xref: /linux/Documentation/userspace-api/media/v4l/dev-encoder.rst (revision a1c613ae4c322ddd58d5a8539dbfba2a0380a8c0)

.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-no-invariants-or-later

.. _encoder:

*************************************************
Memory-to-Memory Stateful Video Encoder Interface
*************************************************

A stateful video encoder takes raw video frames in display order and encodes
them into a bytestream. It generates complete chunks of the bytestream, including
all metadata, headers, etc. The resulting bytestream does not require any
further post-processing by the client.

Performing software stream processing, header generation etc. in the driver
in order to support this interface is strongly discouraged. In case such
operations are needed, use of the Stateless Video Encoder Interface (in
development) is strongly advised.

Conventions and Notations Used in This Document
===============================================

1. The general V4L2 API rules apply if not specified in this document
   otherwise.

2. The meaning of words "must", "may", "should", etc. is as per `RFC
   2119 <https://tools.ietf.org/html/rfc2119>`_.

3. All steps not marked "optional" are required.

4. :c:func:`VIDIOC_G_EXT_CTRLS` and :c:func:`VIDIOC_S_EXT_CTRLS` may be used
   interchangeably with :c:func:`VIDIOC_G_CTRL` and :c:func:`VIDIOC_S_CTRL`,
   unless specified otherwise.

5. Single-planar API (see :ref:`planar-apis`) and applicable structures may be
   used interchangeably with multi-planar API, unless specified otherwise,
   depending on encoder capabilities and following the general V4L2 guidelines.

6. i = [a..b]: sequence of integers from a to b, inclusive, i.e. i =
   [0..2]: i = 0, 1, 2.

7. Given an ``OUTPUT`` buffer A, then A' represents a buffer on the ``CAPTURE``
   queue containing data that resulted from processing buffer A.

Glossary
========

Refer to :ref:`decoder-glossary`.

State Machine
=============

.. kernel-render:: DOT
   :alt: DOT digraph of encoder state machine
   :caption: Encoder State Machine

   digraph encoder_state_machine {
       node [shape = doublecircle, label="Encoding"] Encoding;

       node [shape = circle, label="Initialization"] Initialization;
       node [shape = circle, label="Stopped"] Stopped;
       node [shape = circle, label="Drain"] Drain;
       node [shape = circle, label="Reset"] Reset;

       node [shape = point]; qi
       qi -> Initialization [ label = "open()" ];

       Initialization -> Encoding [ label = "Both queues streaming" ];

       Encoding -> Drain [ label = "V4L2_ENC_CMD_STOP" ];
       Encoding -> Reset [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
       Encoding -> Stopped [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
       Encoding -> Encoding;

       Drain -> Stopped [ label = "All CAPTURE\nbuffers dequeued\nor\nVIDIOC_STREAMOFF(OUTPUT)" ];
       Drain -> Reset [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];

       Reset -> Encoding [ label = "VIDIOC_STREAMON(CAPTURE)" ];
       Reset -> Initialization [ label = "VIDIOC_REQBUFS(OUTPUT, 0)" ];

       Stopped -> Encoding [ label = "V4L2_ENC_CMD_START\nor\nVIDIOC_STREAMON(OUTPUT)" ];
       Stopped -> Reset [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
   }

Querying Capabilities
=====================

1. To enumerate the set of coded formats supported by the encoder, the
   client may call :c:func:`VIDIOC_ENUM_FMT` on ``CAPTURE``.

   * The full set of supported formats will be returned, regardless of the
     format set on ``OUTPUT``.

2. To enumerate the set of supported raw formats, the client may call
   :c:func:`VIDIOC_ENUM_FMT` on ``OUTPUT``.

   * Only the formats supported for the format currently active on ``CAPTURE``
     will be returned.

   * In order to enumerate raw formats supported by a given coded format,
     the client must first set that coded format on ``CAPTURE`` and then
     enumerate the formats on ``OUTPUT``.

