', 'SkipSyncTests', 2); .\n");
        printf("PTB-INFO: You won't get proper stimulus onset timestamps though, so windowed mode may be of limited use.\n");
    }

    // Check for availability of VSYNC extension:

    // First we try if the MESA variant of the swap control extensions is available. It has two advantages:
    // First, it also provides a function to query the current swap interval. Second it allows to set a
    // zero swap interval to dynamically disable sync to retrace, just as on OS/X and Windows:
    if (strstr(glXQueryExtensionsString(dpy, scrnum), "GLX_MESA_swap_control")) {
        // Bingo! Bind Mesa variant of setup call to sgi setup call, just to simplify the code
        // that actually uses the setup call -- no special cases or extra code needed there :-)
        // This special glXSwapIntervalSGI() call will simply accept an input value of zero for
        // disabling vsync'ed bufferswaps as a valid input parameter:
        glXSwapIntervalSGI = (PFNGLXSWAPINTERVALSGIPROC) glXGetProcAddressARB((const GLubyte *) "glXSwapIntervalMESA");

        // Additionally bind the Mesa query call:
        glXGetSwapIntervalMESA = (PFNGLXGETSWAPINTERVALMESAPROC) glXGetProcAddressARB((const GLubyte *) "glXGetSwapIntervalMESA");
        if (PsychPrefStateGet_Verbosity() > 3) printf("PTB-INFO: Using GLX_MESA_swap_control extension for control of vsync.\n");
    }
    else {
        // Unsupported. Disable the get call:
        glXGetSwapIntervalMESA = NULL;
    }

    // Special case: Buggy ATI driver: Supports the VSync extension and glXSwapIntervalSGI, but provides the
    // wrong extension namestring "WGL_EXT_swap_control" (from MS-Windows!), so GLEW doesn't auto-detect and
    // bind the extension. If this special case is present, we do it here manually ourselves:
    if ((glXSwapIntervalSGI == NULL) && (strstr((const char *) glGetString(GL_EXTENSIONS), "WGL_EXT_swap_control") != NULL)) {
        // Looks so: Bind manually...
        glXSwapIntervalSGI = (PFNGLXSWAPINTERVALSGIPROC) glXGetProcAddressARB((const GLubyte *) "glXSwapIntervalSGI");
    }

    // Extension finally supported?
    if (glXSwapIntervalSGI==NULL || ( strstr((const char *) glXQueryExtensionsString(dpy, scrnum), "GLX_SGI_swap_control")==NULL &&
        strstr((const char *) glGetString(GL_EXTENSIONS), "WGL_EXT_swap_control")==NULL && strstr(glXQueryExtensionsString(dpy, scrnum), "GLX_MESA_swap_control")==NULL )) {
        // No, total failure to bind extension:
        glXSwapIntervalSGI = NULL;

        if (PsychPrefStateGet_Verbosity() > 1) {
            printf("PTB-WARNING: Your graphics driver doesn't allow me to control syncing wrt. vertical retrace!\n");
            printf("PTB-WARNING: Please update your display graphics driver as soon as possible to fix this.\n");
            printf("PTB-WARNING: Until then, you can manually enable syncing to VBL somehow in a manner that is\n");
            printf("PTB-WARNING: dependent on the type of gfx-card and driver. Google is your friend...\n");
        }
    }

    PsychUnlockDisplay();

    // Ok, we should be ready for OS independent setup...
    fflush(NULL);

    // Wait for X-Server to settle...
    PsychLockDisplay();
    XSync(dpy, 1);
    PsychUnlockDisplay();

    // Wait 250 msecs extra to give desktop compositor a chance to settle:
    PsychYieldIntervalSeconds(0.25);

    PsychLockDisplay();

    // Retrieve modeline of current video mode on primary crtc for the screen to which
    // this onscreen window is assigned. Could also query useful info about crtc, but let's not
    // overdo it in the first iteration...
    XRRCrtcInfo *crtc_info = NULL;
    XRRModeInfo *mode = PsychOSGetModeLine(screenSettings->screenNumber, 0, &crtc_info);
    if (mode) {
        // Assign modes display height aka vactive or vdisplay as startline of vblank interval:
        windowRecord->VBL_Startline = mode->height;
        // Assign vbl endline as vtotal - 1:
        windowRecord->VBL_Endline   = mode->vTotal - 1;

        // Check for output display rotation enabled. Will likely impair timing/timestamping
        // because it uses copy-swaps via an intermediate shadow framebuffer to do rotation
        // during copy-swap blit, instead of via rotated crtc scanout, as most crtc's don't
        // support this in hardware:
        if ((crtc_info->rotation != RR_Rotate_0) && (PsychPrefStateGet_Verbosity() > 1)) {
            printf("PTB-WARNING: Your primary output display has hardware rotation enabled. It is not displaying in upright orientation.\n");
            printf("PTB-WARNING: On many graphics cards, this will cause unreliable stimulus presentation timing and timestamping.\n");
            printf("PTB-WARNING: If you want non-upright stimulus presentation, look at 'help PsychImaging' on how to achieve this in\n");
            printf("PTB-WARNING: a way that doesn't impair timing. The subfunctions 'FlipHorizontal' and 'FlipVertical' are what you probably need.\n");
        }

        XRRFreeCrtcInfo(crtc_info);
    }

    PsychUnlockDisplay();

    // Try to enable swap event delivery to us:
    if (PsychOSSwapCompletionLogging(windowRecord, 2, 0) && (PsychPrefStateGet_Verbosity() > 3)) {
        printf("PTB-INFO: INTEL_swap_event support for additional swap completion correctness checks enabled.\n");
    }

    // Well Done!
    return(TRUE);
}

/*
 *    PsychOSGetPostSwapSBC() -- Internal method for now, used in close window path.
 *    Caution: Blocks with display lock held! Do not use outside close window path!!!
 */
static psych_int64 PsychOSGetPostSwapSBC(PsychWindowRecordType *windowRecord)
{
    psych_int64 ust, msc, sbc;
    sbc = 0;

    #ifdef GLX_OML_sync_control
    // Extension unsupported or known to be defective? Return "damage neutral" 0 in that case:
    if ((NULL == glXWaitForSbcOML) || (windowRecord->specialflags & kPsychOpenMLDefective)) return(0);

    // Extension supported: Perform query and error check.
    PsychLockDisplay();
    if (!glXWaitForSbcOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, 0, &ust, &msc, &sbc)) {
        // Failed! Return a "damage neutral" result:
        sbc = 0;
    }
    PsychUnlockDisplay();
    #endif

    return(sbc);
}

void PsychOSCloseWindow(PsychWindowRecordType *windowRecord)
{
    Display* dpy = windowRecord->targetSpecific.deviceContext;

    // We have to rebind the OpenGL context for this swapbuffers call to work around some
    // mesa bug for intel drivers which would cause a crash without context:
    PsychLockDisplay();
    glXMakeCurrent(dpy, windowRecord->targetSpecific.windowHandle, windowRecord->targetSpecific.contextObject);
    PsychUnlockDisplay();

    // Perform a fully synced flip with backbuffer cleared to black, to have a defined final
    // frontbuffer color for switching back to windowing system. Avoids leaving pixel trash
    // behind on some multi-x-screen setups with some drivers:
    glClearColor(0,0,0,1);
    glClear(GL_COLOR_BUFFER_BIT);
    PsychOSFlipWindowBuffers(windowRecord);
    PsychOSGetPostSwapSBC(windowRecord);

    // Check if we are trying to close the window after it had an "odd" (== non-even)
    // number of bufferswaps. If so, we execute one last bufferswap to make the count
    // even. This means that if this window was swapped via page-flipping, the system
    // should end with the same backbuffer-frontbuffer assignment as the one prior
    // to opening the window. This may help sidestep certain bugs in compositing desktop
    // managers (e.g., Compiz).
    if (PsychOSGetPostSwapSBC(windowRecord) % 2) {
        // Uneven count. Submit a swapbuffers request and wait for it to truly finish:

        // A glClear to touch the framebuffer before flip. Why? To accomodate some quirks of
        // the Intel ddx as of 2.99.917 with DRI2+SNA and triple-buffering enabled. Makes
        // triple-buffered mode at least marginally useful for some restricted use cases:
        glClear(GL_COLOR_BUFFER_BIT);

        PsychOSFlipWindowBuffers(windowRecord);
        PsychOSGetPostSwapSBC(windowRecord);
    }

    if (PsychPrefStateGet_Verbosity() > 5) {
        printf("PTB-DEBUG:PsychOSCloseWindow: Closing with a final swapbuffers count of %i.\n", (int) PsychOSGetPostSwapSBC(windowRecord));
    }

    PsychLockDisplay();

    // Detach OpenGL rendering context again - just to be safe!
    glXMakeCurrent(dpy, None, NULL);

    // Delete rendering context:
    glXDestroyContext(dpy, windowRecord->targetSpecific.contextObject);
    windowRecord->targetSpecific.contextObject=NULL;

    // Delete swap context:
    glXDestroyContext(dpy, windowRecord->targetSpecific.glswapcontextObject);
    windowRecord->targetSpecific.glswapcontextObject=NULL;

    // Delete userspace context, if any:
    if (windowRecord->targetSpecific.glusercontextObject) {
        glXDestroyContext(dpy, windowRecord->targetSpecific.glusercontextObject);
        windowRecord->targetSpecific.glusercontextObject = NULL;
    }

    // Wait for X-Server to settle...
    XSync(dpy, 0);

    if (useGLX13) glXDestroyWindow(dpy, windowRecord->targetSpecific.windowHandle);
    windowRecord->targetSpecific.windowHandle = 0;

    // Close & Destroy the window:
    XUnmapWindow(dpy, windowRecord->targetSpecific.xwindowHandle);

    // Wait for X-Server to settle...
    XSync(dpy, 0);

    XDestroyWindow(dpy, windowRecord->targetSpecific.xwindowHandle);
    windowRecord->targetSpecific.xwindowHandle=0;

    // Wait for X-Server to settle...
    XSync(dpy, 0);

    // Release device context: We just release the reference. The connection to the display is
    // closed below.
    windowRecord->targetSpecific.deviceContext=NULL;

    // Decrement global count of open onscreen windows:
    x11_windowcount--;

    // Was this the last window?
    if (x11_windowcount<=0) {
        int dummy;

        x11_windowcount=0;

        // (Re-)enable X-Windows screensavers if they were enabled before opening windows:
        // Set screensaver to previous settings, potentially enabling it:
        XSetScreenSaver(dpy, -1, 0, DefaultBlanking, DefaultExposures);

        // And just for safety, do it via DPMS enable as well:
        if (DPMSQueryExtension(dpy, &dummy, &dummy)) DPMSEnable(dpy);

        // Unmap/release possibly mapped device memory: Defined in PsychScreenGlue.c
        PsychScreenUnmapDeviceMemory();
    }

