Re: [PATCHv4 1/2] media: docs-rst: Document memory-to-memory video decoder interface

[Nicolas Dufresne]

Le mardi 11 juin 2019 Ã 10:29 +0200, Hans Verkuil a Ãcrit :
> On 6/10/19 9:54 PM, Nicolas Dufresne wrote:
> > Hi Hans,
> > 
> > I went through it, and I think we are close to ready. Unfortunately, I
> > believe the SOURCE_CHANGE event is still under specified. There was a
> > post recently to try and add a new flag for changes in color space
> > which is not included here. We are also missing a workflow for changes
> > that don't affect the allocation but will affect the rendering on a
> > display (like colorimetry). Userspace needs to know at which frames
> > these parameter changes in order to signal that to the following
> > processing block. I think we must have a plane for this.
> 
> Yes, we need a new flag for the SOURCE_CHANGE event to indicate colorimetry
> changes. That's actually useful for e.g. HDMI receivers as well.
> 
> But I don't see why that should make much of a change to the spec: you still
> have to drain the capture queue, the only difference is that you don't need
> to reallocate buffers, you can just restart the decoder.

I don't think you need to drain the queue if the change is just
metadata that have no impact on the buffers allocation or layout. I
think we should have a way to communicate these changed while
streaming. Basically, something like the event, but serialized.

> 
> My intention was to get this spec in first, then extend it once we have a new
> colorimetry change flag.
> 
> > Another aspect, I'd like if we could relax a bit the bits about dynamic
> > resolutions, making some space for the "static resolution" cases.
> > Something to keep in mind, these "static resolution" HW might not be
> > able to inform userspace of the display size of the picture, we should
> > check with Maxime what happens with the Amlogic.
> 
> I've asked him about that in another email thread:
> 
> https://www.mail-archive.com/linux-media@xxxxxxxxxxxxxxx/msg147652.html
> 
> Maxime, related to that: can the amlogic detect midstream colorimetry
> changes? Or is that also impossible?
> 
> > Le lundi 03 juin 2019 Ã 13:28 +0200, Hans Verkuil a Ãcrit :
> > > From: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> > > 
> > > Due to complexity of the video decoding process, the V4L2 drivers of
> > > stateful decoder hardware require specific sequences of V4L2 API calls
> > > to be followed. These include capability enumeration, initialization,
> > > decoding, seek, pause, dynamic resolution change, drain and end of
> > > stream.
> > > 
> > > Specifics of the above have been discussed during Media Workshops at
> > > LinuxCon Europe 2012 in Barcelona and then later Embedded Linux
> > > Conference Europe 2014 in DÃsseldorf. The de facto Codec API that
> > > originated at those events was later implemented by the drivers we already
> > > have merged in mainline, such as s5p-mfc or coda.
> > > 
> > > The only thing missing was the real specification included as a part of
> > > Linux Media documentation. Fix it now and document the decoder part of
> > > the Codec API.
> > > 
> > > Signed-off-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> > > Signed-off-by: Hans Verkuil <hverkuil-cisco@xxxxxxxxx>
> > > ---
> > >  Documentation/media/uapi/v4l/dev-decoder.rst  | 1084 +++++++++++++++++
> > >  Documentation/media/uapi/v4l/dev-mem2mem.rst  |    8 +-
> > >  Documentation/media/uapi/v4l/pixfmt-v4l2.rst  |    5 +
> > >  Documentation/media/uapi/v4l/v4l2.rst         |   10 +-
> > >  .../media/uapi/v4l/vidioc-decoder-cmd.rst     |   41 +-
> > >  5 files changed, 1132 insertions(+), 16 deletions(-)
> > >  create mode 100644 Documentation/media/uapi/v4l/dev-decoder.rst
> > > 
> > > diff --git a/Documentation/media/uapi/v4l/dev-decoder.rst b/Documentation/media/uapi/v4l/dev-decoder.rst
> > > new file mode 100644
> > > index 000000000000..b106f2d97c48
> > > --- /dev/null
> > > +++ b/Documentation/media/uapi/v4l/dev-decoder.rst
> > > @@ -0,0 +1,1084 @@
> > > +.. SPDX-License-Identifier: GPL-2.0
> > > +
> > > +.. _decoder:
> > > +
> > > +*************************************************
> > > +Memory-to-memory Stateful Video Decoder Interface
> > > +*************************************************
> > > +
> > > +A stateful video decoder takes complete chunks of the bitstream (e.g. Annex-B
> > > +H.264/HEVC stream, raw VP8/9 stream) and decodes them into raw video frames in
> > 
> > (minor, may ignore)
> > This isn't very precise, do we expect to document this per-format ? I
> > think we should maybe say "a complete frame of the bitstream", that
> > being said, I think stateful CODEC should be allowed to implement the
> > per-slice/per-frame control we are adding to stateless CODEC. Maybe
> > something like "documented complete chunks of bitstream" ?