3. The client may use :c:func:`VIDIOC_ENUM_FRAMESIZES` to detect supported
   resolutions for a given format, passing the desired pixel format in
   :c:type:`v4l2_frmsizeenum` ``pixel_format``.

   * Values returned by :c:func:`VIDIOC_ENUM_FRAMESIZES` for a coded pixel
     format will include all possible coded resolutions supported by the
     encoder for the given coded pixel format.

   * Values returned by :c:func:`VIDIOC_ENUM_FRAMESIZES` for a raw pixel format
     will include all possible frame buffer resolutions supported by the
     encoder for the given raw pixel format and coded format currently set on
     ``CAPTURE``.

4. The client may use :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` to detect supported
   frame intervals for a given format and resolution, passing the desired pixel
   format in :c:type:`v4l2_frmivalenum` ``pixel_format`` and the resolution
   in :c:type:`v4l2_frmivalenum` ``width`` and :c:type:`v4l2_frmivalenum`
   ``height``.

   * Values returned by :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` for a coded pixel
     format and coded resolution will include all possible frame intervals
     supported by the encoder for the given coded pixel format and resolution.

   * Values returned by :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` for a raw pixel
     format and resolution will include all possible frame intervals supported
     by the encoder for the given raw pixel format and resolution and for the
     coded format, coded resolution and coded frame interval currently set on
     ``CAPTURE``.

   * Support for :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` is optional. If it is
     not implemented, then there are no special restrictions other than the
     limits of the codec itself.

5. Supported profiles and levels for the coded format currently set on
   ``CAPTURE``, if applicable, may be queried using their respective controls
   via :c:func:`VIDIOC_QUERYCTRL`.

6. Any additional encoder capabilities may be discovered by querying
   their respective controls.

Initialization
==============

1. Set the coded format on the ``CAPTURE`` queue via :c:func:`VIDIOC_S_FMT`.

   * **Required fields:**

     ``type``
         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.

     ``pixelformat``
         the coded format to be produced.

     ``sizeimage``
         desired size of ``CAPTURE`` buffers; the encoder may adjust it to
         match hardware requirements.

     ``width``, ``height``
         ignored (read-only).

     other fields
         follow standard semantics.

   * **Returned fields:**

     ``sizeimage``
         adjusted size of ``CAPTURE`` buffers.

     ``width``, ``height``
         the coded size selected by the encoder based on current state, e.g.
         ``OUTPUT`` format, selection rectangles, etc. (read-only).

   .. important::

      Changing the ``CAPTURE`` format may change the currently set ``OUTPUT``
      format. How the new ``OUTPUT`` format is determined is up to the encoder
      and the client must ensure it matches its needs afterwards.

2. **Optional.** Enumerate supported ``OUTPUT`` formats (raw formats for
   source) for the selected coded format via :c:func:`VIDIOC_ENUM_FMT`.

   * **Required fields:**

     ``type``
         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.

     other fields
         follow standard semantics.

   * **Returned fields:**

     ``pixelformat``
         raw format supported for the coded format currently selected on
         the ``CAPTURE`` queue.

     other fields
         follow standard semantics.

3. Set the raw source format on the ``OUTPUT`` queue via
   :c:func:`VIDIOC_S_FMT`.

   * **Required fields:**

     ``type``
         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.

     ``pixelformat``
         raw format of the source.

     ``width``, ``height``
         source resolution.

     other fields
         follow standard semantics.

   * **Returned fields:**

     ``width``, ``height``
         may be adjusted to match encoder minimums, maximums and alignment
         requirements, as required by the currently selected formats, as
         reported by :c:func:`VIDIOC_ENUM_FRAMESIZES`.

     other fields
         follow standard semantics.

   * Setting the ``OUTPUT`` format will reset the selection rectangles to their
     default values, based on the new resolution, as described in the next
     step.

