Re: [RFC PATCH 1/2] media: docs-rst: Add decoder UAPI specification to Codec Interfaces

[Tomasz Figa <tfiga@chromium.org>]

On Tue, Sep 11, 2018 at 11:27 AM Nicolas Dufresne <nicolas@ndufresne.ca> wrote:
>
> Le mardi 05 juin 2018 à 19:33 +0900, Tomasz Figa a écrit :
> > Due to complexity of the video decoding process, the V4L2 drivers of
> > stateful decoder hardware require specific sequencies of V4L2 API calls
> > to be followed. These include capability enumeration, initialization,
> > decoding, seek, pause, dynamic resolution change, flush 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 Düsseldorf. 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 mtk-vcodec.
> >
> > 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@chromium.org>
> > ---
> >  Documentation/media/uapi/v4l/dev-codec.rst | 771 +++++++++++++++++++++
> >  Documentation/media/uapi/v4l/v4l2.rst      |  14 +-
> >  2 files changed, 784 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/media/uapi/v4l/dev-codec.rst b/Documentation/media/uapi/v4l/dev-codec.rst
> > index c61e938bd8dc..0483b10c205e 100644
> > --- a/Documentation/media/uapi/v4l/dev-codec.rst
> > +++ b/Documentation/media/uapi/v4l/dev-codec.rst
> > @@ -34,3 +34,774 @@ the codec and reprogram it whenever another file handler gets access.
> >  This is different from the usual video node behavior where the video
> >  properties are global to the device (i.e. changing something through one
> >  file handle is visible through another file handle).
> > +
> > +This interface is generally appropriate for hardware that does not
> > +require additional software involvement to parse/partially decode/manage
> > +the stream before/after processing in hardware.
> > +
> > +Input data to the Stream API are buffers containing unprocessed video
> > +stream (Annex-B H264/H265 stream, raw VP8/9 stream) only. The driver is
> > +expected not to require any additional information from the client to
> > +process these buffers, and to return decoded frames on the CAPTURE queue
> > +in display order.
> > +
> > +Performing software parsing, processing etc. of the stream in the driver
> > +in order to support stream API is strongly discouraged. In such case use
> > +of Stateless Codec Interface (in development) is preferred.
> > +
> > +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.
> > +
> > +3. All steps not marked “optional” are required.
> > +
> > +4. :c:func:`VIDIOC_G_EXT_CTRLS`, :c:func:`VIDIOC_S_EXT_CTRLS` may be used interchangeably with
> > +   :c:func:`VIDIOC_G_CTRL`, :c:func:`VIDIOC_S_CTRL`, unless specified otherwise.
> > +
> > +5. Single-plane API (see spec) and applicable structures may be used
> > +   interchangeably with Multi-plane API, unless specified otherwise.
> > +
> > +6. i = [a..b]: sequence of integers from a to b, inclusive, i.e. i =
> > +   [0..2]: i = 0, 1, 2.
> > +
> > +7. For OUTPUT buffer A, A’ represents a buffer on the CAPTURE queue
> > +   containing data (decoded or encoded frame/stream) that resulted
> > +   from processing buffer A.
> > +
> > +Glossary
> > +========
> > +
> > +CAPTURE
> > +   the destination buffer queue, decoded frames for
> > +   decoders, encoded bitstream for encoders;
> > +   ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or
> > +   ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``
> > +
> > +client
> > +   application client communicating with the driver
> > +   implementing this API
> > +
> > +coded format
> > +   encoded/compressed video bitstream format (e.g.
> > +   H.264, VP8, etc.); see raw format; this is not equivalent to fourcc
> > +   (V4L2 pixelformat), as each coded format may be supported by multiple
> > +   fourccs (e.g. ``V4L2_PIX_FMT_H264``, ``V4L2_PIX_FMT_H264_SLICE``, etc.)