> > 
> > > +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 notation 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 Aâ 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
> > > +   bitstream; ``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 bitstream 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 bitstream; 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 bitstream.
> > > +
> > > +raw format
> > > +   uncompressed format containing raw pixel data (e.g. YUV, RGB formats).
> > > +
> > > +resume point
> > > +   a point in the bitstream 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 bitstream.
> > > +
> > > +stream metadata
> > > +   additional (non-visual) information contained inside encoded bitstream;
> > > +   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``.
> > > +
> > > +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 (bitstream) 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 bitstream 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 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 bitstream 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 bitstream.
> > > +
> > > +    * **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.
> > > +
> > > +   * 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 last 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.
> > > +
> > > +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 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 bitstream 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:
> > > +
> > > +     {Aâ, Bâ, Gâ, Hâ}, {Aâ, Gâ, Hâ}, {Gâ, Hâ}.
> > > +
> > > +   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 bitstream may require switching
> > > +to a different resolution during the decoding.
> > > +
> > > +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.
> > > +
> > > +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``.
> > 
> > What about a format changes that does now affect the resolution, like a
> > pixel format change ? Also, what about changes that don't require a
> 
> A pixel format change always implies a resolution change.
> 
> > reallocation ? Like a change in the colorimetry parameters ? I don't
> > think the spec need to initially cover it all, but we need to make sure
> > there is a mechanism for userspace to know at which frames to read back
> > the new format (G_FMT). Maybe we need both an event and a buffer flag
> > to mark it ?
> 
> After a SOURCE_CHANGE event is received, G_FMT returns the *new* format,
> including colorimetry. This will become the active format once the drain
> operation finished.
> 
> If that's not clear from the spec, then I need to improve it.
> 
> Regards,
> 
> 	Hans
> 
> > > +
> > > +    .. 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 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.
> > > diff --git a/Documentation/media/uapi/v4l/dev-mem2mem.rst b/Documentation/media/uapi/v4l/dev-mem2mem.rst
> > > index 67a980818dc8..caa05f5f6380 100644
> > > --- a/Documentation/media/uapi/v4l/dev-mem2mem.rst
> > > +++ b/Documentation/media/uapi/v4l/dev-mem2mem.rst
> > > @@ -39,4 +39,10 @@ file handle is visible through another file handle).
> > >  One of the most common memory-to-memory device is the codec. Codecs
> > >  are more complicated than most and require additional setup for
> > >  their codec parameters. This is done through codec controls.
> > > -See :ref:`mpeg-controls`.
> > > +See :ref:`mpeg-controls`. More details on how to use codec memory-to-memory
> > > +devices are given in the following sections.
> > > +
> > > +.. toctree::
> > > +    :maxdepth: 1
> > > +
> > > +    dev-decoder
> > > diff --git a/Documentation/media/uapi/v4l/pixfmt-v4l2.rst b/Documentation/media/uapi/v4l/pixfmt-v4l2.rst
> > > index 71eebfc6d853..caf14e440447 100644
> > > --- a/Documentation/media/uapi/v4l/pixfmt-v4l2.rst
> > > +++ b/Documentation/media/uapi/v4l/pixfmt-v4l2.rst
> > > @@ -39,6 +39,11 @@ Single-planar format structure
> > >  	to a multiple of the scale factor of any smaller planes. For
> > >  	example when the image format is YUV 4:2:0, ``width`` and
> > >  	``height`` must be multiples of two.