4. Set the raw frame interval on the ``OUTPUT`` queue via
   :c:func:`VIDIOC_S_PARM`. This also sets the coded frame interval on the
   ``CAPTURE`` queue to the same value.

   * **Required fields:**

     ``type``
	 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.

     ``parm.output``
	 set all fields except ``parm.output.timeperframe`` to 0.

     ``parm.output.timeperframe``
	 the desired frame interval; the encoder may adjust it to
	 match hardware requirements.

   * **Returned fields:**

     ``parm.output.timeperframe``
	 the adjusted frame interval.

   .. important::

      Changing the ``OUTPUT`` frame interval *also* sets the framerate that
      the encoder uses to encode the video. So setting the frame interval
      to 1/24 (or 24 frames per second) will produce a coded video stream
      that can be played back at that speed. The frame interval for the
      ``OUTPUT`` queue is just a hint, the application may provide raw
      frames at a different rate. It can be used by the driver to help
      schedule multiple encoders running in parallel.

      In the next step the ``CAPTURE`` frame interval can optionally be
      changed to a different value. This is useful for off-line encoding
      were the coded frame interval can be different from the rate at
      which raw frames are supplied.

   .. important::

      ``timeperframe`` deals with *frames*, not fields. So for interlaced
      formats this is the time per two fields, since a frame consists of
      a top and a bottom field.

   .. note::

      It is due to historical reasons that changing the ``OUTPUT`` frame
      interval also changes the coded frame interval on the ``CAPTURE``
      queue. Ideally these would be independent settings, but that would
      break the existing API.

5. **Optional** Set the coded frame interval on the ``CAPTURE`` queue via
   :c:func:`VIDIOC_S_PARM`. This is only necessary if the coded frame
   interval is different from the raw frame interval, which is typically
   the case for off-line encoding. Support for this feature is signalled
   by the :ref:`V4L2_FMT_FLAG_ENC_CAP_FRAME_INTERVAL <fmtdesc-flags>` format flag.

   * **Required fields:**

     ``type``
	 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.

     ``parm.capture``
	 set all fields except ``parm.capture.timeperframe`` to 0.

     ``parm.capture.timeperframe``
	 the desired coded frame interval; the encoder may adjust it to
	 match hardware requirements.

   * **Returned fields:**

     ``parm.capture.timeperframe``
	 the adjusted frame interval.

   .. important::

      Changing the ``CAPTURE`` frame interval sets the framerate for the
      coded video. It does *not* set the rate at which buffers arrive on the
      ``CAPTURE`` queue, that depends on how fast the encoder is and how
      fast raw frames are queued on the ``OUTPUT`` queue.

   .. important::

      ``timeperframe`` deals with *frames*, not fields. So for interlaced
      formats this is the time per two fields, since a frame consists of
      a top and a bottom field.

   .. note::

      Not all drivers support this functionality, in that case just set
      the desired coded frame interval for the ``OUTPUT`` queue.

      However, drivers that can schedule multiple encoders based on the
      ``OUTPUT`` frame interval must support this optional feature.

6. **Optional.** Set the visible resolution for the stream metadata via
   :c:func:`VIDIOC_S_SELECTION` on the ``OUTPUT`` queue if it is desired
   to be different than the full OUTPUT resolution.

   * **Required fields:**

     ``type``
         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.

     ``target``
         set to ``V4L2_SEL_TGT_CROP``.

     ``r.left``, ``r.top``, ``r.width``, ``r.height``
         visible rectangle; this must fit within the `V4L2_SEL_TGT_CROP_BOUNDS`
         rectangle and may be subject to adjustment to match codec and
         hardware constraints.

   * **Returned fields:**

     ``r.left``, ``r.top``, ``r.width``, ``r.height``
         visible rectangle adjusted by the encoder.

   * The following selection targets are supported on ``OUTPUT``:

     ``V4L2_SEL_TGT_CROP_BOUNDS``
         equal to the full source frame, matching the active ``OUTPUT``
         format.

