VIDIOC_DECODER_CMD, VIDIOC_TRY_DECODER_CMD — Execute an decoder command
int ioctl( | int fd, |
| int request, | |
struct v4l2_decoder_cmd *argp); |
These ioctls control an audio/video (usually MPEG-) decoder.
VIDIOC_DECODER_CMD sends a command to the
decoder, VIDIOC_TRY_DECODER_CMD can be used to
try a command without actually executing it. To send a command applications
must initialize all fields of a struct v4l2_decoder_cmd and call
VIDIOC_DECODER_CMD or VIDIOC_TRY_DECODER_CMD
with a pointer to this structure.
The cmd field must contain the
command code. Some commands use the flags field for
additional information.
A write() or VIDIOC_STREAMON call sends an implicit
START command to the decoder if it has not been started yet.
A close() or VIDIOC_STREAMOFF call of a streaming
file descriptor sends an implicit immediate STOP command to the decoder, and all
buffered data is discarded.
These ioctls are optional, not all drivers may support them. They were introduced in Linux 3.3.
Table A.10. struct v4l2_decoder_cmd
| __u32 | cmd | The decoder command, see Table A.11, “Decoder Commands”. | ||
| __u32 | flags | Flags to go with the command. If no flags are defined for this command, drivers and applications must set this field to zero. | ||
| union | (anonymous) | |||
| struct | start | Structure containing additional data for the
V4L2_DEC_CMD_START command. | ||
| __s32 | speed | Playback speed and direction. The playback speed is defined as
speed/1000 of the normal speed. So 1000 is normal playback.
Negative numbers denote reverse playback, so -1000 does reverse playback at normal
speed. Speeds -1, 0 and 1 have special meanings: speed 0 is shorthand for 1000
(normal playback). A speed of 1 steps just one frame forward, a speed of -1 steps
just one frame back.
| ||
| __u32 | format | Format restrictions. This field is set by the driver, not the
application. Possible values are V4L2_DEC_START_FMT_NONE if
there are no format restrictions or V4L2_DEC_START_FMT_GOP
if the decoder operates on full GOPs (Group Of Pictures).
This is usually the case for reverse playback: the decoder needs full GOPs, which
it can then play in reverse order. So to implement reverse playback the application
must feed the decoder the last GOP in the video file, then the GOP before that, etc. etc.
| ||
| struct | stop | Structure containing additional data for the
V4L2_DEC_CMD_STOP command. | ||
| __u64 | pts | Stop playback at this pts or immediately
if the playback is already past that timestamp. Leave to 0 if you want to stop after the
last frame was decoded.
| ||
| struct | raw | |||
| __u32 | data[16] | Reserved for future extensions. Drivers and applications must set the array to zero. |
Table A.11. Decoder Commands
V4L2_DEC_CMD_START | 0 | Start the decoder. When the decoder is already
running or paused, this command will just change the playback speed.
That means that calling V4L2_DEC_CMD_START when
the decoder was paused will not resume the decoder.
You have to explicitly call V4L2_DEC_CMD_RESUME for that.
This command has one flag:
V4L2_DEC_CMD_START_MUTE_AUDIO. If set, then audio will
be muted when playing back at a non-standard speed.
|
V4L2_DEC_CMD_STOP | 1 | Stop the decoder. When the decoder is already stopped,
this command does nothing. This command has two flags:
if V4L2_DEC_CMD_STOP_TO_BLACK is set, then the decoder will
set the picture to black after it stopped decoding. Otherwise the last image will
repeat. If V4L2_DEC_CMD_STOP_IMMEDIATELY is set, then the decoder
stops immediately (ignoring the pts value), otherwise it
will keep decoding until timestamp >= pts or until the last of the pending data from
its internal buffers was decoded.
|
V4L2_DEC_CMD_PAUSE | 2 | Pause the decoder. When the decoder has not been
started yet, the driver will return an EPERM error code. When the decoder is
already paused, this command does nothing. This command has one flag:
if V4L2_DEC_CMD_PAUSE_TO_BLACK is set, then set the
decoder output to black when paused.
|
V4L2_DEC_CMD_RESUME | 3 | Resume decoding after a PAUSE command. When the decoder has not been started yet, the driver will return an EPERM error code. When the decoder is already running, this command does nothing. No flags are defined for this command. |
On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.
The cmd field is invalid.
The application sent a PAUSE or RESUME command when the decoder was not running.