xref: /kernel/linux/linux-5.10/Documentation/userspace-api/media/v4l/vidioc-decoder-cmd.rst (revision 8c2ecf20)

.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
.. c:namespace:: V4L

.. _VIDIOC_DECODER_CMD:

************************************************
ioctl VIDIOC_DECODER_CMD, VIDIOC_TRY_DECODER_CMD
************************************************

Name
====

VIDIOC_DECODER_CMD - VIDIOC_TRY_DECODER_CMD - Execute an decoder command

Synopsis
========

.. c:macro:: VIDIOC_DECODER_CMD

``int ioctl(int fd, VIDIOC_DECODER_CMD, struct v4l2_decoder_cmd *argp)``

.. c:macro:: VIDIOC_TRY_DECODER_CMD

``int ioctl(int fd, VIDIOC_TRY_DECODER_CMD, struct v4l2_decoder_cmd *argp)``

Arguments
=========

``fd``
    File descriptor returned by :c:func:`open()`.

``argp``
    pointer to struct :c:type:`v4l2_decoder_cmd`.

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 :c:type:`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 :c:func:`write()` or :ref:`VIDIOC_STREAMON`
call sends an implicit START command to the decoder if it has not been
started yet. Applies to both queues of mem2mem decoders.

A :c:func:`close()` or :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
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 :ref:`decoder`).

.. tabularcolumns:: |p{1.1cm}|p{2.4cm}|p{1.2cm}|p{1.6cm}|p{10.6cm}|

.. c:type:: v4l2_decoder_cmd

.. cssclass:: longtable

.. flat-table:: struct v4l2_decoder_cmd
    :header-rows:  0
    :stub-columns: 0
    :widths: 1 1 1 3

    * - __u32
      - ``cmd``
      -
      - The decoder command, see :ref:`decoder-cmds`.
    * - __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.
    * - }
      -


.. tabularcolumns:: |p{5.6cm}|p{0.6cm}|p{11.3cm}|

.. _decoder-cmds:

.. flat-table:: Decoder Commands
    :header-rows:  0
    :stub-columns: 0
    :widths: 56 6 113

    * - ``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.

	For a device implementing the :ref:`decoder`, once the drain sequence
	is initiated with the ``V4L2_DEC_CMD_STOP`` command, it must be driven
	to completion before this command can be invoked.  Any attempt to
	invoke the command while the drain sequence is in progress will trigger
	an ``EBUSY`` error code.  The command may be also used to restart the
	decoder in case of an implicit stop initiated by the decoder itself,
	without the ``V4L2_DEC_CMD_STOP`` being called explicitly. See
	:ref:`decoder` for more details.
    * - ``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.

	For a device implementing the :ref:`decoder`, the command will initiate
	the drain sequence as documented in :ref:`decoder`.  No flags or other
	arguments are accepted in this case. Any attempt to invoke the command
	again before the sequence completes will trigger an ``EBUSY`` error
	code.
    * - ``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.

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
:ref:`Generic Error Codes <gen-errors>` chapter.

EBUSY
    A drain sequence of a device implementing the :ref:`decoder` 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.