    // Release dedicated x-display connection for the dead window:
    if (usePerWindowXConnections) {
        XCloseDisplay(windowRecord->targetSpecific.privDpy);
    }

    PsychUnlockDisplay();

    // Shut down the PRIME UDP socket if any was in use:
    if (prime_sockfd[PsychGetXScreenIdForScreen(windowRecord->screenNumber)] > 0)
        close(prime_sockfd[PsychGetXScreenIdForScreen(windowRecord->screenNumber)]);
    prime_sockfd[PsychGetXScreenIdForScreen(windowRecord->screenNumber)] = 0;
    if (prime_sockfd2[PsychGetXScreenIdForScreen(windowRecord->screenNumber)] > 0)
        close(prime_sockfd2[PsychGetXScreenIdForScreen(windowRecord->screenNumber)]);
    prime_sockfd2[PsychGetXScreenIdForScreen(windowRecord->screenNumber)] = 0;

    // Done.
    return;
}

/*
 *    PsychOSGetVBLTimeAndCount()
 *
 *    Returns absolute system time of last VBL and current total count of VBL interrupts since
 *    startup of gfx-system for the given screen. Returns a time of -1 and a count of 0 if this
 *    feature is unavailable on the given OS/Hardware configuration.
 */
double  PsychOSGetVBLTimeAndCount(PsychWindowRecordType *windowRecord, psych_uint64* vblCount)
{
    unsigned int	vsync_counter = 0;
    psych_uint64	ust, msc, sbc;

    PsychLockDisplay();

    #ifdef GLX_OML_sync_control
    // Ok, this will return VBL count and last VBL time via the OML GetSyncValuesOML call
    // if that extension is supported on this setup. The Linux FOSS graphics stack (DRI2/DRI3)
    // supports this on all gpu's since at least the year 2010:
    if ((NULL != glXGetSyncValuesOML) && !(windowRecord->specialflags & kPsychOpenMLDefective) && (glXGetSyncValuesOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, (int64_t*) &ust, (int64_t*) &msc, (int64_t*) &sbc))) {
        PsychUnlockDisplay();
        *vblCount = msc;
        if ((PsychGetKernelTimebaseFrequencyHz() > 10000) && !(windowRecord->specialflags & kPsychNeedOpenMLWorkaround1)) {
            // Convert ust into regular GetSecs timestamp:
            // At least we hope this conversion is correct...
            return( PsychOSMonotonicToRefTime(((double) ust) / PsychGetKernelTimebaseFrequencyHz()) );
        }
        else {
            // Last VBL timestamp unavailable:
            return(-1);
        }
    }
    #else
    #warning GLX_OML_sync_control unsupported! Compile with -std=gnu99 to enable it!
    #endif

    // Do we have SGI video sync extensions?
    if (NULL != glXGetVideoSyncSGI) {
        // Retrieve absolute count of vbl's since startup:
        glXGetVideoSyncSGI(&vsync_counter);
        PsychUnlockDisplay();
        *vblCount = (psych_uint64) vsync_counter;

        // Retrieve absolute system time of last retrace, convert into PTB standard time system and return it:
        // Not yet supported on Linux:
        return(-1);
    }
    else {
        // Unsupported :(
        PsychUnlockDisplay();
        *vblCount = 0;
        return(-1);
    }
}

/* PsychOSGetSwapCompletionTimestamp()
 *
 * Retrieve a very precise timestamp of doublebuffer swap completion by means
 * of OS specific facilities. This function is optional. If the underlying
 * OS/driver/GPU combo doesn't support a high-precision, high-reliability method
 * to query such timestamps, the function should return -1 as a signal that it
 * is unsupported or (temporarily) unavailable. Higher level timestamping code
 * should use/prefer timestamps returned by this function over other timestamps
 * provided by other mechanisms if possible. Calling code must be prepared to
 * use alternate timestamping methods if this method fails or returns a -1
 * unsupported error. Calling code must expect this function to block until
 * swap completion.
 *
 * Input argument targetSBC: Swapbuffers count for which to wait for. A value
 * of zero means to block until all pending bufferswaps for windowRecord have
 * completed, then return the timestamp of the most recently completed swap.
 *
 * A value of zero is recommended.
 *
 * Returns: Precise and reliable swap completion timestamp in seconds of
 * system time in variable referenced by 'tSwap', and msc value of completed swap,
 * or a negative value on error (-1 == unsupported, -2/-3 == Query failed).
 *
 */
psych_int64 PsychOSGetSwapCompletionTimestamp(PsychWindowRecordType *windowRecord, psych_int64 targetSBC, double* tSwap)
{
    psych_int64 ust, msc, sbc;
    msc = -1;

    #ifdef GLX_OML_sync_control

    // DRI Prime hybridGraphics setup in outputSource -> outputSink mode? Typically a NVidia Optimus
    // Laptop under the proprietary NVidia driver, which can't do the much better Prime DRI3/Present gpu renderoffload.
    // If OML_sync_control is not supported natively, then use our custom UDP protocol between the modesetting-ddx and us.
    // This only works for unredirected fullscreen windows, as only then we can be somewhat certain that
    // pageflips on the outputSink iGPU are triggered by our OpenGL bufferswaps.
    if (((NULL == glXWaitForSbcOML) || (windowRecord->specialflags & kPsychOpenMLDefective)) &&
        (windowRecord->hybridGraphics == 3 || windowRecord->hybridGraphics == 4) &&
        (windowRecord->specialflags & kPsychIsFullscreenWindow)) {
        // Type 4 setup uses wait for true flip completion, type 3 setup only waits for flip scheduled:
        int targetCompletionMode = (windowRecord->hybridGraphics == 4) ? 1 : 0;
        int xscreen = PsychGetXScreenIdForScreen(windowRecord->screenNumber);

        // Yes. Try to use our own custom UDP network protocol to get swap completion timestamps
        // from the specially hacked modesetting ddx.
        if (prime_sockfd[xscreen] == 0) {
            struct _buf buf;
            struct sockaddr_in client_addr;
            memset(&client_addr, 0, sizeof(client_addr));
            client_addr.sin_family = AF_INET;
            client_addr.sin_addr.s_addr = htonl(INADDR_ANY);
            client_addr.sin_port = htons(10000 + xscreen);

            // Get socket:
            prime_sockfd[xscreen] = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
            if (prime_sockfd[xscreen] < 0) {
                if (PsychPrefStateGet_Verbosity() > 0)
                    printf("PTB-ERROR:PsychOSGetSwapCompletionTimestamp: Failed to create UDP receive socket for Prime slave-output timestampig on screen %i! [%s]\n",
                           windowRecord->screenNumber, strerror(errno));
                return(-1);
            }

            // Try to bind it to localhost loopback receive port 10000 + X-Screen numbeer:
            if (bind(prime_sockfd[xscreen], (struct sockaddr *) &client_addr, sizeof(client_addr))) {
                // Failed.
                if (PsychPrefStateGet_Verbosity() > 0)
                    printf("PTB-ERROR:PsychOSGetSwapCompletionTimestamp: Failed to bind UDP socket for Prime slave-output timestampig on screen %i to port %i! [%s]\n",
                           windowRecord->screenNumber, 10000 + xscreen, strerror(errno));
                close(prime_sockfd[xscreen]);
                prime_sockfd[xscreen] = -1;
                return(-1);
            }

            prime_sockfd2[xscreen] = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
            if (prime_sockfd2[xscreen] < 0) {
                if (PsychPrefStateGet_Verbosity() > 0)
                    printf("PTB-ERROR:PsychOSGetSwapCompletionTimestamp: Failed to create UDP send socket for Prime slave-output timestampig on screen %i! [%s]\n",
                           windowRecord->screenNumber, strerror(errno));
                close(prime_sockfd[xscreen]);
                prime_sockfd[xscreen] = -1;
                return(-1);
            }

            client_addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
            if(connect(prime_sockfd2[xscreen], (struct sockaddr *) &client_addr, sizeof(client_addr))) {
                // Failed.
                if (PsychPrefStateGet_Verbosity() > 0)
                    printf("PTB-ERROR:PsychOSGetSwapCompletionTimestamp: Failed to connect UDP send socket for Prime slave-output timestampig on screen %i to port %i! [%s]\n",
                           windowRecord->screenNumber, 10000 + xscreen, strerror(errno));
                close(prime_sockfd[xscreen]);
                prime_sockfd[xscreen] = -1;
                close(prime_sockfd2[xscreen]);
                prime_sockfd2[xscreen] = -1;
                return(-1);
            }

            // Ready to rock at next flip:
            windowRecord->specialflags &= ~kPsychOpenMLDefective;

            if (PsychPrefStateGet_Verbosity() > 2)
                printf("PTB-INFO: Custom PRIME Flip completion timestamping for screen %i enabled. Will start at next Flip...\n", windowRecord->screenNumber);

            // Warn about use of multiple video outputs per screen. This cannot possibly work with any level of reliability:
            if ((PsychScreenToHead(windowRecord->screenNumber, 1) >= 0) && (PsychPrefStateGet_Verbosity() > 1))
                printf("PTB-WARNING: Custom PRIME Flip completion timestamping for screen %i will not work reliably, because multiple displays are active on this screen!\n",
                       windowRecord->screenNumber);

            // We must skip this one, as the X-Server may have sent out feedback for this flip already,
            // before we managed to create and bind our UDP socket, so the info may be lost and we would
            // hang if we tried to wait for it to arrive. The next flip will be processed by us:
            windowRecord->reference_sbc = 0;

            // Wait for half a second to make sure that any potential pending flips complete
            // and any in-flight UDP packets have been received by our new socket. Then drain
            // whatever got received, until nothing gets received for at least 10 msecs.
            // We want to make sure the sockets receive queue is in a well defined (=empty)
            // state before we use the connection for actual swap timestamping:
            PsychWaitIntervalSeconds(0.5);
            while (recv(prime_sockfd[xscreen], &buf, sizeof(buf), MSG_DONTWAIT | MSG_WAITALL) >= 0)
                PsychWaitIntervalSeconds(0.010);

            return(-1);
        }

        // Prime custom timestamping ready?
        if (prime_sockfd[xscreen] > 0) {
            struct _buf buf;

            // First wait for swap fence packet via a blocking receive:
            while (TRUE) {
                recv(prime_sockfd[xscreen], &buf, sizeof(buf), MSG_WAITALL);
                if (PsychPrefStateGet_Verbosity() > 12)
                    printf("0-Screen %i: flipcomplete %i : sbc=%lld : msc=%lld : ust=%lld\n", buf.scrnIndex, buf.flags, windowRecord->reference_sbc, buf.frame, buf.usec);

                // glXSwapBuffers fence packet received from ourselves, ie. PsychOSFlipWindowBuffers?
                if (buf.flags == 100) {
                    // Exit blocking receive loop:
                    break;
                }
            }