     ``V4L2_SEL_TGT_CROP_DEFAULT``
         equal to ``V4L2_SEL_TGT_CROP_BOUNDS``.

     ``V4L2_SEL_TGT_CROP``
         rectangle within the source buffer to be encoded into the
         ``CAPTURE`` stream; defaults to ``V4L2_SEL_TGT_CROP_DEFAULT``.

         .. note::

            A common use case for this selection target is encoding a source
            video with a resolution that is not a multiple of a macroblock,
            e.g.  the common 1920x1080 resolution may require the source
            buffers to be aligned to 1920x1088 for codecs with 16x16 macroblock
            size. To avoid encoding the padding, the client needs to explicitly
            configure this selection target to 1920x1080.

   .. warning::

      The encoder may adjust the crop/compose rectangles to the nearest
      supported ones to meet codec and hardware requirements. The client needs
      to check the adjusted rectangle returned by :c:func:`VIDIOC_S_SELECTION`.

7. Allocate buffers for both ``OUTPUT`` and ``CAPTURE`` via
   :c:func:`VIDIOC_REQBUFS`. This may be performed in any order.

   * **Required fields:**

     ``count``
         requested number of buffers to allocate; greater than zero.

     ``type``
         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT`` or
         ``CAPTURE``.

     other fields
         follow standard semantics.

   * **Returned fields:**

     ``count``
          actual number of buffers allocated.

   .. warning::

      The actual number of allocated buffers may differ from the ``count``
      given. The client must check the updated value of ``count`` after the
      call returns.

   .. note::

      To allocate more than the minimum number of OUTPUT buffers (for pipeline
      depth), the client may query the ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT``
      control to get the minimum number of buffers required, and pass the
      obtained value plus the number of additional buffers needed in the
      ``count`` field to :c:func:`VIDIOC_REQBUFS`.

   Alternatively, :c:func:`VIDIOC_CREATE_BUFS` can be used to have more
   control over buffer allocation.

   * **Required fields:**

     ``count``
         requested number of buffers to allocate; greater than zero.

     ``type``
         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.

     other fields
         follow standard semantics.

   * **Returned fields:**

     ``count``
         adjusted to the number of allocated buffers.

8. Begin streaming on both ``OUTPUT`` and ``CAPTURE`` queues via
   :c:func:`VIDIOC_STREAMON`. This may be performed in any order. The actual
   encoding process starts when both queues start streaming.

.. note::

   If the client stops the ``CAPTURE`` queue during the encode process and then
   restarts it again, the encoder will begin generating a stream independent
   from the stream generated before the stop. The exact constraints depend
   on the coded format, but may include the following implications:

   * encoded frames produced after the restart must not reference any
     frames produced before the stop, e.g. no long term references for
     H.264/HEVC,

   * any headers that must be included in a standalone stream must be
     produced again, e.g. SPS and PPS for H.264/HEVC.

Encoding
========

This state is reached after the `Initialization` sequence finishes
successfully.  In this state, the client queues and dequeues buffers to both
queues via :c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`, following the
standard semantics.

The content of encoded ``CAPTURE`` buffers depends on the active coded pixel
format and may be affected by codec-specific extended controls, as stated
in the documentation of each format.

Both queues operate independently, following standard behavior of V4L2 buffer
queues and memory-to-memory devices. In addition, the order of encoded frames
dequeued from the ``CAPTURE`` queue may differ from the order of queuing raw
frames to the ``OUTPUT`` queue, due to properties of the selected coded format,
e.g. frame reordering.

