.. SPDX-License-Identifier: GPL-2.0

.. _decoder:

*************************************************
Memory-to-Memory Stateful Video Decoder Interface
*************************************************

A stateful video decoder takes complete chunks of the bytestream (e.g. Annex-B
H.264/HEVC stream, raw VP8/9 stream) and decodes them into raw video frames in
display order. The decoder is expected not to require any additional information
from the client to process these buffers.

Performing software parsing, processing etc. of the stream in the driver in
order to support this interface is strongly discouraged. In case such
operations are needed, use of the Stateless Video Decoder 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 decoder 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.

.. _decoder-glossary:

Glossary
========

CAPTURE
   the destination buffer queue; for decoders, the queue of buffers containing
   decoded frames; for encoders, the queue of buffers containing an encoded
   bytestream; ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or
   ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``; data is captured from the hardware
   into ``CAPTURE`` buffers.

client
   the application communicating with the decoder or encoder implementing
   this interface.

coded format
   encoded/compressed video bytestream format (e.g. H.264, VP8, etc.); see
   also: raw format.

coded height
   height for given coded resolution.

coded resolution
   stream resolution in pixels aligned to codec and hardware requirements;
   typically visible resolution rounded up to full macroblocks;
   see also: visible resolution.

coded width
   width for given coded resolution.

decode order
   the order in which frames are decoded; may differ from display order if the
   coded format includes a feature of frame reordering; for decoders,
   ``OUTPUT`` buffers must be queued by the client in decode order; for
   encoders ``CAPTURE`` buffers must be returned by the encoder in decode order.

destination
   data resulting from the decode process; see ``CAPTURE``.

display order
   the order in which frames must be displayed; for encoders, ``OUTPUT``
   buffers must be queued by the client in display order; for decoders,
   ``CAPTURE`` buffers must be returned by the decoder in display order.

DPB
   Decoded Picture Buffer; an H.264/HEVC term for a buffer that stores a decoded
   raw frame available for reference in further decoding steps.

EOS
   end of stream.

IDR
   Instantaneous Decoder Refresh; a type of a keyframe in an H.264/HEVC-encoded
   stream, which clears the list of earlier reference frames (DPBs).

keyframe
   an encoded frame that does not reference frames decoded earlier, i.e.
   can be decoded fully on its own.

macroblock
   a processing unit in image and video compression formats based on linear
   block transforms (e.g. H.264, VP8, VP9); codec-specific, but for most of
   popular codecs the size is 16x16 samples (pixels).

OUTPUT
   the source buffer queue; for decoders, the queue of buffers containing
   an encoded bytestream; for encoders, the queue of buffers containing raw
   frames; ``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or
   ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``; the hardware is fed with data
   from ``OUTPUT`` buffers.

PPS
   Picture Parameter Set; a type of metadata entity in an H.264/HEVC bytestream.

raw format
   uncompressed format containing raw pixel data (e.g. YUV, RGB formats).

resume point
   a point in the bytestream from which decoding may start/continue, without
   any previous state/data present, e.g.: a keyframe (VP8/VP9) or
   SPS/PPS/IDR sequence (H.264/HEVC); a resume point is required to start decode
   of a new stream, or to resume decoding after a seek.

source
   data fed to the decoder or encoder; see ``OUTPUT``.

source height
   height in pixels for given source resolution; relevant to encoders only.

source resolution
   resolution in pixels of source frames being source to the encoder and
   subject to further cropping to the bounds of visible resolution; relevant to
   encoders only.

source width
   width in pixels for given source resolution; relevant to encoders only.

SPS
   Sequence Parameter Set; a type of metadata entity in an H.264/HEVC bytestream.

stream metadata
   additional (non-visual) information contained inside encoded bytestream;
   for example: coded resolution, visible resolution, codec profile.

visible height
   height for given visible resolution; display height.

visible resolution
   stream resolution of the visible picture, in pixels, to be used for
   display purposes; must be smaller or equal to coded resolution;
   display resolution.

visible width
   width for given visible resolution; display width.