            // Then, if waitig for flip completion, wait for the preceeding flip-scheduled packet via a blocking receive:
            while (targetCompletionMode == 1) {
                recv(prime_sockfd[xscreen], &buf, sizeof(buf), MSG_WAITALL);
                if (PsychPrefStateGet_Verbosity() > 12)
                    printf("0-Screen %i: flipcomplete %i : sbc=%lld : msc=%lld : ust=%lld\n", buf.scrnIndex, buf.flags, windowRecord->reference_sbc, buf.frame, buf.usec);

                // Proper "flip queued" packet received?
                if (buf.flags == 0) {
                    // Exit blocking receive loop:
                    break;
                }
            }

            // Then wait for desired flip scheduled/completion packet via a blocking receive:
            while (msc == -1) {
                recv(prime_sockfd[xscreen], &buf, sizeof(buf), MSG_WAITALL);
                if (PsychPrefStateGet_Verbosity() > 12)
                    printf("1-Screen %i: flipcomplete %i : sbc=%lld : msc=%lld : ust=%lld\n", buf.scrnIndex, buf.flags, windowRecord->reference_sbc, buf.frame, buf.usec);

                // Proper completion packet of type targetCompletionMode received?
                if (buf.flags == targetCompletionMode) {
                    // Yes. Assign preliminary result:
                    msc = buf.frame;
                    ust = buf.usec;

                    // Exit blocking receive loop:
                    break;
                }
            }

            // We have a useable result. But is it the most recent - and thereby right - one, or stale data?
            // Perform non-blocking receives until the sockets receive queue is drained. If a UDP packet of
            // our targetCompletionMode is received its content will overwrite our previous results:
            while (recv(prime_sockfd[xscreen], &buf, sizeof(buf), MSG_DONTWAIT | MSG_WAITALL) == sizeof(buf)) {
                if (PsychPrefStateGet_Verbosity() > 12)
                    printf("2-Screen %i: flipcomplete %i : sbc=%lld : msc=%lld : ust=%lld\n", buf.scrnIndex, buf.flags, windowRecord->reference_sbc, buf.frame, buf.usec);

                // Proper completion packet of type targetCompletionMode received?
                if (buf.flags == targetCompletionMode) {
                    // Yes. Overwrite previous result with this more recent one:
                    msc = buf.frame;
                    ust = buf.usec;
                }
            }

            // Make sure msc is always positive and incrementing (needed, because msc is defined as signed integer):
            msc &= ~(1ULL << 63);

            // If no actual timestamp / msc was requested, then we return here:
            if (tSwap == NULL) return(msc);

            // Success at least for timestamping. Translate ust into system time in seconds:
            *tSwap = PsychOSMonotonicToRefTime(((double) ust) / PsychGetKernelTimebaseFrequencyHz());

            if (targetCompletionMode == 0) {
                *tSwap += windowRecord->VideoRefreshInterval;
                ust += (psych_int64) (windowRecord->VideoRefreshInterval * 1e6);
            }

            // Update cached reference values for future swaps:
            windowRecord->reference_ust = ust;
            windowRecord->reference_msc = msc;
            windowRecord->reference_sbc++;

            if (PsychPrefStateGet_Verbosity() > 11) printf("PTB-DEBUG:PsychOSGetSwapCompletionTimestamp: Success! refust = %lld, refmsc = %lld, refsbc = %lld.\n", ust, msc, sbc);

            // Return msc of swap completion:
            return(msc);
        }

        // Does not work: Fall through to OML_sync_control timestamping.
    }

    // Extension unsupported or known to be defective? Return -1 "unsupported" in that case:
    if ((NULL == glXWaitForSbcOML) || (windowRecord->specialflags & kPsychOpenMLDefective)) return(-1);

    // Hack to work around bugs in Mesa 10.3.3's DRI3/Present glXWaitForSBC() implementation. Doesn't work properly
    // for targetSBC == 0, so feed it the equivalent non-zero targetSBC from the glXSwapBuffersMscOML()
    // return value to force it into the less broken code path. This bug is fixed in 10.3.4+, 10.4.1+, 10.5+ and later,
    // so mostly of historic value, as distros which enable DRI3 also ship sufficiently modern Mesa versions. Iow.,
    // this workaround is a good candidate for removal in a future cleanup:
    if (targetSBC == 0) {
        targetSBC = windowRecord->target_sbc;
        if (PsychPrefStateGet_Verbosity() > 11) printf("PTB-DEBUG:PsychOSGetSwapCompletionTimestamp: Supported. Calling with overriden targetSBC = %lld.\n", targetSBC);
    }
    else if (PsychPrefStateGet_Verbosity() > 11) printf("PTB-DEBUG:PsychOSGetSwapCompletionTimestamp: Supported. Calling with targetSBC = %lld.\n", targetSBC);

    // If this is a vsync'ed swap which potentially waits until a future point in time before completing, then
    // glXWaitForSbcOML() may block until that future point in time. Doing so, it will block the used x-display
    // connection to the X-Server. If we are not the only onscreen window in existence and use of per-window
    // x-display connections is disabled then we share this connection with all other onscreen windows. If
    // currently any asynchronous swaps are pending via async background flip threads, then us blocking
    // the shared x-display connection in glXWaitForSbcOML() would prevent those other threads from
    // communicating with the X-Server, effectively destroying all parallelism for background swap execution.
    // As a consequence all scheduled swaps on all onscreen windows would execute and finalize in lock-step,
    // rendering the requested stimulus onset presentation times for those windows dysfunctional, therefore
    // massively disrupting the wanted presentation timing!
    //
    // To prevent this, we must only call glXWaitForSbcOML() after we can be certain the swap completed. We
    // do this by waiting via polling. We poll the current sbc value and compare against the target value for
    // confirmed swap completion. Only then we continue to glXWaitForSbcOML() to collect the swap info non-blocking.
    //
    // This is the polling loop:
    PsychLockDisplay();
    while ((windowRecord->vSynced) && ((PsychGetNrAsyncFlipsActive() > 0) || (PsychGetNrFrameSeqStereoWindowsActive() > 0)) &&
        (windowRecord->targetSpecific.privDpy == windowRecord->targetSpecific.deviceContext) &&
        glXGetSyncValuesOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, &ust, &msc, &sbc) &&
        (sbc < windowRecord->target_sbc)) {
        // Wanted 'sbc' value of target_sbc not yet reached -> The bufferswap isn't confirmed to be completed yet.
        // Need to wait a bit to release the cpu for other threads and processes, then repoll for swap completion.
        PsychUnlockDisplay();

        // Make sure msc is always positive and incrementing (needed, because msc is defined as signed integer):
        msc &= ~(1ULL << 63);

        // Is the current video refresh cycle count 'msc' already at or past the expected count of swap completion?
        if (msc < windowRecord->lastSwaptarget_msc) {
            // No: At time 'ust', the 'msc' was at least one refresh cycle duration away from the earliest possible
            // count of swap completion. That means the swap won't complete earlier than at least one refresh
            // duration after 'ust'. Let's go to sleep and wait until almost until that point in time, aka
            // 'ust' + 1 video refresh duration:
            PsychWaitUntilSeconds(PsychOSMonotonicToRefTime(((double) ust) / PsychGetKernelTimebaseFrequencyHz()) + windowRecord->VideoRefreshInterval - 0.001);
        } else {
            // Yes: Swap completion can happen almost any time now. Sleep for a millisecond, then repoll:
            PsychYieldIntervalSeconds(0.001);
        }

        // Repoll for swap completion...
        PsychLockDisplay();
    }

    // Display lock held here...

    // Extension supported: Perform query and error check.
    if (!glXWaitForSbcOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, targetSBC, &ust, &msc, &sbc)) {
        PsychUnlockDisplay();

        // OpenML supposed to be supported and in good working order according to startup check?
        if (windowRecord->gfxcaps & kPsychGfxCapSupportsOpenML) {
            // Yes. Then this is a new failure condition and we report it as such:
            if (PsychPrefStateGet_Verbosity() > 11) {
                printf("PTB-DEBUG:PsychOSGetSwapCompletionTimestamp: glXWaitForSbcOML() failed! Failing with rc = -2.\n");
            }
            return(-2);
        }

        // No. Failing this call is kind'a expected, so we don't make a big fuss on each
        // failure but return "unsupported" rc, so calling code can try fallback-path without
        // making much noise:
        return(-1);
    }

    PsychUnlockDisplay();

    // Make sure msc is always positive and incrementing (needed, because msc is defined as signed integer):
    msc &= ~(1ULL << 63);

    // This disabled codepath implements a workaround for kms drivers which deliver broken/wrong kms pageflip
    // events. It would work on any kms driver, but is currently not needed for any of them (as of all kernel
    // versions starting with Linux 3.16, and all latest stable kernels since 3.13, and all kernels before 3.13).
    //
    // The workaround implemented here is used in PTB releases since April 2014 to work around bugs in kms-pageflip
    // events on nouveau-kms present in the initial releases of Linux 3.13 - 3.15. These bugs have been fixed in
    // Linux 3.16 and the bug fixes have been backported to 3.13 - 3.15, so all latest stable Linux kernels are now
    // bug-free wrt. this issue. However, 3.13-3.15 kernels which haven't been updated to the latest stable kernels
    // will carry the bug and need the workaround, as signalled by specialflags & kPsychNeedOpenMLTSWorkaround.
    // We also allow to manually force the workaround on, via kPsychForceOpenMLTSWorkaround ConserveVRAM setting,
    // in case it needs to be enabled post-release for whatever reason:
    if ((windowRecord->specialflags & kPsychNeedOpenMLTSWorkaround) || (PsychPrefStateGet_ConserveVRAM() & kPsychForceOpenMLTSWorkaround)) {
        int rc;
        double tref;

        // Try to (ab)use glXGetSyncValuesOML() to get kms vblank timestamp for the vblank of
        // swap completion. If this works we can take advantage of kms accurate timestamps.
        // However, due to the nature of the assumed kms pageflip bug there is a small chance that
        // glXWaitForSbcOML() returned after swap completion but *before* the vblank timestamps and
        // counts could get updated by the kernels vblank irq handler, so the values returned by
        // glXGetSyncValuesOML() might be outdated and therefore also wrong. We query the current
        // values and then validate them in a conservative fashion. If they are close enough to
        // current system time, ie., in the future or less than about a video refresh cycle in the
        // past then we can assume them to be correct and useful to us and we can use them. Otherwise
        // we assume we got old and stale values and just fallback to standard mmio beamposition
        // timestamping. This is a conservative approach which rather discards good values than
        // risking to accept wrong values. This way timestamps should always be correct, even though
        // we have to pay a price in terms of higher execution time and more timestamp noise in case
        // of false rejects:
        PsychLockDisplay();
        rc = glXGetSyncValuesOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, &ust, &msc, &sbc);
        PsychUnlockDisplay();

