1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
|
1) Why such library is needed
==========================
Media devices can be very complex. It is not trivial how to detect what's the
other devices associated with a video node.
This API provides the capabilities of getting the associated devices with a
video node.
It is currently implemented at http://git.linuxtv.org/v4l-utils.git, at the
utils/libmedia_dev/. After validating it, it makes sense to move it to be
part of libv4l.
2) Provided functions
==================
The API defines a macro with its current version. Currently, it is:
#define GET_MEDIA_DEVICES_VERSION 0x0104
Each device type that is known by the API is defined inside enum device_type,
currently defined as:
enum device_type {
UNKNOWN = 65535,
NONE = 65534,
MEDIA_V4L_VIDEO = 0,
MEDIA_V4L_VBI,
MEDIA_DVB_FRONTEND,
MEDIA_DVB_DEMUX,
MEDIA_DVB_DVR,
MEDIA_DVB_NET,
MEDIA_DVB_CA,
MEDIA_SND_CARD,
MEDIA_SND_CAP,
MEDIA_SND_OUT,
MEDIA_SND_CONTROL,
MEDIA_SND_HW,
};
The first function discovers the media devices and stores the information
at an internal representation. Such representation should be opaque to
the userspace applications, as it can change from version to version.
2.1) Device discover and release functions
=====================================
The device discover is done by calling:
void *discover_media_devices(void);
In order to release the opaque structure, a free method is provided:
void free_media_devices(void *opaque);
2.2) Functions to help printing the discovered devices
=================================================
In order to allow printing the device type, a function is provided to
convert from enum device_type into string:
char *media_device_type(enum device_type type);
All discovered devices can be displayed by calling:
void display_media_devices(void *opaque);
2.3) Functions to get device associations
====================================
The API provides 3 methods to get the associated devices:
a) get_associated_device: returns the next device associated with another one
char *get_associated_device(void *opaque,
char *last_seek,
enum device_type desired_type,
char *seek_device,
enum device_type seek_type);
The parameters are:
opaque: media devices opaque descriptor
last_seek: last seek result. Use NULL to get the first result
desired_type: type of the desired device
seek_device: name of the device with you want to get an association.
seek_type: type of the seek device. Using NONE produces the same
result of using NULL for the seek_device.
This function seeks inside the media_devices struct for the next device
that it is associated with a seek parameter.
It can be used to get an alsa device associated with a video device. If
the seek_device is NULL or seek_type is NONE, it will just search for
devices of the desired_type.
b) fget_associated_device: returns the next device associated with another one
char *fget_associated_device(void *opaque,
char *last_seek,
enum device_type desired_type,
int fd_seek_device,
enum device_type seek_type);
The parameters are:
opaque: media devices opaque descriptor
last_seek: last seek result. Use NULL to get the first result
desired_type: type of the desired device
fd_seek_device: file handler for the device where the association will
be made
seek_type: type of the seek device. Using NONE produces the same
result of using NULL for the seek_device.
This function seeks inside the media_devices struct for the next device
that it is associated with a seek parameter.
It can be used to get an alsa device associated with an open file descriptor
c) get_not_associated_device: Returns the next device not associated with
an specific device type.
char *get_not_associated_device(void *opaque,
char *last_seek,
enum device_type desired_type,
enum device_type not_desired_type);
The parameters are:
opaque: media devices opaque descriptor
last_seek: last seek result. Use NULL to get the first result
desired_type: type of the desired device
not_desired_type: type of the seek device
This function seeks inside the media_devices struct for the next physical
device that doesn't support a non_desired type.
This method is useful for example to return the audio devices that are
provided by the motherboard.
3) Examples with typical usecases
==============================
a) Just displaying all media devices:
void *md = discover_media_devices();
display_media_devices(md);
free_media_devices(md);
The devices will be shown at the order they appear at the computer buses.
b) For video0, prints the associated alsa capture device(s):
void *md = discover_media_devices();
char *devname = NULL, video0 = "/dev/video0";
do {
devname = get_associated_device(md, devname, MEDIA_SND_CAP,
video0, MEDIA_V4L_VIDEO);
if (devname)
printf("Alsa capture: %s\n", devname);
} while (devname);
free_media_devices(md);
Note: the video0 string can be declarated as "/dev/video0" or as just "video0",
as the search functions will discard any patch on it.
c) Get the alsa capture device associated with an opened file descriptor:
int fd = open("/dev/video0", O_RDWR);
...
void *md = discover_media_devices();
vid = fget_associated_device(md, NULL, MEDIA_SND_CAP, fd,
MEDIA_V4L_VIDEO);
printf("\n\nAlsa device = %s\n", vid);
close(fd);
free_media_devices(md);
d) Get the mainboard alsa playback devices:
char *devname = NULL;
void *md = discover_media_devices();
do {
devname = get_not_associated_device(md, devname, MEDIA_SND_OUT,
MEDIA_V4L_VIDEO);
if (devname)
printf("Alsa playback: %s\n", devname);
} while (devname);
free_media_devices(md);
e) Get all video devices:
void *md = discover_media_devices();
char *vid = NULL;
do {
vid = get_associated_device(md, vid, MEDIA_V4L_VIDEO,
NULL, NONE);
if (!vid)
break;
printf("Video device: %s\n", vid);
} while (vid);
free_media_devices(md);
|