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

.. kernel-render:: DOT
   :alt: DOT digraph of decoder state machine
   :caption: Decoder State Machine

   digraph decoder_state_machine {
       node [shape = doublecircle, label="Decoding"] Decoding;

       node [shape = circle, label="Initialization"] Initialization;
       node [shape = circle, label="Capture\nsetup"] CaptureSetup;
       node [shape = circle, label="Dynamic\nResolution\nChange"] ResChange;
       node [shape = circle, label="Stopped"] Stopped;
       node [shape = circle, label="Drain"] Drain;
       node [shape = circle, label="Seek"] Seek;
       node [shape = circle, label="End of Stream"] EoS;

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

       Initialization -> CaptureSetup [ label = "CAPTURE\nformat\nestablished" ];

       CaptureSetup -> Stopped [ label = "CAPTURE\nbuffers\nready" ];

       Decoding -> ResChange [ label = "Stream\nresolution\nchange" ];
       Decoding -> Drain [ label = "V4L2_DEC_CMD_STOP" ];
       Decoding -> EoS [ label = "EoS mark\nin the stream" ];
       Decoding -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
       Decoding -> Stopped [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
       Decoding -> Decoding;

       ResChange -> CaptureSetup [ label = "CAPTURE\nformat\nestablished" ];
       ResChange -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];

       EoS -> Drain [ label = "Implicit\ndrain" ];

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

       Seek -> Decoding [ label = "VIDIOC_STREAMON(OUTPUT)" ];
       Seek -> Initialization [ label = "VIDIOC_REQBUFS(OUTPUT, 0)" ];

       Stopped -> Decoding [ label = "V4L2_DEC_CMD_START\nor\nVIDIOC_STREAMON(CAPTURE)" ];
       Stopped -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
   }

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

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

   * The full set of supported formats will be returned, regardless of the
     format set on ``CAPTURE``.
   * Check the flags field of :c:type:`v4l2_fmtdesc` for more information
     about the decoder's capabilities with respect to each coded format.
     In particular whether or not the decoder has a full-fledged bytestream
     parser and if the decoder supports dynamic resolution changes.

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

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

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

3. The client may use :c:func:`VIDIOC_ENUM_FRAMESIZES` to detect supported
   resolutions for a given format, passing 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
     decoder for 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
     decoder for given raw pixel format and the coded format currently set on
     ``OUTPUT``.

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

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

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

   * **Required fields:**

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

     ``pixelformat``
         a coded pixel format.

     ``width``, ``height``
         coded resolution of the stream; required only if it cannot be parsed
         from the stream for the given coded format; otherwise the decoder will
         use this resolution as a placeholder resolution that will likely change
         as soon as it can parse the actual coded resolution from the stream.

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

     other fields
         follow standard semantics.

   * **Return fields:**

     ``sizeimage``
         adjusted size of ``OUTPUT`` buffers.

   * The ``CAPTURE`` format will be updated with an appropriate frame buffer
     resolution instantly based on the width and height returned by
     :c:func:`VIDIOC_S_FMT`.
     However, for coded formats that include stream resolution information,
     after the decoder is done parsing the information from the stream, it will
     update the ``CAPTURE`` format with new values and signal a source change
     event, regardless of whether they match the values set by the client or
     not.

   .. important::

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

2.  Allocate source (bytestream) buffers via :c:func:`VIDIOC_REQBUFS` on
    ``OUTPUT``.

    * **Required fields:**

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

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

      ``memory``
          follows standard semantics.

    * **Return fields:**

      ``count``
          the 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.

    Alternatively, :c:func:`VIDIOC_CREATE_BUFS` on the ``OUTPUT`` queue 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``.

      ``memory``
          follows standard semantics.

      ``format``
          follows standard semantics.

    * **Return fields:**

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

    .. 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.

3.  Start streaming on the ``OUTPUT`` queue via :c:func:`VIDIOC_STREAMON`.

4.  **This step only applies to coded formats that contain resolution information
    in the stream.** Continue queuing/dequeuing bytestream buffers to/from the
    ``OUTPUT`` queue via :c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`. The
    buffers will be processed and returned to the client in order, until
    required metadata to configure the ``CAPTURE`` queue are found. This is
    indicated by the decoder sending a ``V4L2_EVENT_SOURCE_CHANGE`` event with
    ``changes`` set to ``V4L2_EVENT_SRC_CH_RESOLUTION``.

    * It is not an error if the first buffer does not contain enough data for
      this to occur. Processing of the buffers will continue as long as more
      data is needed.

    * If data in a buffer that triggers the event is required to decode the
      first frame, it will not be returned to the client, until the
      initialization sequence completes and the frame is decoded.