        // Make sure msc is always positive and incrementing (needed, because msc is defined as signed integer):
        msc &= ~(1ULL << 63);

        if (rc && (msc >= windowRecord->lastSwaptarget_msc)) {
            PsychGetAdjustedPrecisionTimerSeconds(&tref);
            // Threshold selection for stale timestamp reject: If VideoRefreshInterval is already available post
            // calibration, then we choose 80% video refresh duration - Should catch most timing spikes but still
            // provide enough safety margin against long vblank durations or other jitter. If measurement isn't available,
            // e.g., during initial calibration, we choose a 7 msecs threshold: That's about half a video refresh duration
            // for a 60 Hz display and 7/8th on a 120 Hz display - good enough for calibration, where we are tolerant against
            // outliers anyway.
            tref -= (windowRecord->VideoRefreshInterval > 0) ? (windowRecord->VideoRefreshInterval * 0.8) : 0.007;
            if (PsychOSMonotonicToRefTime(((double) ust) / PsychGetKernelTimebaseFrequencyHz()) < tref) {
                // Yes. Consider the returned ust invalid/outdated/stale. Return with "unsupported" rc to trigger
                // regular mmio beamposition timestamping:
                if (PsychPrefStateGet_Verbosity() > 11) {
                    printf("PTB-DEBUG:PsychOSGetSwapCompletionTimestamp: Workaround provided unreliable result - II. Returning rc=-1 to trigger silent fallback.\n");
                }
                return(-1);
            }
        }
        else {
            if (PsychPrefStateGet_Verbosity() > 11) {
                printf("PTB-DEBUG:PsychOSGetSwapCompletionTimestamp: Workaround provided unreliable result - I. Returning rc=-1 to trigger silent fallback.\n");
            }
            return(-1);
        }
    }

    // Check for valid return values: A zero ust or msc means failure, except for results from nouveau,
    // because there it is "expected" to get a constant zero return value for msc, at least when running
    // on top of a Linux kernel older than 3.13, when this shortcoming was fixed (except for old pre-nv50
    // gpu's = GeForce-7000 and earlier, where the old fallback path is triggered to work around problems
    // with this ancient hardware, and that fallback still returns msc == 0):
    if ((windowRecord->vSynced) && ((ust == 0) || ((msc == 0) && !strstr((char*) glGetString(GL_VENDOR), "nouveau")))) {
        // If this happens at a sbc of less than 20 then it is a known glitch in the intel-ddx which has no
        // practical negative effects, so we paper over it and fail silently with an "unsupported" rc. A
        // pointless warning for a non-issue would just confuse users.
        if (sbc < 20) return(-1);

        if (PsychPrefStateGet_Verbosity() > 1) {
            printf("PTB-DEBUG:PsychOSGetSwapCompletionTimestamp: Invalid return values ust = %lld, msc = %lld from call with success return code (sbc = %lld)! Failing with rc = -2.\n", ust, msc, sbc);
            printf("PTB-DEBUG:PsychOSGetSwapCompletionTimestamp: This likely means a driver bug or malfunction, or that timestamping support has been disabled by the user in the driver!\n");
        }

        // Return with "failure" rc, so calling code can provide more error handling:
        return(-2);
    }

    // Another consistency check: This one is meant to catch the totally broken glXSwapBuffersMscOML()
    // implementation of the Intel-DDX from June 2011 to December 2012. The bug has been fixed in the
    // ddx driver version 2.20.16, released at 15th December 2012.
    //
    // That driver completely ignores the provided targetMSC for fullscreen page-flips!!! It just swaps
    // at next vblank. Iow, the real msc of swap completion can be much lower than the requested targetMSC,
    // killing any kind of stimulus onset timing.
    //
    // Check for this: If the swapcompletion msc is at least 2000 counts, we assume it didn't just wrap
    // around by chance, but is a valid target for concsistency checks. 2000 vblanks ~ 10-33 secs with typical
    // refresh rates. This means a blind spot of about 10-30 seconds every ~ 8 months of system uptime, so
    // at most a dozen trials could get screwed unnoticed if somebody is really really unlucky.
    //
    // If the check is executed, the msc of swap completion should always be >= targetMSC, otherwise something
    // is deeply broken in the driver:
    if ((windowRecord->vSynced) && (msc > 2000) && (windowRecord->lastSwaptarget_msc > 2000) && (msc < windowRecord->lastSwaptarget_msc)) {
        // Utterly broken OML swap scheduling! Disable it, so we can use our old fallback path. Warn user once
        // about broken driver:

        // First detected failure? Skip it on successive failures, as the fallback path will have taken
        // care of it -- One would hope at least.
        if (windowRecord->gfxcaps & kPsychGfxCapSupportsOpenML) {
            // Disable OpenML swap scheduling, we will use the classic wait + glXSwapBuffers path, but
            // still keep glXWaitForSBC() timestamping functional:
            windowRecord->gfxcaps &= ~kPsychGfxCapSupportsOpenML;

            if (PsychPrefStateGet_Verbosity() > 1) {
                printf("\nPTB-WARNING: The flip stimulus onset completed at vblank count %lld before the requested target vblank count %lld !!\n", msc, windowRecord->lastSwaptarget_msc);
                printf("PTB-WARNING: This likely means a serious graphics driver bug or malfunction in the drivers swap scheduling mechanism!\n");
                printf("PTB-WARNING: I will now switch to a fallback / backup method for the remainder of this session, trying to work around this bug.\n");
                printf("PTB-WARNING: All Intel graphics drivers released between June 2011 and up to 14th December 2012 are known to have this bug.\n");
                printf("PTB-WARNING: If you use such a graphics card or driver, please try to update your graphics driver as soon as possible for reliable operation.\n\n");
            }
        }
        else {
            // Failure detected again, *after* our workaround was activated! This must be another independent bug!
            // Note: So far no driver ever exposed this bug, but some of the work the Intel developers are currently
            // doing on their driver has some potential to introduce such a bug, so we are better safe than sorry.
            // Specifically: If the kms pageflip completion handler in the intel-kms driver should return a stale
            // msc and ust from previous vblank, because our special workaround and safety code was removed in Oct. 2012.
            // In theory, the driver has been fixed for all current Intel gpu's, but in practice you never know what
            // kind of hardware bugs may show up or hide in future and current gpus. Better safe than sorry...

            // Disable OpenML completely, in the hope that our old "classic" path can somehow deal with the problem,
            // or perform further diagnostics at least:
            windowRecord->specialflags |= kPsychOpenMLDefective;
            if (PsychPrefStateGet_Verbosity() > 0) {
                printf("\nPTB-ERROR: The flip stimulus onset completed at vblank count %lld before the requested target vblank count %lld !!\n", msc, windowRecord->lastSwaptarget_msc);
                printf("PTB-ERROR: This likely means a serious graphics driver bug or malfunction in the drivers swap scheduling or timestamping mechanism!\n");
                printf("PTB-ERROR: I will now switch to a fallback / backup method for the remainder of this session, trying to work around this bug.\n");
                printf("PTB-ERROR: There are no guarantees though. Your system should be considered *not trustwhorthy* for timing sensitive tasks\n");
                printf("PTB-ERROR: until the problem is properly diagnosed and fixed. Please report this failure to the Psychtoolbox user forum.\n\n");
            }
        }
    }

    // If no actual timestamp / msc was requested, then we return here. This is used by the
    // workaround code for multi-threaded XLib access. It passes NULL to just (ab)use this
    // function to wait for swap completion, before it touches the framebuffer for real.
    // See function PsychLockedTouchFramebufferIfNeeded() in PsychWindowSupport.c
    // It is also used for the startup tests in PsychOSInitializeOpenML() which is why it
    // *has* to be located after the consistency checks directly above this statement.
    if (tSwap == NULL) return(msc);

    // Success at least for timestamping. Translate ust into system time in seconds:
    *tSwap = PsychOSMonotonicToRefTime(((double) ust) / PsychGetKernelTimebaseFrequencyHz());

    // If we are running on a slightly incomplete nouveau-kms driver which always returns a zero msc,
    // we need to get good ust,msc,sbc values for later use as reference and as return value via an
    // extra roundtrip to the kernel. The most important info, the swap completion timestamp, aka ust
    // as returned from glXWaitForSbcOML() has already been converted into GetSecs() timebase and returned
    // in tSwap, so it is ok to overwrite ust here:
    if (msc == 0) {
        PsychLockDisplay();
        if (!glXGetSyncValuesOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, &ust, &msc, &sbc)) {
            PsychUnlockDisplay();

            // Ohoh:
            if (PsychPrefStateGet_Verbosity() > 11) {
                printf("PTB-DEBUG:PsychOSGetSwapCompletionTimestamp: Invalid return values ust = %lld, msc = %lld from glXGetSyncValuesOML() call with success return code (sbc = %lld)! Failing with rc = -1.\n", ust, msc, sbc);
            }

            // Return with "unsupported" rc, so calling code can try fallback-path:
            return(-1);
        }
        PsychUnlockDisplay();
    }

    // Make sure msc is always positive and incrementing (needed, because msc is defined as signed integer):
    msc &= ~(1ULL << 63);

    // Update cached reference values for future swaps:
    windowRecord->reference_ust = ust;
    windowRecord->reference_msc = msc;
    windowRecord->reference_sbc = sbc;

    if (PsychPrefStateGet_Verbosity() > 11) printf("PTB-DEBUG:PsychOSGetSwapCompletionTimestamp: Success! refust = %lld, refmsc = %lld, refsbc = %lld.\n", ust, msc, sbc);

    // Try to get corresponding INTEL_swap_event for cross-checking:
    if (PsychOSSwapCompletionLogging(windowRecord, 4, (int) sbc)) {
        // Got it. We are only interested in one thing: Was this a fullscreen window bufferswap with a non page-flipped swap?
        // For non-fullscreen windows, all bets are off wrt. stimulus onset timing or timestamping, and the user knows this,
        // as we've told so at window creation time.
        //
        // For fullscreen windows however, the user can expect pageflip swaps for best precision. If this doesn't work out,
        // it hints to some configuration problem on the system and we better warn the user about unreliable timing:
        if ((windowRecord->vSynced) && (windowRecord->specialflags & kPsychIsFullscreenWindow) && (PsychPrefStateGet_SkipSyncTests() < 2) && (windowRecord->swapcompletiontype > 1)) {
            // Ohoh: Non-pageflipped fullscreen window swap:
            if (PsychPrefStateGet_Verbosity() > 1) {
                printf("\nPTB-WARNING: Flip for window %i didn't use pageflipping for flip. Visual presentation timing and timestamps are likely unreliable!\n", windowRecord->windowIndex);
                printf("PTB-WARNING: Something is misconfigured on your system, otherwise pageflipping would have been used by the graphics driver for reliable timing.\n");
                printf("PTB-WARNING: Read the Linux specific section of 'help SyncTrouble' for some common causes and fixes for this problem.\n");
            }
        }
    }