> > +
> > +coded height
> > +   height for given coded resolution
> > +
> > +coded resolution
> > +   stream resolution in pixels aligned to codec
> > +   format and hardware requirements; see also visible resolution
> > +
> > +coded width
> > +   width for given coded resolution
> > +
> > +decode order
> > +   the order in which frames are decoded; may differ
> > +   from display (output) order if frame reordering (B frames) is active in
> > +   the stream; OUTPUT buffers must be queued in decode order; for frame
> > +   API, CAPTURE buffers must be returned by the driver in decode order;
> > +
> > +display order
> > +   the order in which frames must be displayed
> > +   (outputted); for stream API, CAPTURE buffers must be returned by the
> > +   driver in display order;
> > +
> > +EOS
> > +   end of stream
> > +
> > +input height
> > +   height in pixels for given input resolution
> > +
> > +input resolution
> > +   resolution in pixels of source frames being input
> > +   to the encoder and subject to further cropping to the bounds of visible
> > +   resolution
> > +
> > +input width
> > +   width in pixels for given input resolution
> > +
> > +OUTPUT
> > +   the source buffer queue, encoded bitstream for
> > +   decoders, raw frames for encoders; ``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or
> > +   ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``
> > +
> > +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 (VPX) or SPS/PPS/IDR sequence (H.264); a resume point is
> > +   required to start decode of a new stream, or to resume decoding after a
> > +   seek;
> > +
> > +source buffer
> > +   buffers allocated for source queue
> > +
> > +source queue
> > +   queue containing buffers used for source data, i.e.
> > +
> > +visible height
> > +   height for given visible resolution
> > +
> > +visible resolution
> > +   stream resolution of the visible picture, in
> > +   pixels, to be used for display purposes; must be smaller or equal to
> > +   coded resolution;
> > +
> > +visible width
> > +   width for given visible resolution
> > +
> > +Decoder
> > +=======
> > +
> > +Querying capabilities
> > +---------------------
> > +
> > +1. To enumerate the set of coded formats supported by the driver, the
> > +   client uses :c:func:`VIDIOC_ENUM_FMT` for OUTPUT. The driver must always
> > +   return the full set of supported formats, irrespective of the
> > +   format set on the CAPTURE queue.
> > +
> > +2. To enumerate the set of supported raw formats, the client uses
> > +   :c:func:`VIDIOC_ENUM_FMT` for CAPTURE. The driver must return only the
> > +   formats supported for the format currently set on the OUTPUT
> > +   queue.
> > +   In order to enumerate raw formats supported by a given coded
> > +   format, the client must first set that coded format on the
> > +   OUTPUT queue and then enumerate the CAPTURE queue.
> > +
> > +3. The client may use :c:func:`VIDIOC_ENUM_FRAMESIZES` to detect supported
> > +   resolutions for a given format, passing its fourcc in
> > +   :c:type:`v4l2_frmivalenum` ``pixel_format``.
> > +
> > +   a. Values returned from :c:func:`VIDIOC_ENUM_FRAMESIZES` for coded formats
> > +      must be maximums for given coded format for all supported raw
> > +      formats.
> > +
> > +   b. Values returned from :c:func:`VIDIOC_ENUM_FRAMESIZES` for raw formats must
> > +      be maximums for given raw format for all supported coded
> > +      formats.
> > +
> > +   c. The client should derive the supported resolution for a
> > +      combination of coded+raw format by calculating the
> > +      intersection of resolutions returned from calls to
> > +      :c:func:`VIDIOC_ENUM_FRAMESIZES` for the given coded and raw formats.
> > +
> > +4. Supported profiles and levels for given format, if applicable, may be
> > +   queried using their respective controls via :c:func:`VIDIOC_QUERYCTRL`.
> > +
> > +5. The client may use :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` to enumerate maximum
> > +   supported framerates by the driver/hardware for a given
> > +   format+resolution combination.
> > +
> > +Initialization sequence
> > +-----------------------
> > +
> > +1. (optional) Enumerate supported OUTPUT formats and resolutions. See
> > +   capability enumeration.
> > +
> > +2. Set a coded format on the source queue via :c:func:`VIDIOC_S_FMT`
> > +
> > +   a. Required fields:
> > +
> > +      i.   type = OUTPUT
> > +
> > +      ii.  fmt.pix_mp.pixelformat set to a coded format
> > +
> > +      iii. fmt.pix_mp.width, fmt.pix_mp.height only if cannot be
> > +           parsed from the stream for the given coded format;
> > +           ignored otherwise;
> > +
> > +   b. Return values:
> > +
> > +      i.  EINVAL: unsupported format.