    * If the client has not set the coded resolution of the stream on its own,
      calling :c:func:`VIDIOC_G_FMT`, :c:func:`VIDIOC_S_FMT`,
      :c:func:`VIDIOC_TRY_FMT` or :c:func:`VIDIOC_REQBUFS` on the ``CAPTURE``
      queue will not return the real values for the stream until a
      ``V4L2_EVENT_SOURCE_CHANGE`` event with ``changes`` set to
      ``V4L2_EVENT_SRC_CH_RESOLUTION`` is signaled.

    .. important::

       Any client query issued after the decoder queues the event will return
       values applying to the just parsed stream, including queue formats,
       selection rectangles and controls.

    .. note::

       A client capable of acquiring stream parameters from the bytestream on
       its own may attempt to set the width and height of the ``OUTPUT`` format
       to non-zero values matching the coded size of the stream, skip this step
       and continue with the `Capture Setup` sequence. However, it must not
       rely on any driver queries regarding stream parameters, such as
       selection rectangles and controls, since the decoder has not parsed them
       from the stream yet. If the values configured by the client do not match
       those parsed by the decoder, a `Dynamic Resolution Change` will be
       triggered to reconfigure them.

    .. note::

       No decoded frames are produced during this phase.

5.  Continue with the `Capture Setup` sequence.

Capture Setup
=============

1.  Call :c:func:`VIDIOC_G_FMT` on the ``CAPTURE`` queue to get format for the
    destination buffers parsed/decoded from the bytestream.

    * **Required fields:**

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

    * **Return fields:**

      ``width``, ``height``
          frame buffer resolution for the decoded frames.

      ``pixelformat``
          pixel format for decoded frames.

      ``num_planes`` (for _MPLANE ``type`` only)
          number of planes for pixelformat.

      ``sizeimage``, ``bytesperline``
          as per standard semantics; matching frame buffer format.

    .. note::

       The value of ``pixelformat`` may be any pixel format supported by the
       decoder for the current stream. The decoder should choose a
       preferred/optimal format for the default configuration. For example, a
       YUV format may be preferred over an RGB format if an additional
       conversion step would be required for the latter.

2.  **Optional.** Acquire the visible resolution via
    :c:func:`VIDIOC_G_SELECTION`.

    * **Required fields:**

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

      ``target``
          set to ``V4L2_SEL_TGT_COMPOSE``.

    * **Return fields:**

      ``r.left``, ``r.top``, ``r.width``, ``r.height``
          the visible rectangle; it must fit within the frame buffer resolution
          returned by :c:func:`VIDIOC_G_FMT` on ``CAPTURE``.

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

      ``V4L2_SEL_TGT_CROP_BOUNDS``
          corresponds to the coded resolution of the stream.

      ``V4L2_SEL_TGT_CROP_DEFAULT``
          the rectangle covering the part of the ``CAPTURE`` buffer that
          contains meaningful picture data (visible area); width and height
          will be equal to the visible resolution of the stream.

      ``V4L2_SEL_TGT_CROP``
          the rectangle within the coded resolution to be output to
          ``CAPTURE``; defaults to ``V4L2_SEL_TGT_CROP_DEFAULT``; read-only on
          hardware without additional compose/scaling capabilities.

      ``V4L2_SEL_TGT_COMPOSE_BOUNDS``
          the maximum rectangle within a ``CAPTURE`` buffer, which the cropped
          frame can be composed into; equal to ``V4L2_SEL_TGT_CROP`` if the
          hardware does not support compose/scaling.

      ``V4L2_SEL_TGT_COMPOSE_DEFAULT``
          equal to ``V4L2_SEL_TGT_CROP``.

      ``V4L2_SEL_TGT_COMPOSE``
          the rectangle inside a ``CAPTURE`` buffer into which the cropped
          frame is written; defaults to ``V4L2_SEL_TGT_COMPOSE_DEFAULT``;
          read-only on hardware without additional compose/scaling capabilities.

      ``V4L2_SEL_TGT_COMPOSE_PADDED``
          the rectangle inside a ``CAPTURE`` buffer which is overwritten by the
          hardware; equal to ``V4L2_SEL_TGT_COMPOSE`` if the hardware does not
          write padding pixels.

    .. warning::

       The values are guaranteed to be meaningful only after the decoder
       successfully parses the stream metadata. The client must not rely on the
       query before that happens.

3.  **Optional.** Enumerate ``CAPTURE`` formats via :c:func:`VIDIOC_ENUM_FMT` on
    the ``CAPTURE`` queue. Once the stream information is parsed and known, the
    client may use this ioctl to discover which raw formats are supported for
    given stream and select one of them via :c:func:`VIDIOC_S_FMT`.