    #endif

    // Return msc of swap completion:
    return(msc);
}

/* PsychOSInitializeOpenML() - Linux specific function.
 *
 * Performs basic initialization of the OpenML OML_sync_control extension.
 * Performs basic and extended correctness tests and disables extension if it
 * is unreliable, or enables workarounds for partially broken extensions.
 *
 * Called from PsychDetectAndAssignGfxCapabilities() as part of the PsychOpenOffscreenWindow()
 * procedure for a window with OpenML support.
 *
 */
void PsychOSInitializeOpenML(PsychWindowRecordType *windowRecord)
{
    #ifdef GLX_OML_sync_control

    psych_int64 ust, msc, sbc, oldmsc, oldust, finalmsc;
    psych_bool failed = FALSE;
    struct utsname unameresult;
    char extraversionsignature[512];
    int major = 0, minor = 0, patchlevel = 0, extralevel = 0;

    // Enable rendering context of window:
    PsychSetGLContext(windowRecord);

    // Check if we are running on a version of Linux 3.13 - 3.15 with broken nouveau-kms pageflip completion events:
    uname(&unameresult);
    sscanf(unameresult.release, "%i.%i.%i", &major, &minor, &patchlevel);

    // We check Linux versions 3.13 to 3.15 for broken kms-pageflip events if we are running on nouveau-kms.
    // Exceptions are -rc release candidate kernels, so MK can still use rc's built from git/source for patch testing.
    if ((((major == 3) && ((minor >= 13) && (minor <= 15))) && !strstr(unameresult.release, "-rc")) && strstr((const char *) glGetString(GL_VENDOR), "nouveau")) {
        // Potentially faulty nouveau-kms. Check against the known kernel patchlevels when the bug was fixed:
        // We know Linux stable kernels 3.13.11.5+, 3.14.12+ and 3.15.5+ are fixed.
        // As far as Ubuntu distribution kernels go, we know the ones based on 3.13.11.5+ are fine,
        // but we need some special case query code below to detect them, as their regular version
        // string identifies them as "3.13.0-something":
        if (((minor == 15) && (patchlevel < 5)) || ((minor == 14) && (patchlevel < 12))) {
            // Not yet fixed pre 3.14.12+ or pre 3.15.5+ kernel. Force workaround:
            windowRecord->specialflags |= kPsychNeedOpenMLTSWorkaround;
        }

        if ((minor == 13) && (patchlevel > 0) && (patchlevel < 11)) {
            // Not fixed 3.13.1 - 3.13.10 kernel:
            windowRecord->specialflags |= kPsychNeedOpenMLTSWorkaround;
        }

        if ((minor == 13) && (patchlevel == 11)) {
            // 3.13.11.x kernel. What is x? Smaller than 5 means a not fixed < 3.13.11.5 kernel:
            sscanf(unameresult.release, "%*i.%*i.%*i.%i", &extralevel);
            if (extralevel < 5) windowRecord->specialflags |= kPsychNeedOpenMLTSWorkaround;
        }

        // Finally a special case check for Ubuntu distribution kernels:
        if ((minor == 13) && (patchlevel == 0)) {
            // Is this a Ubuntu distro kernel?
            FILE* fd = fopen("/proc/version_signature", "rt");
            if (fd && fgets(extraversionsignature, sizeof(extraversionsignature), fd)) {
                // Seems so. Find signature of the stable kernel on which this one is based:
                if (!strstr(extraversionsignature, "3.13.11.")) {
                    // Nope. This is a regular 3.13.11 kernel with the bug:
                    windowRecord->specialflags |= kPsychNeedOpenMLTSWorkaround;
                }
                else {
                    // Definitely an Ubuntu distribution kernel based on the
                    // Ubuntu maintained 3.13.11.x long term stable kernel series.
                    // Kernels based on 3.13.11.5+ are good, earlier ones are bad:
                    sscanf(strstr(extraversionsignature, "3.13.11."), "%*i.%*i.%*i.%i", &extralevel);
                    if (extralevel < 5) {
                        windowRecord->specialflags |= kPsychNeedOpenMLTSWorkaround;
                        if (PsychPrefStateGet_Verbosity() >= 4) printf("PTB-INFO: Ubuntu distribution kernel with nouveau-kms pageflip event bug.\n");
                    }
                    else if (PsychPrefStateGet_Verbosity() >= 4) printf("PTB-INFO: Fixed Ubuntu distribution kernel without nouveau-kms pageflip event bug. Good :)\n");
                }
            }
            else {
                // Nope. This is a regular 3.13.0 kernel with the bug:
                windowRecord->specialflags |= kPsychNeedOpenMLTSWorkaround;
            }

            if (fd) fclose(fd);
        }

        // Yes. kms driver on these kernels delivers faulty data inside its kms-pageflip completion events, so although
        // return from glXWaitForSbcOML() can be trusted to mean swap-completion, the msc and ust timestamp are wrong.
        if ((windowRecord->specialflags & kPsychNeedOpenMLTSWorkaround) && (PsychPrefStateGet_Verbosity() >= 3)) {
            printf("PTB-INFO: Your Linux kernel %s has a slightly faulty nouveau graphics driver. Enabling a workaround.\n", unameresult.release);
            printf("PTB-INFO: Please upgrade your kernel to the latest stable version as soon as possible to avoid the workaround.\n");
        }
    }

    PsychLockDisplay();

    // Perform a wait for 3 video refresh cycles to get valid (ust,msc,sbc)
    // values for initialization of windowRecord's cached values:
    if (!glXGetSyncValuesOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, &ust, &msc, &sbc) || (msc == 0) ||
        !glXWaitForMscOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, msc + 3, 0, 0, &ust, &msc, &sbc) || (ust == 0)) {
        PsychUnlockDisplay();

        // Basic OpenML functions failed?!? Not good! Disable OpenML swap scheduling:
        windowRecord->gfxcaps &= ~kPsychGfxCapSupportsOpenML;

        // OpenML timestamping in PsychOSGetSwapCompletionTimestamp() and PsychOSGetVBLTimeAndCount() disabled:
        windowRecord->specialflags |= kPsychOpenMLDefective;

        // Warn user:
        if (PsychPrefStateGet_Verbosity() > 1) {
            printf("PTB-WARNING: At least one test call for OpenML OML_sync_control extension failed! Will disable OpenML and revert to fallback implementation.\n");
        }

        return;
    }

    // Have a valid (ust, msc) baseline. Store it in windowRecord for future use:
    windowRecord->reference_ust = ust;
    windowRecord->reference_msc = msc;
    windowRecord->reference_sbc = sbc;

    // Perform correctness test for glXGetSyncValuesOML() over a time span
    // of 6 video refresh cycles. This checks for a limitation that is present
    // in all shipping Linux kernels up to at least version 2.6.36, possibly
    // also in 2.6.37 depending on MK's progress with this feature:
    finalmsc = msc + 6;
    oldmsc = msc;
    oldust = ust;

    while ((msc < finalmsc) && !failed) {
        // Wait a quarter millisecond:
        PsychWaitIntervalSeconds(0.000250);

        // Query current (msc, ust):
        if (!glXGetSyncValuesOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, &ust, &msc, &sbc)) {
            // Query failed!
            failed = TRUE;
        }

        // Has msc changed since last query due to a regular msc increment, i.e., a new video refresh interval?
        if (msc != oldmsc) {
            // Yes. Update reference values for test:
            oldmsc = msc;
            oldust = ust;
        }

        // ust must be equal to oldust at this point, either because a msc increment has updated
        // the ust for the new vblank interval in lock-step and our code above has updated oldust
        // accordingly, or because no msc increment has happened, in which case ust should stay
        // unchanged as well, ie., ust == oldust. If ust and oldust are different then that means
        // that ust has changed its value in the middle of a refresh interval without an intervening
        // vblank. This would happen if glXGetSyncValuesOML() is defective and doesn't return ust
        // timestamps locked to vblank / msc increments, but simply system time values.
        if (ust != oldust) {
            // Failure of glXGetSyncValuesOML()! This is a broken implementation which needs
            // our workaround:
            failed = TRUE;
        }

        // Repeat test loop:
    }

    // Failed or succeeded?
    if (failed) {
        // Failed! Enable workaround and optionally inform user:
        windowRecord->specialflags |= kPsychNeedOpenMLWorkaround1;

        if (PsychPrefStateGet_Verbosity() > 1) {
            printf("PTB-INFO: OpenML OML_sync_control implementation with problematic glXGetSyncValuesOML() function detected. Enabling workaround for ok performance.\n");
        }
    }

    if (glXGetSyncValuesOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, &ust, &msc, &sbc)) {
        // Check swap scheduling for reliable operation. Intel ddx drivers from June 2011 to at least October 2012 are known
        // to be seriously buggy here. Schedule a swap a few vblanks in the future, wait for its completion and timestamp it.
        // This will run a consistency check inside PsychOSGetSwapCompletionTimestamp() which would trigger warnings and fallbacks
        // if it detects problems of the driver with sticking to the schedule:
        PsychUnlockDisplay();

        // A glClear to touch the framebuffer before flip. Why? To accomodate some quirks of
        // the Intel ddx as of 2.99.917 with DRI2+SNA and triple-buffering enabled. Makes
        // triple-buffered mode at least marginally useful for some restricted use cases.
        // Without rendering something to the framebuffer, swap scheduling totally falls over
        // if triple-buffering is enabled under DRI2...
        glClear(GL_COLOR_BUFFER_BIT);
        PsychOSScheduleFlipWindowBuffers(windowRecord, 0.0, msc + 5, 0, 0, 0);

        // Just a dummy call to wait for completion and to trigger consistency checks and workarounds if needed:
        PsychOSGetSwapCompletionTimestamp(windowRecord, 0, NULL);
    }
    else {
        PsychUnlockDisplay();

        if (PsychPrefStateGet_Verbosity() > 1) {
            printf("PTB-WARNING: Spurious failure of glXGetSyncValuesOML(). Could not perform some correctness tests. Something may be broken in your systems timestamping!\n");
        }
    }

