7.7. ioctl VIDIOC_DECODER_CMD, VIDIOC_TRY_DECODER_CMD¶
7.7.1. Name¶
VIDIOC_DECODER_CMD - VIDIOC_TRY_DECODER_CMD - Execute an decoder command
7.7.2. Synopsis¶
-
int
ioctl
(int fd, VIDIOC_DECODER_CMD, struct v4l2_decoder_cmd *argp)¶
-
int
ioctl
(int fd, VIDIOC_TRY_DECODER_CMD, struct v4l2_decoder_cmd *argp)¶
7.7.3. Arguments¶
fd
- File descriptor returned by open().
argp
- pointer to struct
v4l2_decoder_cmd
.
7.7.4. 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. Applies to both queues of mem2mem decoders.
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. Applies to both queues of mem2mem decoders.
In principle, these ioctls are optional, not all drivers may support them. They were introduced in Linux 3.3. They are, however, mandatory for stateful mem2mem decoders (as further documented in Memory-to-Memory Stateful Video Decoder Interface).
-
v4l2_decoder_cmd
¶
__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 For a device implementing the Memory-to-Memory Stateful Video Decoder Interface, once the drain sequence
is initiated with the |
V4L2_DEC_CMD_STOP |
1 | Stop the decoder. When the decoder is already stopped, this
command does nothing. This command has two flags: if
For a device implementing the Memory-to-Memory Stateful Video Decoder Interface, the command will initiate
the drain sequence as documented in Memory-to-Memory Stateful Video Decoder Interface. No flags or other
arguments are accepted in this case. Any attempt to invoke the command
again before the sequence completes will trigger an |
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. |
V4L2_DEC_CMD_FLUSH |
4 | Flush any held capture buffers. Only valid for stateless decoders.
This command is typically used when the application reached the
end of the stream and the last output buffer had the
V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF flag set. This would prevent
dequeueing the capture buffer containing the last decoded frame.
So this command can be used to explicitly flush that final decoded
frame. This command does nothing if there are no held capture buffers. |
7.7.5. 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.
- EBUSY
- A drain sequence of a device implementing the Memory-to-Memory Stateful Video Decoder Interface is still in progress. It is not allowed to issue another decoder command until it completes.
- EINVAL
- The
cmd
field is invalid. - EPERM
- The application sent a PAUSE or RESUME command when the decoder was not running.