File: dri3proto.txt

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			  The DRI3 Extension
			     Version 1.2
			      2018-02-28

			    Keith Packard
			  keithp@keithp.com
			  Intel Corporation

			    Daniel Stone
		       daniels@collabora.com
			     Collabora


1. Introduction

The DRI3 extension provides mechanisms to translate between direct
rendered buffers and X pixmaps. When combined with the Present extension,
a complete direct rendering solution for hardware-accelerated devices
such as GPUs is provided.

The direct rendered buffers are passed across the protocol via
standard POSIX file descriptor passing mechanisms. On Linux, these
buffers are DMA-BUF objects.

DRI3 also includes a mechanism to translate between Linux Futexes
and X Sync extension Fences. This provides a synchronization mechanism
which can be used to serialize access to shared render buffers.

1.1. Acknowledgments

Eric Anholt <eric@anholt.net>
Dave Airlie <airlied@redhat.com>
Kristian Høgsberg <krh@bitplanet.net>
James Jones <jajones@nvidia.com>
Arthur Huillet <arthur.huillet@free.fr>
Louis-Francis Ratté-Boulianne <lfrb@collabora.com>

			     ❄ ❄ ❄  ❄  ❄ ❄ ❄ 

2. Data Types

The DRI3 extension uses the RandR extension Provider data type to
select among multiple GPUs on a single screen and the Sync extension
fence object to provide graphics object synchronization.

			     ❄ ❄ ❄  ❄  ❄ ❄ ❄ 

4. Errors

DRI3 defines no errors.

			     ❄ ❄ ❄  ❄  ❄ ❄ ❄ 

5. Events

DRI3 defines no events.

			     ❄ ❄ ❄  ❄  ❄ ❄ ❄ 

6. Protocol Types

DRI3 defines no new protocol types.

			     ❄ ❄ ❄  ❄  ❄ ❄ ❄ 

7. Extension Initialization

The name of this extension is "DRI3"

┌───
    DRI3QueryVersion
	client-major-version:	CARD32
	client-minor-version:	CARD32
	major-version:		CARD32
	minor-version:		CARD32
└───

	The client sends the highest supported version to the server
	and the server sends the highest version it supports, but no
	higher than the requested version. Major versions changes can
	introduce incompatibilities in existing functionality, minor
	version changes introduce only backward compatible changes.
	It is the clients responsibility to ensure that the server
	supports a version which is compatible with its expectations.

	Backwards compatible changes included addition of new
	requests.


			     ❄ ❄ ❄  ❄  ❄ ❄ ❄ 

8. Extension Requests

┌───
    DRI3Open
	drawable: DRAWABLE
	provider: PROVIDER
	nfd: CARD8
	device: FD
└───
	Errors: Drawable, Value, Match

	This requests that the X server open the direct rendering
	device associated with drawable and RandR provider. The
	provider must support SourceOutput or SourceOffload.

	The file descriptor for the device is returned in
	'device'. 'nfd' will be set to one (this is strictly a
	convenience for XCB which otherwise would need
	request-specific information about how many file descriptors
	were associated with this reply).

┌───
    DRI3PixmapFromBuffer
	pixmap: PIXMAP
	drawable: DRAWABLE
	size: CARD32
	width, height, stride: CARD16
	depth, bpp: CARD8
	buffer: FD
└───
	Errors: Alloc, Drawable, IDChoice, Value, Match

	Creates a pixmap for the direct rendering object associated
	with 'buffer' and the screen associated with 'drawable'.
	Changes to pixmap will be visible in that direct rendered
	object and changes to the direct rendered object will be
	visible in the pixmap.

	'size' specifies the total size of the buffer bytes. 'width',
	'height' describe the geometry (in pixels) of the underlying
	buffer. 'stride' specifies the number of bytes per scanline in
	the buffer. The pixels within the buffer are not required to
	be arranged in a simple linear fashion, but 'size' will be at
	least 'height' * 'stride'.

	Precisely how any additional information about the buffer is
	shared is outside the scope of this extension.

	If buffer cannot be used with the screen associated with
	drawable, a Match error is returned.

	If depth or bpp are not supported by the screen, a Value error
	is returned.

	For information on synchronization of buffer access between
	the client and the X server, please see section 12.

┌───
    DRI3BufferFromPixmap
	pixmap: PIXMAP
	nfd: CARD8
	size: CARD32
	width, height, stride: CARD16
	depth, bpp: CARD8
	buffer: FD
└───
	Errors: Pixmap, Match

	Pass back a direct rendering object associated with
	pixmap. Changes to pixmap will be visible in that
	direct rendered object and changes to the direct rendered
	object will be visible in the pixmap.