    #else
    // Disable extension:
    windowRecord->gfxcaps &= ~kPsychGfxCapSupportsOpenML;
    #endif

    return;
}

/*
 *    PsychOSScheduleFlipWindowBuffers()
 *
 *    Schedules a double buffer swap operation for given window at a given
 *    specific target time or target refresh count in a specified way.
 *
 *    This uses OS specific API's and algorithms to schedule the asynchronous
 *    swap. This function is optional, target platforms are free to not implement
 *    it but simply return a "not supported" status code.
 *
 *    Arguments:
 *
 *    windowRecord - The window to be swapped.
 *    tWhen        - Requested target system time for swap. Swap shall happen at first
 *                VSync >= tWhen.
 *    targetMSC	 - If non-zero, specifies target msc count for swap. Overrides tWhen, unless tWhen == DBL_MAX.
 *    divisor, remainder - If set to non-zero, msc at swap must satisfy (msc % divisor) == remainder.
 *    specialFlags - Additional options, a bit field consisting of single bits that can be or'ed together:
 *                1 = Constrain swaps to even msc values, 2 = Constrain swaps to odd msc values. (Used for frame-seq. stereo field selection)
 *
 *    Return value:
 *
 *    Value greater than or equal to zero on success: The target msc for which swap is scheduled.
 *    Negative value: Error. Function failed. -1 == Function unsupported on current system configuration.
 *    -2 ... -x == Error condition.
 *
 */
psych_int64 PsychOSScheduleFlipWindowBuffers(PsychWindowRecordType *windowRecord, double tWhen, psych_int64 targetMSC, psych_int64 divisor, psych_int64 remainder, unsigned int specialFlags)
{
    psych_int64 ust, msc, sbc, rc;
    double tNow, tMsc;

    // Linux: If this is implemented then it is implemented via the OpenML OML_sync_control extension.
    // Is the extension supported by the system and enabled by Psychtoolbox? If not, we return
    // a "not-supported" status code of -1 and turn into a no-op:
    if (!(windowRecord->gfxcaps & kPsychGfxCapSupportsOpenML)) return(-1);

    // Extension supported and enabled. Use it.
    #ifdef GLX_OML_sync_control

    // Enable rendering context of window:
    PsychSetGLContext(windowRecord);

    // According to OpenML spec, a glFlush() isn't implicitely performed by
    // glXSwapBuffersMscOML(). Therefore need to do it ourselves, although
    // some implementations may do it anyway:
    if (!windowRecord->PipelineFlushDone) glFlush();
    windowRecord->PipelineFlushDone = TRUE;

    // Non-Zero targetMSC provided to directy specify the msc on which swap should happen?
    // If so, then we can skip computation and directly call with that targetMSC:
    if ((targetMSC == 0) && (tWhen != DBL_MAX)) {
        // No: targetMSC shall be computed from given valid tWhen system target time.

        // Target time in the past? If so, advance it to "now", so we always lock onto
        // future vblanks. Why? Because the Intel ddx (as of v2.99.917) tries to optimize
        // bufferswaps away if their targetMSC is in the past! It turns them into buffer
        // exchanges (GLX_EXCHANGE_COMPLETE_INTEL style) that seem to have completed at
        // the most recent *past* vblank. Naturally this triggers all kinds our consistency
        // checks and makes PTB very angry. Now this only happens on DRI2 when the driver
        // is set to triple-buffered mode, which is unfortunately the default setting.
        // Switching triple-buffering off reliably solves all problems and all is good.
        // However, there are certain special usage scenarios for PTB in which we would love
        // to take advantage of Intels DRI2 triple-buffering, so we try to accomodate the driver
        // and help it to do the right thing for us even under triple-buffering. Easiest no-impact
        // way to do it is to correct and target times from the past into target times for next
        // vblank:
        PsychGetAdjustedPrecisionTimerSeconds(&tNow);
        if (windowRecord->vSynced && (tWhen < tNow)) tWhen = tNow;

        // Get current (msc,ust) reference values for computation.
        // Get current values for (msc, ust, sbc) the textbook way: Return error code -2 on failure:
        PsychLockDisplay();
        if (!glXGetSyncValuesOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, &ust, &msc, &sbc)) {
            PsychUnlockDisplay();
            return(-2);
        }

        // glXGetSyncValuesOML() known to return totally bogus ust timestamps? Or ust <= 0 returned,
        // which means a temporary (EAGAIN style) failure?
        if ((windowRecord->specialflags & kPsychNeedOpenMLWorkaround1) || (ust <= 0)) {
            // Useless ust returned. We need to recover a useable reference (msc, ust) pair via
            // trickery instead. Check if tWhen is at least 4 video refresh cycles in the future.
            if ((ust <= 0) && (PsychPrefStateGet_Verbosity() > 11)) printf("PTB-DEBUG:PsychOSScheduleFlipWindowBuffers: Invalid ust %lld returned by query. Current msc = %lld.\n", ust, msc);

            PsychGetAdjustedPrecisionTimerSeconds(&tNow);
            if (((tWhen - tNow) / windowRecord->VideoRefreshInterval) > 4.0) {
                // Yes. We have some time until deadline. Use a blocking wait for at
                // least 2 video refresh cycles. glXWaitForMscOML() doesn't have known
                // issues iff it has to wait for a msc that is in the future, ie., it has
                // to perform a blocking wait. In that case it will return a valid (msc, ust)
                // pair on return from blocking wait. Wait until msc+2 is reached and retrieve
                // updated (msc, ust):
                if (PsychPrefStateGet_Verbosity() > 11) printf("PTB-DEBUG:PsychOSScheduleFlipWindowBuffers: glXWaitForMscOML until msc = %lld, now msc = %lld.\n", msc + 2, msc);
                if (!glXWaitForMscOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, msc + 2, 0, 0, &ust, &msc, &sbc)) {
                    PsychUnlockDisplay();
                    return(-3);
                }
            }
            else {
                // No. Swap deadline is too close to current time. We have no option other than
                // simply using the last cached values in our windowRecord and hoping that they
                // will be good enough:
                ust = windowRecord->reference_ust;
                msc = windowRecord->reference_msc;
                sbc = windowRecord->reference_sbc;
            }
        }

        // Have a valid (ust, msc) baseline. Store it in windowRecord for future use:
        windowRecord->reference_ust = ust;
        windowRecord->reference_msc = msc;
        windowRecord->reference_sbc = sbc;

        // Compute targetMSC for given baseline and target time tWhen:
        tMsc = PsychOSMonotonicToRefTime(((double) ust) / PsychGetKernelTimebaseFrequencyHz());
        targetMSC = msc + ((psych_int64)(floor((tWhen - tMsc) / windowRecord->VideoRefreshInterval) + 1));
        if (windowRecord->vSynced && (targetMSC <= msc)) targetMSC = msc + 1;
    }

    // Clamp targetMSC to a positive non-zero value, unless special case
    // for glXSwapBuffers() semantic is requested, in which case we pass
    // the zero targetMSC on. A zero targetMSC, divisor and remainder will
    // trigger good old glXSwapBuffers() semantics:
    if ((targetMSC <= 0) && (tWhen != DBL_MAX)) targetMSC = 1;

    // Swap at specific even or odd frame requested? This is useful for frame-sequential stereo
    // presentation that shall start its presentation at a specific eye-view:
    if (specialFlags & (0x1 | 0x2)) {
        // Yes. Setup (divisor,remainder) constraint so that
        // 0x1 maps to even target frames, and 0x2 maps to odd
        // target frames:
        divisor = 2;
        remainder = (specialFlags & 0x1) ? 0 : 1;
        // Make sure initial targetMSC obeys (divisor,remainder) constraint:
        targetMSC += (targetMSC % divisor == remainder) ? 0 : 1;
    }

    if (PsychPrefStateGet_Verbosity() > 12) printf("PTB-DEBUG:PsychOSScheduleFlipWindowBuffers: Submitting swap request for targetMSC = %lld, divisor = %lld, remainder = %lld.\n", targetMSC, divisor, remainder);

    // Ok, we have a valid final targetMSC. Schedule a bufferswap for that targetMSC, taking a potential
    // (divisor, remainder) constraint into account:
    rc = glXSwapBuffersMscOML(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle, targetMSC, divisor, remainder);

    PsychUnlockDisplay();

    // Failed? Return -4 error code if so:
    if (rc == -1) return(-4);

    // Keep track of target_sbc and targetMSC, who knows for what they might be good for?
    windowRecord->target_sbc = rc;
    windowRecord->lastSwaptarget_msc = targetMSC;

    #else
    // No op branch in case OML_sync_control isn't enabled at compile time:
    return(-1);
    #endif

    // Successfully scheduled the swap request: Return targetMSC for which it was scheduled:
    return(targetMSC);
}

/*
 *    PsychOSFlipWindowBuffers()
 *
 *    Performs OS specific double buffer swap call.
 */
void PsychOSFlipWindowBuffers(PsychWindowRecordType *windowRecord)
{
    // Execute OS neutral bufferswap code first:
    PsychExecuteBufferSwapPrefix(windowRecord);

    // Prefer use of scheduled swap api (glXSwapBuffersMscOML()) on Linux, if supported,
    // to achieve the same effect as a simple glXSwapBuffers() call would have.
    // Mesa, for versions < 10.3.4, has a bug in its new DRI3/Present implementation of
    // glXWaitForSbcOML() in that it malfunctions with targetSBC == 0: It doesn't wait for
    // all pending swaps to complete, but falls through and returns stale values from previous
    // swaps! By using PsychOSScheduleFlipWindowBuffers() we get the proper targetSBC for this
    // swap request, so a corresponding workaround in PsychOSGetSwapCompletionTimestamp() can
    // use it to call glXWaitForSbcOML() with a proper non-zero targetSBC -- problem solved.
    // If the function is unsupported we fall back to good ol' glXSwapBuffers below.
    // This should not have any negative side effects compared to the old implementation, so
    // can be safely used on DRI2 and other graphics stacks as well.
    //
    // Note another fun-bug: A targetMSC of zero causes a hang of the bufferswap mechanism for
    // XOrg < 1.16.3 + Mesa < 10.3.4. We solve this in PsychOSScheduleFlipWindowBuffers() by
    // requesting a tWhen of 0.0 with targetMSC = 0, so the routine will try to compute true
    // targetMSC from tWhen and when detecting it is in the past, will push tWhen to "now", which
    // will schedule the swap for the targetMSC ( >> 0 ) of the next refresh cycle, at least if
    // vsync'ed swap is requested.
    //
    // Note: These bugs are fixed in current XOrg and Mesa, and distributions with older
    // XOrg/Mesa versions don't have DRI3 enabled by default, so this workaround is a bit
    // of historic value for odd cases. However, the current Intel ddx 2.99.917 creates some
    // new problems under DRI2 + triple-buffering. See PsychOSScheduleFlipWindowBuffers() for
    // why we want to work around these if possible for some performance gain...
    if (PsychOSScheduleFlipWindowBuffers(windowRecord, 0.0, 0, 0, 0, 0) >= 0) return;

    // For NVidia Optimus outputSink style Prime sync emit a "swap submitted" marker packet, so
    // our completion code in PsychOSGetSwapCompletionTimestamp can find the start of the most
    // recent swaps data in the UDP receive queue, filtering out spurious packets from page-flips
    // which were not triggered by ourselves, e.g., instead triggered by mouse cursor visual
    // updates (NVidia proprietary driver uses a SW cursor on Optimus setups):
    if (prime_sockfd2[PsychGetXScreenIdForScreen(windowRecord->screenNumber)] > 0) {
        struct _buf buf;
        buf.frame = 0;
        buf.usec = 0;
        buf.scrnIndex = PsychGetXScreenIdForScreen(windowRecord->screenNumber);
        buf.flags = 100;
        if (send(prime_sockfd2[buf.scrnIndex], &buf, sizeof(buf), MSG_DONTWAIT) != sizeof(buf))
            printf("PTB-ERROR:PsychOSFlipWindowBuffers: Failed to send UDP swap fence for screen %i.\n", windowRecord->screenNumber);
    }