The client must not assume any direct relationship between ``CAPTURE`` and
``OUTPUT`` buffers and any specific timing of buffers becoming
available to dequeue. Specifically:

* a buffer queued to ``OUTPUT`` may result in more than one buffer produced on
  ``CAPTURE`` (for example, if returning an encoded frame allowed the encoder
  to return a frame that preceded it in display, but succeeded it in the decode
  order; however, there may be other reasons for this as well),

* a buffer queued to ``OUTPUT`` may result in a buffer being produced on
  ``CAPTURE`` later into encode process, and/or after processing further
  ``OUTPUT`` buffers, or be returned out of order, e.g. if display
  reordering is used,

* buffers may become available on the ``CAPTURE`` queue without additional
  buffers queued to ``OUTPUT`` (e.g. during drain or ``EOS``), because of the
  ``OUTPUT`` buffers queued in the past whose encoding results are only
  available at later time, due to specifics of the encoding process,

* buffers queued to ``OUTPUT`` may not become available to dequeue instantly
  after being encoded into a corresponding ``CAPTURE`` buffer, e.g. if the
  encoder needs to use the frame as a reference for encoding further frames.

.. note::

   To allow matching encoded ``CAPTURE`` buffers with ``OUTPUT`` buffers they
   originated from, the client can set the ``timestamp`` field of the
   :c:type:`v4l2_buffer` struct when queuing an ``OUTPUT`` buffer. The
   ``CAPTURE`` buffer(s), which resulted from encoding that ``OUTPUT`` buffer
   will have their ``timestamp`` field set to the same value when dequeued.

   In addition to the straightforward case of one ``OUTPUT`` buffer producing
   one ``CAPTURE`` buffer, the following cases are defined:

   * one ``OUTPUT`` buffer generates multiple ``CAPTURE`` buffers: the same
     ``OUTPUT`` timestamp will be copied to multiple ``CAPTURE`` buffers,

   * the encoding order differs from the presentation order (i.e. the
     ``CAPTURE`` buffers are out-of-order compared to the ``OUTPUT`` buffers):
     ``CAPTURE`` timestamps will not retain the order of ``OUTPUT`` timestamps.

.. note::

   To let the client distinguish between frame types (keyframes, intermediate
   frames; the exact list of types depends on the coded format), the
   ``CAPTURE`` buffers will have corresponding flag bits set in their
   :c:type:`v4l2_buffer` struct when dequeued. See the documentation of
   :c:type:`v4l2_buffer` and each coded pixel format for exact list of flags
   and their meanings.

Should an encoding error occur, it will be reported to the client with the level
of details depending on the encoder capabilities. Specifically:

* the ``CAPTURE`` buffer (if any) that contains the results of the failed encode
  operation will be returned with the ``V4L2_BUF_FLAG_ERROR`` flag set,

* if the encoder is able to precisely report the ``OUTPUT`` buffer(s) that triggered
  the error, such buffer(s) will be returned with the ``V4L2_BUF_FLAG_ERROR`` flag
  set.

.. note::

   If a ``CAPTURE`` buffer is too small then it is just returned with the
   ``V4L2_BUF_FLAG_ERROR`` flag set. More work is needed to detect that this
   error occurred because the buffer was too small, and to provide support to
   free existing buffers that were too small.

In case of a fatal failure that does not allow the encoding to continue, any
further operations on corresponding encoder file handle will return the -EIO
error code. The client may close the file handle and open a new one, or
alternatively reinitialize the instance by stopping streaming on both queues,
releasing all buffers and performing the Initialization sequence again.

Encoding Parameter Changes
==========================

The client is allowed to use :c:func:`VIDIOC_S_CTRL` to change encoder
parameters at any time. The availability of parameters is encoder-specific
and the client must query the encoder to find the set of available controls.

The ability to change each parameter during encoding is encoder-specific, as
per the standard semantics of the V4L2 control interface. The client may
attempt to set a control during encoding and if the operation fails with the
-EBUSY error code, the ``CAPTURE`` queue needs to be stopped for the
configuration change to be allowed. To do this, it may follow the `Drain`
sequence to avoid losing the already queued/encoded frames.