	'size' specifies the total size of the buffer bytes. 'width',
	'height' describe the geometry (in pixels) of the underlying
	buffer. 'stride' specifies the number of bytes per scanline in
	the buffer. The pixels within the buffer are not required to
	be arranged in a simple linear fashion, but 'size' will be at
	least 'height' * 'stride'.

	Precisely how any additional information about the buffer is
	shared is outside the scope of this extension.

	If buffer cannot be used with the screen associated with
	drawable, a Match error is returned.

	For information on synchronization of buffer access between
	the client and the X server, please see section 12.

┌───
    DRI3FenceFromFD
	drawable: DRAWABLE
	fence: FENCE
	initially-triggered: BOOL
	fd: FD
└───
	Errors: IDchoice, Drawable

	Creates a Sync extension Fence that provides the regular Sync
	extension semantics along with a file descriptor that provides
	a device-specific mechanism to manipulate the fence directly.
	Details about the mechanism used with this file descriptor are
	outside the scope of the DRI3 extension.

	For information on synchronization of buffer access between
	the client and the X server, please see section 12.

┌───
    DRI3FDFromFence
	drawable: DRAWABLE
	fence: FENCE
	fd: FD
└───
	Errors: IDchoice, Drawable, Match

	Given a Sync extension Fence that provides the regular Sync
	extension semantics, returns a file descriptor that provides a
	device-specific mechanism to manipulate the fence directly.
	Details about the mechanism used with this file descriptor are
	outside the scope of the DRI3 extension. 'drawable' must be
	associated with a direct rendering device that 'fence' can
	work with, otherwise a Match error results.

	For information on synchronization of buffer access between
	the client and the X server, please see section 12.

┌───
    DRI3GetSupportedModifiers
	window: WINDOW
	depth: CARD8
	bpp: CARD8
	num_window_modifiers: CARD32
	num_screen_modifiers: CARD32
	window_modifiers: ListOfCARD64
	screen_modifiers: ListOfCARD64
└───
	Errors: Window, Match

	Return supported DRM FourCC modifiers for the specified
	'window'.

	The first list of 'window_modifiers' contains a set of
	modifiers which the server considers optimal for the window's
	current configuration. Using these modifiers to allocate, even
	if locally suboptimal to the client driver, may result in a
	more optimal display pipeline, e.g. by avoiding composition.

	The second list of 'screen_modifiers', is the total set of
	modifiers which are acceptable for use on the Screen associated
	with 'window'. This set of modifiers will not change over the
	lifetime of the client. Using this set of modifiers to allocate
	may not result in a globally optimal pipeline, if separate
	'window_modifiers' are available.

	It is expected that a client calling this request will obtain
	the modifiers for a particular window, allocate buffers using
	the preferred modifier set as described above, create a Pixmap
	referring to the storage of those buffers using the
	DRI3BuffersFromPixmap request, then make the content visible
	in the storage of those buffers visible with a request such as
	the Present extension's PresentPixmap.

	The meaning of any modifier is canonically defined in
	drm_fourcc.h.

┌───
    DRI3PixmapFromBuffers
	pixmap: PIXMAP
	window: WINDOW
	num_buffers: CARD8
	width, height: CARD16
	stride0, offset0: CARD32
	stride1, offset1: CARD32
	stride2, offset2: CARD32
	stride3, offset3: CARD32
	depth, bpp: CARD8
	modifier: CARD64
	buffers: ListOfFD
└───
	Errors: Alloc, Window, IDChoice, Value, Match

	Creates a pixmap for the direct rendering object associated
	with 'buffers' and the screen associated with 'window'.
	Changes to pixmap will be visible in that direct rendered
	object and changes to the direct rendered object will be
	visible in the pixmap. The pixmap will be available for
	presentation to the window.

	In contrast to PixmapFromBuffer, multiple buffers may be
	combined to specify a single logical source for pixel
	sampling: 'num_buffers' may be set from 1 (single buffer,
	akin to PixmapFromBuffer) to 4. This is the number of file
	descriptors which will be sent with this request; one per
	buffer.

	Modifiers allow explicit specification of non-linear sources,
	such as tiled or compressed buffers. The combination of bpp,
	depth, and modifier allows unambiguous declaration of the
	buffer layout in a manner defined by the DRM tokens.

	If 'modifier' is DRM_FORMAT_MOD_INVALID, the client does
	not have information on the buffer layout. In this case, the
	buffer may only have a single plane. The driver may make its
	own inference through unspecified means to determine the exact
	buffer layout, however this is neither required nor defined
	by the specification, and is considered an implementation
	detail of the particular driver.

	'width' and 'height' describe the geometry (in pixels) of the
	logical pixel-sample source.

	'strideN' and 'offsetN' define the number of bytes per logical
	scanline, and the distance in bytes from the beginning of the
	buffer passed for that plane until the start of the sample
	source for that plane, respectively for plane N. If the plane
	is not used according to the format and modifier specification,
	both values for that plane must be zero.