    .. important::

       The decoder will return only formats supported for the currently
       established coded format, as per the ``OUTPUT`` format and/or stream
       metadata parsed in this initialization sequence, even if more formats
       may be supported by the decoder in general. In other words, the set
       returned will be a subset of the initial query mentioned in the
       `Querying Capabilities` section.

       For example, a decoder may support YUV and RGB formats for resolutions
       1920x1088 and lower, but only YUV for higher resolutions (due to
       hardware limitations). After parsing a resolution of 1920x1088 or lower,
       :c:func:`VIDIOC_ENUM_FMT` may return a set of YUV and RGB pixel formats,
       but after parsing resolution higher than 1920x1088, the decoder will not
       return RGB, unsupported for this resolution.

       However, subsequent resolution change event triggered after
       discovering a resolution change within the same stream may switch
       the stream into a lower resolution and :c:func:`VIDIOC_ENUM_FMT`
       would return RGB formats again in that case.

4.  **Optional.** Set the ``CAPTURE`` format via :c:func:`VIDIOC_S_FMT` on the
    ``CAPTURE`` queue. The client may choose a different format than
    selected/suggested by the decoder in :c:func:`VIDIOC_G_FMT`.

    * **Required fields:**

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

      ``pixelformat``
          a raw pixel format.

      ``width``, ``height``
         frame buffer resolution of the decoded stream; typically unchanged from
	 what was returned with :c:func:`VIDIOC_G_FMT`, but it may be different
	 if the hardware supports composition and/or scaling.

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

5. **Optional.** Set the compose rectangle via :c:func:`VIDIOC_S_SELECTION` on
   the ``CAPTURE`` queue if it is desired and if the decoder has compose and/or
   scaling capabilities.

   * **Required fields:**

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

     ``target``
         set to ``V4L2_SEL_TGT_COMPOSE``.

     ``r.left``, ``r.top``, ``r.width``, ``r.height``
         the rectangle inside a ``CAPTURE`` buffer into which the cropped
         frame is written; defaults to ``V4L2_SEL_TGT_COMPOSE_DEFAULT``;
         read-only on hardware without additional compose/scaling capabilities.

   * **Return fields:**

     ``r.left``, ``r.top``, ``r.width``, ``r.height``
         the visible rectangle; it must fit within the frame buffer resolution
         returned by :c:func:`VIDIOC_G_FMT` on ``CAPTURE``.

   .. warning::

      The decoder may adjust the compose rectangle to the nearest
      supported one to meet codec and hardware requirements. The client needs
      to check the adjusted rectangle returned by :c:func:`VIDIOC_S_SELECTION`.

6.  If all the following conditions are met, the client may resume the decoding
    instantly:

    * ``sizeimage`` of the new format (determined in previous steps) is less
      than or equal to the size of currently allocated buffers,

    * the number of buffers currently allocated is greater than or equal to the
      minimum number of buffers acquired in previous steps. To fulfill this
      requirement, the client may use :c:func:`VIDIOC_CREATE_BUFS` to add new
      buffers.

    In that case, the remaining steps do not apply and the client may resume
    the decoding by one of the following actions:

    * if the ``CAPTURE`` queue is streaming, call :c:func:`VIDIOC_DECODER_CMD`
      with the ``V4L2_DEC_CMD_START`` command,

    * if the ``CAPTURE`` queue is not streaming, call :c:func:`VIDIOC_STREAMON`
      on the ``CAPTURE`` queue.

    However, if the client intends to change the buffer set, to lower
    memory usage or for any other reasons, it may be achieved by following
    the steps below.

7.  **If the** ``CAPTURE`` **queue is streaming,** keep queuing and dequeuing
    buffers on the ``CAPTURE`` queue until a buffer marked with the
    ``V4L2_BUF_FLAG_LAST`` flag is dequeued.

8.  **If the** ``CAPTURE`` **queue is streaming,** call :c:func:`VIDIOC_STREAMOFF`
    on the ``CAPTURE`` queue to stop streaming.

    .. warning::

       The ``OUTPUT`` queue must remain streaming. Calling
       :c:func:`VIDIOC_STREAMOFF` on it would abort the sequence and trigger a
       seek.