The timing of parameter updates is encoder-specific, as per the standard
semantics of the V4L2 control interface. If the client needs to apply the
parameters exactly at specific frame, using the Request API
(:ref:`media-request-api`) should be considered, if supported by the encoder.

Drain
=====

To ensure that all the queued ``OUTPUT`` buffers have been processed and the
related ``CAPTURE`` buffers are given to the client, the client must follow the
drain sequence described below. After the drain sequence ends, the client has
received all encoded frames for all ``OUTPUT`` buffers queued before the
sequence was started.

1. Begin the drain sequence by issuing :c:func:`VIDIOC_ENCODER_CMD`.

   * **Required fields:**

     ``cmd``
         set to ``V4L2_ENC_CMD_STOP``.

     ``flags``
         set to 0.

     ``pts``
         set to 0.

   .. warning::

      The sequence can be only initiated if both ``OUTPUT`` and ``CAPTURE``
      queues are streaming. For compatibility reasons, the call to
      :c:func:`VIDIOC_ENCODER_CMD` will not fail even if any of the queues is
      not streaming, but at the same time it will not initiate the `Drain`
      sequence and so the steps described below would not be applicable.

2. Any ``OUTPUT`` buffers queued by the client before the
   :c:func:`VIDIOC_ENCODER_CMD` was issued will be processed and encoded as
   normal. The client must continue to handle both queues independently,
   similarly to normal encode operation. This includes:

   * queuing and dequeuing ``CAPTURE`` buffers, until a buffer marked with the
     ``V4L2_BUF_FLAG_LAST`` flag is dequeued,

     .. warning::

        The last buffer may be empty (with :c:type:`v4l2_buffer`
        ``bytesused`` = 0) and in that case it must be ignored by the client,
        as it does not contain an encoded frame.

     .. note::

        Any attempt to dequeue more ``CAPTURE`` buffers beyond the buffer
        marked with ``V4L2_BUF_FLAG_LAST`` will result in a -EPIPE error from
        :c:func:`VIDIOC_DQBUF`.

   * dequeuing processed ``OUTPUT`` buffers, until all the buffers queued
     before the ``V4L2_ENC_CMD_STOP`` command are dequeued,

   * dequeuing the ``V4L2_EVENT_EOS`` event, if the client subscribes to it.

   .. note::

      For backwards compatibility, the encoder will signal a ``V4L2_EVENT_EOS``
      event when the last frame has been encoded and all frames are ready to be
      dequeued. It is deprecated behavior and the client must not rely on it.
      The ``V4L2_BUF_FLAG_LAST`` buffer flag should be used instead.

3. Once all ``OUTPUT`` buffers queued before the ``V4L2_ENC_CMD_STOP`` call are
   dequeued and the last ``CAPTURE`` buffer is dequeued, the encoder is stopped
   and it will accept, but not process any newly queued ``OUTPUT`` buffers
   until the client issues any of the following operations:

   * ``V4L2_ENC_CMD_START`` - the encoder will not be reset and will resume
     operation normally, with all the state from before the drain,

   * a pair of :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the
     ``CAPTURE`` queue - the encoder will be reset (see the `Reset` sequence)
     and then resume encoding,

   * a pair of :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the
     ``OUTPUT`` queue - the encoder will resume operation normally, however any
     source frames queued to the ``OUTPUT`` queue between ``V4L2_ENC_CMD_STOP``
     and :c:func:`VIDIOC_STREAMOFF` will be discarded.

.. note::

   Once the drain sequence is initiated, the client needs to drive it to
   completion, as described by the steps above, unless it aborts the process by
   issuing :c:func:`VIDIOC_STREAMOFF` on any of the ``OUTPUT`` or ``CAPTURE``
   queues.  The client is not allowed to issue ``V4L2_ENC_CMD_START`` or
   ``V4L2_ENC_CMD_STOP`` again while the drain sequence is in progress and they
   will fail with -EBUSY error code if attempted.

