8.7. ioctl VIDIOC_DECODER_CMD, VIDIOC_TRY_DECODER_CMD¶
8.7.1. Name¶
VIDIOC_DECODER_CMD - VIDIOC_TRY_DECODER_CMD - Execute an decoder command
8.7.3. Arguments¶
fd
- File descriptor returned by open().
request
- VIDIOC_DECODER_CMD, VIDIOC_TRY_DECODER_CMD
argp
8.7.4. Description¶
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 ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF 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.
__u32 | cmd |
The decoder command, see 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. |
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. mem2mem decoders will stop producing new frames
altogether. They will send a V4L2_EVENT_EOS event when the
last frame has been decoded and all frames are ready to be
dequeued and will set the V4L2_BUF_FLAG_LAST buffer flag on
the last buffer of the capture queue to indicate there will be no
new buffers produced to dequeue. This buffer may be empty,
indicated by the driver setting the bytesused field to 0. Once
the V4L2_BUF_FLAG_LAST flag was set, the
VIDIOC_DQBUF ioctl will not block anymore,
but return an EPIPE error code. 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. |
8.7.5. Return Value¶
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.
- EINVAL
- The
cmd
field is invalid. - EPERM
- The application sent a PAUSE or RESUME command when the decoder was not running.