	Precisely how any additional information about the buffer (such
	as memory placement) is shared is outside the scope of this
	extension.

	If the buffer(s) cannot be used with the screen associated with
	'window', a Match error is returned.

	If the bpp, depth, and modifier combination is not supported by
	the screen, a Value error is returned.

	For information on synchronization of buffer access between
	the client and the X server, please see section 12.

┌───
    DRI3BuffersFromPixmap
	pixmap: PIXMAP
	nfd: CARD8
	width, height: CARD16
	depth, bpp: CARD8
	modifier: CARD64
	strides: ListOfCARD32
	offsets: ListOfCARD32
	buffers: ListOfFD
└───
	Errors: Pixmap, Match

	Returns direct rendering objects associated with 'pixmap'.
	Changes to 'pixmap' will be visible in the direct rendered
	objects and changes to the direct rendered objects will be
	visible in 'pixmap' after flushing and synchronization.

	'width' and 'height' describe the geometry (in pixels) of the
	logical pixel-sample source from combining the direct rendering
	objects.

	See PixmapFromBuffers for more details on DRM modifiers usage.

	'nfd' describes the number of buffers returned for the pixmap,
	which must be combined together according to 'depth', 'bpp', and
	'modifier'.

	For each buffer, there is an entry in the 'strides',
	'offsets', and 'buffers' list. 'buffer' contains a single file
	descriptor referring to the buffer. 'stride' specifies the
	number of bytes per logical scanline for this plane, and
	'offset' specifies the distance in bytes from the beginning
	of 'buffer' until the start of the sample source for that
	plane.

	Precisely how any additional information about the buffer is
	shared is outside the scope of this extension.

	If buffers cannot be exported from the the screen associated
	with 'pixmap', a Match error is returned.

	For information on synchronization of buffer access between
	the client and the X server, please see section 12.


			     ❄ ❄ ❄  ❄  ❄ ❄ ❄

9. Extension Events

DRI3 defines no events.

			     ❄ ❄ ❄  ❄  ❄ ❄ ❄

10. Extension Versioning

The DRI3 extension is adapted from the DRI2 extension.

	1.0: First published version

	1.1: Cosmetic changes

	1.2: Add GetSupportedModifiers,
	     PixmapFromBuffers, and BuffersFromPixmap requests.

			     ❄ ❄ ❄  ❄  ❄ ❄ ❄


11. Relationship with other extensions

As an extension designed to support other extensions, there is
naturally some interactions with other extensions.

11.1 GLX

GLX is both an application interface and an X extension. OpenGL
applications using the GLX API will use the GLX extension, DRI3 and
Present when doing direct rendering.

11.2 Present

The Present extension provides a way to synchronize the display of pixmap
contents to the screen. When used in conjunction with DRI3, they
provide a complete direct rendering solution for OpenGL or other APIs.

11.3 DRI2

DRI3 provides similar functionality to the DRI2Connect and
DRI2GetBuffersWithFormat requests, however DRI3 uses file descriptors
to refer to the direct rendering device and buffers.

Present and DRI3 are designed in conjunction to replace DRI2

11.2 XvMC / Xv

It might be nice to be able to reference YUV formatted direct rendered
objects from the X server.

			     ❄ ❄ ❄  ❄  ❄ ❄ ❄

12. Synchronization

Synchronization of access to buffers shared between processes is not
currently explicitly controlled by this protocol.

Without the use of additional extensions not defined by the DRI3
protocol as of version 1.2, synchronization between multiple
processes and contexts is considered to follow the implicit model.

In this model, the driver is required to have a global view of
access requests issued by all processes with a reference to the
buffer, and control scheduling of all operations on that buffer,
whether performed by the CPU or auxiliary hardware.

The driver is responsible for enforcing a strict ordering to protect
against write-after-read or read-after-write hazards, such that any
reads requested by one process or context, are fulfilled before any
writes requested by another process or context, as long as that read
was definitively submitted before the write.

A similar dependency exists for reads submitted after writes: the
driver must ensure that the write is fully visible and coherent to
the read request.

As a purely illustrative example, if two processes share a buffer,
where one process reads from a buffer using an OpenGL texture
sampler and submits this work by calling 'glFlush', and the other
process submits work to the driver to write to that buffer, the
driver is responsible for ensuring that the results of the latter
write are not visible to the texture sampler.

The Sync fences provided by DRI3 control only this submission of
work and ensuing global visibility of the requests, rather than the
completion of the work within any hardware. To further the example
above, a fence used to prevent any writes to the buffer before the
sampler had completed access, the fence would be signaled when
'glFlush' had been called, at which point the request has become
globally visible to the driver's request-scheduling and
synchronization mechanisms. The logical ordering of requests made
by software has been preserved, and the driver then takes care
to ensure that these requests are scheduled such they do not
observe effects from requests made later in time.