   For reference, handling of various corner cases is described below:

   * In case of no buffer in the ``OUTPUT`` queue at the time the
     ``V4L2_ENC_CMD_STOP`` command was issued, the drain sequence completes
     immediately and the encoder returns an empty ``CAPTURE`` buffer with the
     ``V4L2_BUF_FLAG_LAST`` flag set.

   * In case of no buffer in the ``CAPTURE`` queue at the time the drain
     sequence completes, the next time the client queues a ``CAPTURE`` buffer
     it is returned at once as an empty buffer with the ``V4L2_BUF_FLAG_LAST``
     flag set.

   * If :c:func:`VIDIOC_STREAMOFF` is called on the ``CAPTURE`` queue in the
     middle of the drain sequence, the drain sequence is canceled and all
     ``CAPTURE`` buffers are implicitly returned to the client.

   * If :c:func:`VIDIOC_STREAMOFF` is called on the ``OUTPUT`` queue in the
     middle of the drain sequence, the drain sequence completes immediately and
     next ``CAPTURE`` buffer will be returned empty with the
     ``V4L2_BUF_FLAG_LAST`` flag set.

   Although not mandatory, the availability of encoder commands may be queried
   using :c:func:`VIDIOC_TRY_ENCODER_CMD`.

Reset
=====

The client may want to request the encoder to reinitialize the encoding, so
that the following stream data becomes independent from the stream data
generated before. Depending on the coded format, that may imply that:

* encoded frames produced after the restart must not reference any frames
  produced before the stop, e.g. no long term references for H.264/HEVC,

* any headers that must be included in a standalone stream must be produced
  again, e.g. SPS and PPS for H.264/HEVC.

This can be achieved by performing the reset sequence.

1. Perform the `Drain` sequence to ensure all the in-flight encoding finishes
   and respective buffers are dequeued.

2. Stop streaming on the ``CAPTURE`` queue via :c:func:`VIDIOC_STREAMOFF`. This
   will return all currently queued ``CAPTURE`` buffers to the client, without
   valid frame data.

3. Start streaming on the ``CAPTURE`` queue via :c:func:`VIDIOC_STREAMON` and
   continue with regular encoding sequence. The encoded frames produced into
   ``CAPTURE`` buffers from now on will contain a standalone stream that can be
   decoded without the need for frames encoded before the reset sequence,
   starting at the first ``OUTPUT`` buffer queued after issuing the
   `V4L2_ENC_CMD_STOP` of the `Drain` sequence.

This sequence may be also used to change encoding parameters for encoders
without the ability to change the parameters on the fly.

Commit Points
=============

Setting formats and allocating buffers triggers changes in the behavior of the
encoder.

1. Setting the format on the ``CAPTURE`` queue may change the set of formats
   supported/advertised on the ``OUTPUT`` queue. In particular, it also means
   that the ``OUTPUT`` format may be reset and the client must not rely on the
   previously set format being preserved.

2. Enumerating formats on the ``OUTPUT`` queue always returns only formats
   supported for the current ``CAPTURE`` format.

3. Setting the format on the ``OUTPUT`` queue does not change the list of
   formats available on the ``CAPTURE`` queue. An attempt to set the ``OUTPUT``
   format that is not supported for the currently selected ``CAPTURE`` format
   will result in the encoder adjusting the requested ``OUTPUT`` format to a
   supported one.

4. Enumerating formats on the ``CAPTURE`` queue always returns the full set of
   supported coded formats, irrespective of the current ``OUTPUT`` format.

5. While buffers are allocated on any of the ``OUTPUT`` or ``CAPTURE`` queues,
   the client must not change the format on the ``CAPTURE`` queue. Drivers will
   return the -EBUSY error code for any such format change attempt.

To summarize, setting formats and allocation must always start with the
``CAPTURE`` queue and the ``CAPTURE`` queue is the master that governs the
set of supported formats for the ``OUTPUT`` queue.