    // Trigger the "Front <-> Back buffer swap (flip) (on next vertical retrace)":
    PsychLockDisplay();
    glXSwapBuffers(windowRecord->targetSpecific.privDpy, windowRecord->targetSpecific.windowHandle);
    PsychUnlockDisplay();

    windowRecord->target_sbc = 0;
}

/* Enable/disable syncing of buffer-swaps to vertical retrace. */
void PsychOSSetVBLSyncLevel(PsychWindowRecordType *windowRecord, int swapInterval)
{
    int error, myinterval;

    // Enable rendering context of window:
    PsychSetGLContext(windowRecord);

    // Store new setting also in internal helper variable, e.g., to allow workarounds to work:
    windowRecord->vSynced = (swapInterval > 0) ? TRUE : FALSE;

    // Try to set requested swapInterval if swap-control extension is supported on
    // this Linux machine. Otherwise this will be a no-op...
    // Note: On Mesa, glXSwapIntervalSGI() is actually a redirected call to glXSwapIntervalMESA()!
    if (glXSwapIntervalSGI) {
        PsychLockDisplay();
        error = glXSwapIntervalSGI(swapInterval);
        PsychUnlockDisplay();
        if (error) {
            if (PsychPrefStateGet_Verbosity()>1) printf("\nPTB-WARNING: FAILED to %s synchronization to vertical retrace!\n\n", (swapInterval > 0) ? "enable" : "disable");
        }
    }

    // If Mesa query is supported, double-check if the system accepted our settings:
    if (glXGetSwapIntervalMESA) {
        PsychLockDisplay();
        myinterval = glXGetSwapIntervalMESA();
        PsychUnlockDisplay();
        if (myinterval != swapInterval) {
            if (PsychPrefStateGet_Verbosity()>1) printf("\nPTB-WARNING: FAILED to %s synchronization to vertical retrace (System ignored setting [Req %i != Actual %i])!\n\n", (swapInterval > 0) ? "enable" : "disable", swapInterval, myinterval);
        }
    }

    return;
}

/*
 *    PsychOSSetGLContext()
 *
 *    Set the window to which GL drawing commands are sent.
 */
void PsychOSSetGLContext(PsychWindowRecordType *windowRecord)
{
    // glXGetCurrentContext() should be safe to use unlocked, as it accesses thread
    // local state:
    if (glXGetCurrentContext() != windowRecord->targetSpecific.contextObject) {
        if (glXGetCurrentContext() != NULL) {
            // We need to glFlush the context before switching, otherwise race-conditions may occur:
            glFlush();

            // Need to unbind any FBO's in old context before switch, otherwise bad things can happen...
            if (glBindFramebufferEXT) glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
        }

        // Switch to new context:
        PsychLockDisplay();
        glXMakeCurrent(windowRecord->targetSpecific.deviceContext, windowRecord->targetSpecific.windowHandle, windowRecord->targetSpecific.contextObject);
        PsychUnlockDisplay();
    }
}

/*
 *    PsychOSUnsetGLContext()
 *
 *    Clear the drawing context.
 */
void PsychOSUnsetGLContext(PsychWindowRecordType* windowRecord)
{
    if (glXGetCurrentContext() != NULL) {
        // We need to glFlush the context before switching, otherwise race-conditions may occur:
        glFlush();

        // Need to unbind any FBO's in old context before switch, otherwise bad things can happen...
        if (glBindFramebufferEXT) glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
    }

    PsychLockDisplay();
    glXMakeCurrent(windowRecord->targetSpecific.deviceContext, None, NULL);
    PsychUnlockDisplay();
}

/* Same as PsychOSSetGLContext() but for selecting userspace rendering context,
 * optionally copying state from PTBs context.
 */
void PsychOSSetUserGLContext(PsychWindowRecordType *windowRecord, psych_bool copyfromPTBContext)
{
    // Child protection:
    if (windowRecord->targetSpecific.glusercontextObject == NULL) PsychErrorExitMsg(PsychError_user,"GL Userspace context unavailable! Call InitializeMatlabOpenGL *before* Screen('OpenWindow')!");

    if (copyfromPTBContext) {
        // This unbind is probably not needed on X11/GLX, but better safe than sorry...
        PsychLockDisplay();
        glXMakeCurrent(windowRecord->targetSpecific.deviceContext, None, NULL);

        // Copy render context state:
        glXCopyContext(windowRecord->targetSpecific.deviceContext, windowRecord->targetSpecific.contextObject, windowRecord->targetSpecific.glusercontextObject, GL_ALL_ATTRIB_BITS);
        PsychUnlockDisplay();
    }

    // Setup new context if it isn't already setup. -> Avoid redundant context switch.
    if (glXGetCurrentContext() != windowRecord->targetSpecific.glusercontextObject) {
        PsychLockDisplay();
        glXMakeCurrent(windowRecord->targetSpecific.deviceContext, windowRecord->targetSpecific.windowHandle, windowRecord->targetSpecific.glusercontextObject);
        PsychUnlockDisplay();
    }
}

/* PsychOSSetupFrameLock - Check if framelock / swaplock support is available on
 * the given graphics system implementation and try to enable it for the given
 * pair of onscreen windows.
 *
 * If possible, will try to add slaveWindow to the swap group and/or swap barrier
 * of which masterWindow is already a member, putting slaveWindow into a swap-lock
 * with the masterWindow. If masterWindow isn't yet part of a swap group, create a
 * new swap group and attach masterWindow to it, before joining slaveWindow into the
 * new group. If masterWindow is part of a swap group and slaveWindow is NULL, then
 * remove masterWindow from the swap group.
 *
 * The swap lock mechanism used is operating system and GPU dependent. Many systems
 * will not support framelock/swaplock at all.
 *
 * Returns TRUE on success, FALSE on failure.
 */
psych_bool PsychOSSetupFrameLock(PsychWindowRecordType *masterWindow, PsychWindowRecordType *slaveWindow)
{
    GLuint maxGroups, maxBarriers, targetGroup;
    psych_bool rc = FALSE;

    // GNU/Linux: Try NV_swap_group support first, then SGI swap group support.
    PsychLockDisplay();

    // NVidia swap group extension supported?
    if ((glxewIsSupported("GLX_NV_swap_group") || glewIsSupported("GLX_NV_swap_group")) && (NULL != glXQueryMaxSwapGroupsNV)) {
        // Yes. Check if given GPU really supports it:
        if (PsychPrefStateGet_Verbosity() > 5) printf("PTB-DEBUG: NV_swap_group supported. Querying available groups...\n");

        if (glXQueryMaxSwapGroupsNV(masterWindow->targetSpecific.deviceContext, PsychGetXScreenIdForScreen(masterWindow->screenNumber), &maxGroups, &maxBarriers) && (maxGroups > 0)) {
            // Yes. What to do?
            if (PsychPrefStateGet_Verbosity() > 5) printf("PTB-DEBUG: NV_swap_group supported. Implementation supports up to %i swap groups. Trying to join or unjoin group.\n", maxGroups);

            if (NULL == slaveWindow) {
                // Asked to remove master from swap group:
                glXJoinSwapGroupNV(masterWindow->targetSpecific.deviceContext, masterWindow->targetSpecific.windowHandle, 0);
                masterWindow->swapGroup = 0;
                PsychUnlockDisplay();
                return(TRUE);
            }
            else {
                // Non-NULL slaveWindow: Shall attach to swap group.
                // Master already part of a swap group?
                if (0 == masterWindow->swapGroup) {
                    // Nope. Try to attach it to first available one:
                    targetGroup = (GLuint) PsychFindFreeSwapGroupId(maxGroups);

                    if ((targetGroup == 0) || !glXJoinSwapGroupNV(masterWindow->targetSpecific.deviceContext, masterWindow->targetSpecific.windowHandle, targetGroup)) {
                        // Failed!
                        if (PsychPrefStateGet_Verbosity() > 1) {
                            printf("PTB-WARNING: Tried to enable framelock support for master-slave window pair, but masterWindow failed to join swapgroup %i! Skipped.\n", targetGroup);
                        }

                        goto try_sgi_swapgroup;
                    }

                    // Sucess for master!
                    masterWindow->swapGroup = targetGroup;
                }

                // Now try to join the masters swapgroup with the slave:
                if (!glXJoinSwapGroupNV(slaveWindow->targetSpecific.deviceContext, slaveWindow->targetSpecific.windowHandle,  masterWindow->swapGroup)) {
                    // Failed!
                    if (PsychPrefStateGet_Verbosity() > 1) {
                        printf("PTB-WARNING: Tried to enable framelock support for master-slave window pair, but slaveWindow failed to join swapgroup %i of master! Skipped.\n", masterWindow->swapGroup);
                    }

                    goto try_sgi_swapgroup;
                }

                // Success! Now both windows are in a common swapgroup and framelock should work!
                slaveWindow->swapGroup = masterWindow->swapGroup;

                if (PsychPrefStateGet_Verbosity() > 1) {
                    printf("PTB-INFO: Framelock support for master-slave window pair via NV_swap_group extension enabled! Joined swap group %i.\n", masterWindow->swapGroup);
                }

                PsychUnlockDisplay();
                return(TRUE);
            }
        }
    }

    // If we reach this point, then NV_swap groups are unsupported, or setup failed.
    try_sgi_swapgroup:

    // Try if we have more luck with SGIX_swap_group extension:
    if (PsychPrefStateGet_Verbosity() > 5) printf("PTB-DEBUG: NV_swap_group unsupported or join operation failed. Trying GLX_SGIX_swap_group support...\n");

    // SGIX swap group extension supported?
    if((glxewIsSupported("GLX_SGIX_swap_group") || glewIsSupported("GLX_SGIX_swap_group")) && (NULL != glXJoinSwapGroupSGIX)) {
        // Yes. What to do?
        if (NULL == slaveWindow) {
            // Asked to remove master from swap group:
            glXJoinSwapGroupSGIX(masterWindow->targetSpecific.deviceContext, masterWindow->targetSpecific.windowHandle, None);
            masterWindow->swapGroup = 0;
            PsychUnlockDisplay();
            return(TRUE);
        }
        else {
            // Non-NULL slaveWindow: Shall attach to swap group.