9.  **If the** ``CAPTURE`` **queue has buffers allocated,** free the ``CAPTURE``
    buffers using :c:func:`VIDIOC_REQBUFS`.

    * **Required fields:**

      ``count``
          set to 0.

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

      ``memory``
          follows standard semantics.

10. Allocate ``CAPTURE`` buffers via :c:func:`VIDIOC_REQBUFS` on the
    ``CAPTURE`` queue.

    * **Required fields:**

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

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

      ``memory``
          follows standard semantics.

    * **Return 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 buffers (for pipeline
       depth), the client may query the ``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE``
       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` on the ``CAPTURE`` queue can be
    used to have more control over buffer allocation. For example, by
    allocating buffers larger than the current ``CAPTURE`` format, future
    resolution changes can be accommodated.

    * **Required fields:**

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

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

      ``memory``
          follows standard semantics.

      ``format``
          a format representing the maximum framebuffer resolution to be
          accommodated by newly allocated buffers.

    * **Return fields:**

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

    .. 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 buffers for a format different than parsed from the stream
       metadata, the client must proceed as follows, before the metadata
       parsing is initiated:

       * set width and height of the ``OUTPUT`` format to desired coded resolution to
         let the decoder configure the ``CAPTURE`` format appropriately,

       * query the ``CAPTURE`` format using :c:func:`VIDIOC_G_FMT` and save it
         until this step.

       The format obtained in the query may be then used with
       :c:func:`VIDIOC_CREATE_BUFS` in this step to allocate the buffers.

11. Call :c:func:`VIDIOC_STREAMON` on the ``CAPTURE`` queue to start decoding
    frames.

Decoding
========

This state is reached after the `Capture Setup` 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 the source ``OUTPUT`` 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 the standard behavior of V4L2
buffer queues and memory-to-memory devices. In addition, the order of decoded
frames dequeued from the ``CAPTURE`` queue may differ from the order of queuing
coded 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 no buffers being produced
  on ``CAPTURE`` (e.g. if it does not contain encoded data, or if only
  metadata syntax structures are present in it),

* a buffer queued to ``OUTPUT`` may result in more than one buffer produced
  on ``CAPTURE`` (if the encoded data contained more than one frame, or if
  returning a decoded frame allowed the decoder to return a frame that
  preceded it in decode, but succeeded it in the display order),

* a buffer queued to ``OUTPUT`` may result in a buffer being produced on
  ``CAPTURE`` later into decode 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 decoding results are only
  available at later time, due to specifics of the decoding process.

.. note::

   To allow matching decoded ``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 decoding 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.

   * multiple ``OUTPUT`` buffers generate one ``CAPTURE`` buffer: timestamp of
     the ``OUTPUT`` buffer queued first will be copied.

   * the decoding order differs from the display 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::

   The backing memory of ``CAPTURE`` buffers that are used as reference frames
   by the stream may be read by the hardware even after they are dequeued.
   Consequently, the client should avoid writing into this memory while the
   ``CAPTURE`` queue is streaming. Failure to observe this may result in
   corruption of decoded frames.

   Similarly, when using a memory type other than ``V4L2_MEMORY_MMAP``, the
   client should make sure that each ``CAPTURE`` buffer is always queued with
   the same backing memory for as long as the ``CAPTURE`` queue is streaming.
   The reason for this is that V4L2 buffer indices can be used by drivers to
   identify frames. Thus, if the backing memory of a reference frame is
   submitted under a different buffer ID, the driver may misidentify it and
   decode a new frame into it while it is still in use, resulting in corruption
   of the following frames.

During the decoding, the decoder may initiate one of the special sequences, as
listed below. The sequences will result in the decoder returning all the
``CAPTURE`` buffers that originated from all the ``OUTPUT`` buffers processed
before the sequence started. Last of the buffers will have the
``V4L2_BUF_FLAG_LAST`` flag set. To determine the sequence to follow, the client
must check if there is any pending event and:

* if a ``V4L2_EVENT_SOURCE_CHANGE`` event with ``changes`` set to
  ``V4L2_EVENT_SRC_CH_RESOLUTION`` is pending, the `Dynamic Resolution
  Change` sequence needs to be followed,

* if a ``V4L2_EVENT_EOS`` event is pending, the `End of Stream` sequence needs
  to be followed.

Some of the sequences can be intermixed with each other and need to be handled
as they happen. The exact operation is documented for each sequence.