> > > +
> > > +	For compressed formats that contain the resolution information encoded
> > > +	inside the stream, when fed to a stateful mem2mem decoder, the fields
> > > +	may be zero to rely on the decoder to detect the right values. For more
> > > +	details see :ref:`decoder` and format descriptions.
> > >      * - __u32
> > >        - ``pixelformat``
> > >        - The pixel format or type of compression, set by the application.
> > > diff --git a/Documentation/media/uapi/v4l/v4l2.rst b/Documentation/media/uapi/v4l/v4l2.rst
> > > index 004ec00db6bd..97015b9b40b8 100644
> > > --- a/Documentation/media/uapi/v4l/v4l2.rst
> > > +++ b/Documentation/media/uapi/v4l/v4l2.rst
> > > @@ -60,6 +60,10 @@ Authors, in alphabetical order:
> > >  
> > >    - Original author of the V4L2 API and documentation.
> > >  
> > > +- Figa, Tomasz <tfiga@xxxxxxxxxxxx>
> > > +
> > > +  - Documented the memory-to-memory decoder interface.
> > > +
> > >  - H Schimek, Michael <mschimek@xxxxxx>
> > >  
> > >    - Original author of the V4L2 API and documentation.
> > > @@ -68,6 +72,10 @@ Authors, in alphabetical order:
> > >  
> > >    - Documented the Digital Video timings API.
> > >  
> > > +- Osciak, Pawel <posciak@xxxxxxxxxxxx>
> > > +
> > > +  - Documented the memory-to-memory decoder interface.
> > > +
> > >  - Osciak, Pawel <pawel@xxxxxxxxxx>
> > >  
> > >    - Designed and documented the multi-planar API.
> > > @@ -92,7 +100,7 @@ Authors, in alphabetical order:
> > >  
> > >    - Designed and documented the VIDIOC_LOG_STATUS ioctl, the extended control ioctls, major parts of the sliced VBI API, the MPEG encoder and decoder APIs and the DV Timings API.
> > >  
> > > -**Copyright** |copy| 1999-2016: Bill Dirks, Michael H. Schimek, Hans Verkuil, Martin Rubli, Andy Walls, Muralidharan Karicheri, Mauro Carvalho Chehab, Pawel Osciak, Sakari Ailus & Antti Palosaari.
> > > +**Copyright** |copy| 1999-2018: Bill Dirks, Michael H. Schimek, Hans Verkuil, Martin Rubli, Andy Walls, Muralidharan Karicheri, Mauro Carvalho Chehab, Pawel Osciak, Sakari Ailus & Antti Palosaari, Tomasz Figa
> > >  
> > >  Except when explicitly stated as GPL, programming examples within this
> > >  part can be used and distributed without restrictions.
> > > diff --git a/Documentation/media/uapi/v4l/vidioc-decoder-cmd.rst b/Documentation/media/uapi/v4l/vidioc-decoder-cmd.rst
> > > index ccf83b05afa7..57f0066f4cff 100644
> > > --- a/Documentation/media/uapi/v4l/vidioc-decoder-cmd.rst
> > > +++ b/Documentation/media/uapi/v4l/vidioc-decoder-cmd.rst
> > > @@ -56,14 +56,16 @@ The ``cmd`` field must contain the command code. Some commands use the
> > >  
> > >  A :ref:`write() <func-write>` or :ref:`VIDIOC_STREAMON`
> > >  call sends an implicit START command to the decoder if it has not been
> > > -started yet.
> > > +started yet. Applies to both queues of mem2mem decoders.
> > >  
> > >  A :ref:`close() <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.
> > > +command to the decoder, and all buffered data is discarded. Applies to both
> > > +queues of mem2mem decoders.
> > >  
> > > -These ioctls are optional, not all drivers may support them. They were
> > > -introduced in Linux 3.3.
> > > +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}|
> > > @@ -167,26 +169,32 @@ introduced in Linux 3.3.
> > >  	``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. 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
> > > -	:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl will not block anymore,
> > > -	but return an ``EPIPE`` error code. If
> > > +	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
> > > @@ -209,6 +217,11 @@ 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.
> > >