This presents a fully coherent in-order FIFO-like model across
processes, where synchronzation is handled externally to the DRI3
client with no explicit intervention.

This restriction also applies for cross-device usage.

			     ❄ ❄ ❄  ❄  ❄ ❄ ❄

Appendix A. Protocol Encoding

Syntactic Conventions

This document uses the same syntactic conventions as the core X
protocol encoding document.


A.1 Common Types

None.

A.2 Protocol Requests

┌───
    DRI3QueryVersion
	1	CARD8			major opcode
	1	0			DRI3 opcode
	2	3			length
	4	CARD32			major version
	4	CARD32			minor version
	1	1			Reply
        1				unused
	2	CARD16			sequence number
	4	0			reply length
	4	CARD32			major version
        4	CARD32			minor version
	16				unused	
└───


┌───
    DRI3Open
	1	CARD8			major opcode
	1	1			DRI3 opcode
	2	4			length
	4	DRAWABLE		drawable
	4	PROVIDER		provider
	1	1			Reply
        1	1			nfd
	2	CARD16			sequence number
	4	0			reply length
	24				unused

	0	FD			device
└───

┌───
    DRI3PixmapFromBuffer
	1	CARD8			major opcode
	1	2			DRI3 opcode
	2	6			length
	4	Pixmap			pixmap
	4	Drawable		drawable
	4	CARD32			size
	2	CARD16			width
	2	CARD16			height
	2	CARD16			stride
	1	CARD8			depth
	1	CARD8			bpp

	0	FD			buffer
└───

┌───
    DRI3BufferFromPixmap
	1	CARD8			major opcode
	1	3			DRI3 opcode
	2	2			length
	4	Pixmap			pixmap
	1	1			Reply
        1	1			nfd
	2	CARD16			sequence number
	4	0			reply length
	4	CARD32			size
	2	CARD16			width
	2	CARD16			height
	2	CARD16			stride
	1	CARD8			depth
	1	CARD8			bpp
	12				unused

	0	FD			buffer
└───

┌───
    DRI3FenceFromFD
	1	CARD8			major opcode
	1	4			DRI3 opcode
	2	4			length
	4	Drawable		drawable
	4	Fence			fence
	1	BOOL			initially triggered
	3				unused

	0	FD			fence fd
└───

┌───
    DRI3FDFromFence
	1	CARD8			major opcode
	1	5			DRI3 opcode
	2	3			length
	4	Drawable		drawable
	4	Fence			fence
	1	1			Reply
        1	1			nfd
	2	CARD16			sequence number
	4	0			reply length
	24				unused

	0	FD			fence fd
└───

┌───
    DRI3GetSupportedModifiers
	1	CARD8			major opcode
	1	7			DRI3 opcode
	2	3			length
	4	Window			window
	1	CARD8			depth
	1	CARD8			bpp
	2				unused
	1	1			Reply
        1	0			unused
	2	CARD16			sequence number
	4	CARD32			reply length
	4	CARD32			num_window_modifiers
	4	CARD32			num_screen_modifiers
	16				unused

	4	ListOfCARD64		window_modifiers[num_window_modifiers]
	4	ListOfCARD64		screen_modifiers[num_screen_modifiers]
└───

┌───
    DRI3PixmapFromBuffers
	1	CARD8			major opcode
	1	8			DRI3 opcode
	2	8			length
	4	Pixmap			pixmap
	4	Window			window
	1	CARD8			num_buffers
	3				unused
	2	CARD16			width
	2	CARD16			height
	4	CARD32			stride0
	4	CARD32			offset0
	4	CARD32			stride1
	4	CARD32			offset1
	4	CARD32			stride2
	4	CARD32			offset2
	4	CARD32			stride3
	4	CARD32			offset3
	1	CARD8			depth
	1	CARD8			bpp
	2				unused
	8	CARD64			modifier

	0	ListOfFD		buffers[num_buffers]
└───

┌───
    DRI3BuffersFromPixmap
	1	CARD8			major opcode
	1	9			DRI3 opcode
	2	2			length
	4	Pixmap			pixmap
	1	1			Reply
        1	CARD8			nfd
	2	CARD16			sequence number
	4	CARD32			reply length
	2	CARD16			width
	2	CARD16			height
	4	CARD8			unused
	8	CARD64			modifier
	1	CARD8			depth
	1	CARD8			bpp
	6				unused

	0	ListOfFD		buffer[nfd]
	4	ListOfCARD32		strides[nfd]
	4	ListOfCARD32		offsets[nfd]
└───

A.3 Protocol Events

The DRI3 extension defines no events.

A.4 Protocol Errors

The DRI3 extension defines no errors.

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