Should a decoding error occur, it will be reported to the client with the level
of details depending on the decoder capabilities. Specifically:

* the CAPTURE buffer that contains the results of the failed decode operation
  will be returned with the V4L2_BUF_FLAG_ERROR flag set,

* if the decoder is able to precisely report the OUTPUT buffer that triggered
  the error, such buffer will be returned with the V4L2_BUF_FLAG_ERROR flag
  set.

In case of a fatal failure that does not allow the decoding to continue, any
further operations on corresponding decoder 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.

Seek
====

Seek is controlled by the ``OUTPUT`` queue, as it is the source of coded data.
The seek does not require any specific operation on the ``CAPTURE`` queue, but
it may be affected as per normal decoder operation.

1. Stop the ``OUTPUT`` queue to begin the seek sequence via
   :c:func:`VIDIOC_STREAMOFF`.

   * **Required fields:**

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

   * The decoder will drop all the pending ``OUTPUT`` buffers and they must be
     treated as returned to the client (following standard semantics).

2. Restart the ``OUTPUT`` queue via :c:func:`VIDIOC_STREAMON`.

   * **Required fields:**

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

   * The decoder will start accepting new source bytestream buffers after the
     call returns.

3. Start queuing buffers containing coded data after the seek to the ``OUTPUT``
   queue until a suitable resume point is found.

   .. note::

      There is no requirement to begin queuing coded data starting exactly
      from a resume point (e.g. SPS or a keyframe). Any queued ``OUTPUT``
      buffers will be processed and returned to the client until a suitable
      resume point is found.  While looking for a resume point, the decoder
      should not produce any decoded frames into ``CAPTURE`` buffers.

      Some hardware is known to mishandle seeks to a non-resume point. Such an
      operation may result in an unspecified number of corrupted decoded frames
      being made available on the ``CAPTURE`` queue. Drivers must ensure that
      no fatal decoding errors or crashes occur, and implement any necessary
      handling and workarounds for hardware issues related to seek operations.

   .. warning::

      In case of the H.264/HEVC codec, the client must take care not to seek
      over a change of SPS/PPS. Even though the target frame could be a
      keyframe, the stale SPS/PPS inside decoder state would lead to undefined
      results when decoding. Although the decoder must handle that case without
      a crash or a fatal decode error, the client must not expect a sensible
      decode output.

      If the hardware can detect such corrupted decoded frames, then
      corresponding buffers will be returned to the client with the
      V4L2_BUF_FLAG_ERROR set. See the `Decoding` section for further
      description of decode error reporting.

4. After a resume point is found, the decoder will start returning ``CAPTURE``
   buffers containing decoded frames.

.. important::

   A seek may result in the `Dynamic Resolution Change` sequence being
   initiated, due to the seek target having decoding parameters different from
   the part of the stream decoded before the seek. The sequence must be handled
   as per normal decoder operation.

.. warning::

   It is not specified when the ``CAPTURE`` queue starts producing buffers
   containing decoded data from the ``OUTPUT`` buffers queued after the seek,
   as it operates independently from the ``OUTPUT`` queue.

   The decoder may return a number of remaining ``CAPTURE`` buffers containing
   decoded frames originating from the ``OUTPUT`` buffers queued before the
   seek sequence is performed.

   The ``VIDIOC_STREAMOFF`` operation discards any remaining queued
   ``OUTPUT`` buffers, which means that not all of the ``OUTPUT`` buffers
   queued before the seek sequence may have matching ``CAPTURE`` buffers
   produced.  For example, given the sequence of operations on the
   ``OUTPUT`` queue:

     QBUF(A), QBUF(B), STREAMOFF(), STREAMON(), QBUF(G), QBUF(H),

   any of the following results on the ``CAPTURE`` queue is allowed:

     {A', B', G', H'}, {A', G', H'}, {G', H'}.

   To determine the CAPTURE buffer containing the first decoded frame after the
   seek, the client may observe the timestamps to match the CAPTURE and OUTPUT
   buffers or use V4L2_DEC_CMD_STOP and V4L2_DEC_CMD_START to drain the
   decoder.

.. note::

   To achieve instantaneous seek, the client may restart streaming on the
   ``CAPTURE`` queue too to discard decoded, but not yet dequeued buffers.

Dynamic Resolution Change
=========================

Streams that include resolution metadata in the bytestream may require switching
to a different resolution during the decoding.