            // Sucess for master by definition. Master is member of its own swapgroup, obviously...
            masterWindow->swapGroup = 1;

            // Now try to join the masters swapgroup with the slave. This can't fail in a non-fatal way.
            // Either it succeeds, or the whole runtime will abort with some GLX command stream error :-I
            glXJoinSwapGroupSGIX(slaveWindow->targetSpecific.deviceContext, slaveWindow->targetSpecific.windowHandle,  masterWindow->targetSpecific.windowHandle);

            // Success! Now both windows are in a common swapgroup and framelock should work!
            slaveWindow->swapGroup = masterWindow->swapGroup;

            if (PsychPrefStateGet_Verbosity() > 1) {
                printf("PTB-INFO: Framelock support for master-slave window pair via GLX_SGIX_swap_group extension enabled!\n");
            }

            PsychUnlockDisplay();
            return(TRUE);
        }
    }

    PsychUnlockDisplay();

    if (PsychPrefStateGet_Verbosity() > 5) printf("PTB-DEBUG: NV_swap_group and GLX_SGIX_swap_group unsupported or join operations failed.\n");

    return(rc);
}

psych_bool PsychOSSwapCompletionLogging(PsychWindowRecordType *windowRecord, int cmd, int aux1)
{
    const char *FieldNames[] = { "OnsetTime", "OnsetVBLCount", "SwapbuffersCount", "SwapType", "BackendFeedbackString" };
    const int  fieldCount = 5;
    PsychGenericScriptType	*s;
    unsigned long glxmask = 0;
    XEvent evt;
    int scrnum;
    int event_type;

    // Invalidate stored swap completion type for this window:
    windowRecord->swapcompletiontype = 0;

    if (cmd == 0 || cmd == 1 || cmd == 2) {
        // Check if GLX_INTEL_swap_event extension is supported. Enable/Disable swap completion event
        // delivery for our window, if so:
        // We enable if override env var "PSYCH_FORCE_INTEL_swap_event" is set, or if the extension is
        // in the glXQueryExtensionsString() or it is in both the server- and client-extension string.
        // Update November 2014: Under DRI3/Present + GLAMOR acceleration, e.g., with nouveau-ddx, it
        // is possible that the extension is only advertised in the client string, not the server or
        // unified string, but the extension still works. Therefore we are now more lenient and enable
        // the extension if it is advertised in any of the query strings. Code will still detect missing
        // extension support and fallback to the right thing if this doesn't work out, so all should be good.
        scrnum = PsychGetXScreenIdForScreen(windowRecord->screenNumber);
        PsychLockDisplay();
        if (useGLX13 && (strstr(glXQueryExtensionsString(windowRecord->targetSpecific.deviceContext, scrnum), "GLX_INTEL_swap_event") || getenv("PSYCH_FORCE_INTEL_swap_event") ||
            strstr(glXGetClientString(windowRecord->targetSpecific.deviceContext, GLX_EXTENSIONS), "GLX_INTEL_swap_event") ||
            strstr(glXQueryServerString(windowRecord->targetSpecific.deviceContext, scrnum, GLX_EXTENSIONS), "GLX_INTEL_swap_event"))) {

            // Always enable the swap event delivery, either to us or to user code:
            glXSelectEvent(windowRecord->targetSpecific.deviceContext, windowRecord->targetSpecific.windowHandle, (unsigned long) GLX_BUFFER_SWAP_COMPLETE_INTEL_MASK);

            // Logical enable state: Usercode has precedence. If it enables it goes to it. If it disabled,
            // it gets directed to us:
            if (cmd == 0 || cmd == 1) windowRecord->swapevents_enabled = (cmd == 1) ? 1 : 2;

            // If we want the data and usercode doesn't have exclusive access to it already, then redirect to us:
            if (cmd == 2 && (windowRecord->swapevents_enabled != 1)) windowRecord->swapevents_enabled = 2;

            PsychUnlockDisplay();
            return(TRUE);
        } else {
            // Failed to enable swap events, possibly because they're unsupported:
            windowRecord->swapevents_enabled = 0;
            PsychUnlockDisplay();
            return(FALSE);
        }
    }

    if (cmd == 3 || 4) {
        // Support for INTEL_swap_event extension enabled? Process swap events if so:
        if (useGLX13) {
            PsychLockDisplay();
            glXGetSelectedEvent(windowRecord->targetSpecific.deviceContext, windowRecord->targetSpecific.windowHandle, &glxmask);
            PsychUnlockDisplay();

            if (glxmask & GLX_BUFFER_SWAP_COMPLETE_INTEL_MASK) {
                // INTEL_swap_event delivery enabled and requested.

                // Delivery to user-code?
                if (cmd == 3 && windowRecord->swapevents_enabled == 1) {
                    // Try to fetch oldest pending one for this window:
                    PsychLockDisplay();
                    if (XCheckTypedWindowEvent(windowRecord->targetSpecific.deviceContext, windowRecord->targetSpecific.xwindowHandle, glx_event_base + GLX_BufferSwapComplete, &evt)) {
                        PsychUnlockDisplay();
                        // Cast to proper event type:
                        GLXBufferSwapComplete *sce = (GLXBufferSwapComplete*) &evt;
                        if (PsychPrefStateGet_Verbosity() > 5) {
                            printf("SWAPEVENT: OurWin=%i ust = %lld, msc = %lld, sbc = %lld, type %s.\n", (int) (sce->drawable == windowRecord->targetSpecific.xwindowHandle),
                                   sce->ust, sce->msc, sce->sbc, (sce->event_type == GLX_FLIP_COMPLETE_INTEL) ? "PAGEFLIP" : (sce->event_type == GLX_COPY_COMPLETE_INTEL) ? "BLIT" : "EXCHANGE");
                        }

                        PsychAllocOutStructArray(aux1, FALSE, 1, fieldCount, FieldNames, &s);
                        PsychSetStructArrayDoubleElement("OnsetTime", 0, PsychOSMonotonicToRefTime(((double) sce->ust) / PsychGetKernelTimebaseFrequencyHz()), s);
                        PsychSetStructArrayDoubleElement("OnsetVBLCount", 0, (double) sce->msc, s);
                        PsychSetStructArrayDoubleElement("SwapbuffersCount", 0, (double) sce->sbc, s);
                        switch (sce->event_type) {
                            case GLX_FLIP_COMPLETE_INTEL:
                                PsychSetStructArrayStringElement("SwapType", 0, "Pageflip", s);
                                PsychSetStructArrayStringElement("BackendFeedbackString", 0, "?cez", s);
                                break;

                            case GLX_EXCHANGE_COMPLETE_INTEL:
                                PsychSetStructArrayStringElement("SwapType", 0, "Exchange", s);
                                PsychSetStructArrayStringElement("BackendFeedbackString", 0, "___z", s);
                                break;

                            case GLX_COPY_COMPLETE_INTEL:
                                PsychSetStructArrayStringElement("SwapType", 0, "Copy", s);
                                PsychSetStructArrayStringElement("BackendFeedbackString", 0, "?c__", s);
                                break;

                            default:
                                PsychSetStructArrayStringElement("SwapType", 0, "Unknown", s);
                                PsychSetStructArrayStringElement("BackendFeedbackString", 0, "", s);
                        }

                        return(TRUE);
                    }
                    PsychUnlockDisplay();
                }

                // Delivery to internal code "us"?
                if (cmd == 4 && windowRecord->swapevents_enabled == 2) {
                    // Get the most recent event in the queue, old ones are not interesting to us atm.:
                    event_type = 0; // Init to "undefined"

                    // Fetch until exhausted:
                    PsychLockDisplay();
                    while (XCheckTypedWindowEvent(windowRecord->targetSpecific.deviceContext, windowRecord->targetSpecific.xwindowHandle, glx_event_base + GLX_BufferSwapComplete, &evt)) {
                        // Cast to proper event type:
                        GLXBufferSwapComplete *sce = (GLXBufferSwapComplete*) &evt;
                        if (PsychPrefStateGet_Verbosity() > 10) {
                            printf("SWAPEVENT: OurWin=%i ust = %lld, msc = %lld, sbc = %lld, type %s.\n", (int) (sce->drawable == windowRecord->targetSpecific.xwindowHandle),
                                   sce->ust, sce->msc, sce->sbc, (sce->event_type == GLX_FLIP_COMPLETE_INTEL) ? "PAGEFLIP" : (sce->event_type == GLX_COPY_COMPLETE_INTEL) ? "BLIT" : "EXCHANGE");
                        }

                        // Assign the one that matches our last 'sbc' for swap completion on our windowRecord:
                        if ((sce->drawable == windowRecord->targetSpecific.xwindowHandle) && (((int) sce->sbc) == aux1)) event_type = sce->event_type;
                    }
                    PsychUnlockDisplay();

                    // event_type is either zero if nothing fetched, or the swap type of the most
                    // recent bufferswap:
                    switch (event_type) {
                        case GLX_FLIP_COMPLETE_INTEL:
                            windowRecord->swapcompletiontype = 1;
                            break;

                        case GLX_EXCHANGE_COMPLETE_INTEL:
                            windowRecord->swapcompletiontype = 2;
                            break;

                        case GLX_COPY_COMPLETE_INTEL:
                            windowRecord->swapcompletiontype = 3;
                            break;

                        default:
                            windowRecord->swapcompletiontype = 0;
                            return(FALSE);
                    }

                    return(TRUE);
                }
            }
        }
    }

    // Invalid cmd or failed cmd:
    return(FALSE);
}

/* PsychOSAdjustForCompositorDelay()
 *
 * Compute OS and desktop compositor specific delay that needs to be subtracted from the
 * target time for a OpenGL doublebuffer swap when conventional swap scheduling is used.
 * Subtract the delay, if any, from the given targetTime and return the corrected targetTime.
 *
 */
double PsychOSAdjustForCompositorDelay(PsychWindowRecordType *windowRecord, double targetTime, psych_bool onlyForCalibration)
{
    // Nothing to do for classic X11/GLX. Just return identity. However, if this
    // is a NVidia Optimus Laptop with Intel iGPU + NVidia dGPU and the proprietary
    // NVidia binary graphics driver is used for output source -> output sink PRIME
    // mode, then we have to work around a NVidia oddity: There is always 1 frame extra
    // delay after a bufferswap request until flip at minimum. Ergo, subtract one video
    // refresh duration from the target time to Increase our chance of hitting the proper
    // target frame. This as of the design of driver version 370.23 with XOrg 1.19-rc1.
    if (!onlyForCalibration && ((windowRecord->hybridGraphics == 2) || (windowRecord->hybridGraphics == 3))) {
        if (PsychPrefStateGet_Verbosity() > 14)
            printf("PTB-DEBUG: PsychOSAdjustForCompositorDelay: Optimus Pre-targetTime: %f secs. VideoRefreshInterval %f secs.\n",
                   targetTime, windowRecord->VideoRefreshInterval);
        targetTime -= windowRecord->VideoRefreshInterval;
    }

    return(targetTime);
}

/* End of classic X11/GLX backend: */
#endif
#endif