.. note::

   Not all decoders can detect resolution changes. Those that do set the
   ``V4L2_FMT_FLAG_DYN_RESOLUTION`` flag for the coded format when
   :c:func:`VIDIOC_ENUM_FMT` is called.

The sequence starts when the decoder detects a coded frame with one or more of
the following parameters different from those previously established (and
reflected by corresponding queries):

* coded resolution (``OUTPUT`` width and height),

* visible resolution (selection rectangles),

* the minimum number of buffers needed for decoding,

* bit-depth of the bitstream has been changed.

Whenever that happens, the decoder must proceed as follows:

1.  After encountering a resolution change in the stream, the decoder sends a
    ``V4L2_EVENT_SOURCE_CHANGE`` event with ``changes`` set to
    ``V4L2_EVENT_SRC_CH_RESOLUTION``.

    .. important::

       Any client query issued after the decoder queues the event will return
       values applying to the stream after the resolution change, including
       queue formats, selection rectangles and controls.

2.  The decoder will then process and decode all remaining buffers from before
    the resolution change point.

    * The last buffer from before the change must be marked with the
      ``V4L2_BUF_FLAG_LAST`` flag, similarly to the `Drain` sequence above.

    .. 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 a decoded 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`.

The client must continue the sequence as described below to continue the
decoding process.

1.  Dequeue the source change event.

    .. important::

       A source change triggers an implicit decoder drain, similar to the
       explicit `Drain` sequence. The decoder is stopped after it completes.
       The decoding process must be resumed with either a pair of calls to
       :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the
       ``CAPTURE`` queue, or a call to :c:func:`VIDIOC_DECODER_CMD` with the
       ``V4L2_DEC_CMD_START`` command.

2.  Continue with the `Capture Setup` sequence.

.. note::

   During the resolution change sequence, the ``OUTPUT`` queue must remain
   streaming. Calling :c:func:`VIDIOC_STREAMOFF` on the ``OUTPUT`` queue would
   abort the sequence and initiate a seek.

   In principle, the ``OUTPUT`` queue operates separately from the ``CAPTURE``
   queue and this remains true for the duration of the entire resolution change
   sequence as well.

   The client should, for best performance and simplicity, keep queuing/dequeuing
   buffers to/from the ``OUTPUT`` queue even while processing this sequence.

Drain
=====

To ensure that all queued ``OUTPUT`` buffers have been processed and 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 decoded frames for all ``OUTPUT`` buffers queued before the
sequence was started.

1. Begin drain by issuing :c:func:`VIDIOC_DECODER_CMD`.

   * **Required fields:**

     ``cmd``
         set to ``V4L2_DEC_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_DECODER_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_DECODER_CMD` was issued will be processed and decoded as
   normal. The client must continue to handle both queues independently,
   similarly to normal decode operation. This includes:

   * handling any operations triggered as a result of processing those buffers,
     such as the `Dynamic Resolution Change` sequence, before continuing with
     the drain sequence,

   * 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 a decoded 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_DEC_CMD_STOP`` command are dequeued,

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

   .. note::

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

3. Once all the ``OUTPUT`` buffers queued before the ``V4L2_DEC_CMD_STOP`` call
   are dequeued and the last ``CAPTURE`` buffer is dequeued, the decoder is
   stopped and it will accept, but not process, any newly queued ``OUTPUT``
   buffers until the client issues any of the following operations:

   * ``V4L2_DEC_CMD_START`` - the decoder 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 decoder will resume the operation normally,
     however any ``CAPTURE`` buffers still in the queue will be returned to the
     client,

   * a pair of :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the
     ``OUTPUT`` queue - any pending source buffers will be returned to the
     client and the `Seek` sequence will be triggered.

.. 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_DEC_CMD_START`` or
   ``V4L2_DEC_CMD_STOP`` again while the drain sequence is in progress and they
   will fail with -EBUSY error code if attempted.

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

End of Stream
=============

If the decoder encounters an end of stream marking in the stream, the decoder
will initiate the `Drain` sequence, which the client must handle as described
above, skipping the initial :c:func:`VIDIOC_DECODER_CMD`.

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

Setting formats and allocating buffers trigger changes in the behavior of the
decoder.

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

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

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

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

5. While buffers are allocated on any of the ``OUTPUT`` or ``CAPTURE`` queues,
   the client must not change the format on the ``OUTPUT`` 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
``OUTPUT`` queue and the ``OUTPUT`` queue is the master that governs the
set of supported formats for the ``CAPTURE`` queue.