> > +
> > +      ii. Others: per spec
> > +
> > +   .. note::
> > +
> > +      The driver must not adjust pixelformat, so if
> > +      ``V4L2_PIX_FMT_H264`` is passed but only
> > +      ``V4L2_PIX_FMT_H264_SLICE`` is supported, S_FMT will return
> > +      -EINVAL. If both are acceptable by client, calling S_FMT for
> > +      the other after one gets rejected may be required (or use
> > +      :c:func:`VIDIOC_ENUM_FMT` to discover beforehand, see Capability
> > +      enumeration).
> > +
> > +3.  (optional) Get minimum number of buffers required for OUTPUT queue
> > +    via :c:func:`VIDIOC_G_CTRL`. This is useful if client intends to use
> > +    more buffers than minimum required by hardware/format (see
> > +    allocation).
> > +
> > +    a. Required fields:
> > +
> > +       i. id = ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT``
> > +
> > +    b. Return values: per spec.
> > +
> > +    c. Return fields:
> > +
> > +       i. value: required number of OUTPUT buffers for the currently set
> > +          format;
> > +
> > +4.  Allocate source (bitstream) buffers via :c:func:`VIDIOC_REQBUFS` on OUTPUT
> > +    queue.
> > +
> > +    a. Required fields:
> > +
> > +       i.   count = n, where n > 0.
> > +
> > +       ii.  type = OUTPUT
> > +
> > +       iii. memory = as per spec
> > +
> > +    b. Return values: Per spec.
> > +
> > +    c. Return fields:
> > +
> > +       i. count: adjusted to allocated number of buffers
> > +
> > +    d. The driver must adjust count to minimum of required number of
> > +       source buffers for given format and count passed. The client
> > +       must check this value after the ioctl returns to get the
> > +       number of buffers allocated.
> > +
> > +    .. note::
> > +
> > +       Passing count = 1 is useful for letting the driver choose
> > +       the minimum according to the selected format/hardware
> > +       requirements.
> > +
> > +    .. note::
> > +
> > +       To allocate more than minimum number of buffers (for pipeline
> > +       depth), use G_CTRL(``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT)`` to
> > +       get minimum number of buffers required by the driver/format,
> > +       and pass the obtained value plus the number of additional
> > +       buffers needed in count to :c:func:`VIDIOC_REQBUFS`.
> > +
> > +5.  Begin parsing the stream for stream metadata via :c:func:`VIDIOC_STREAMON` on
> > +    OUTPUT queue. This step allows the driver to parse/decode
> > +    initial stream metadata until enough information to allocate
> > +    CAPTURE buffers is found. This is indicated by the driver by
> > +    sending a ``V4L2_EVENT_SOURCE_CHANGE`` event, which the client
> > +    must handle.
> > +
> > +    a. Required fields: as per spec.
> > +
> > +    b. Return values: as per spec.
> > +
> > +    .. note::
> > +
> > +       Calling :c:func:`VIDIOC_REQBUFS`, :c:func:`VIDIOC_STREAMON`
> > +       or :c:func:`VIDIOC_G_FMT` on the CAPTURE queue at this time is not
> > +       allowed and must return EINVAL.
> > +
> > +6.  This step only applies for 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 driver
> > +    must keep processing and returning each buffer to the client
> > +    until required metadata to send a ``V4L2_EVENT_SOURCE_CHANGE``
> > +    for source change type ``V4L2_EVENT_SRC_CH_RESOLUTION`` is
> > +    found. There is no requirement to pass enough data for this to
> > +    occur in the first buffer and the driver must be able to
> > +    process any number
> > +
> > +    a. Required fields: as per spec.
> > +
> > +    b. Return values: as per spec.
> > +
> > +    c. If data in a buffer that triggers the event is required to decode
> > +       the first frame, the driver must not return it to the client,
> > +       but must retain it for further decoding.
> > +
> > +    d. Until the resolution source event is sent to the client, calling
> > +       :c:func:`VIDIOC_G_FMT` on the CAPTURE queue must return -EINVAL.
> > +
> > +    .. note::
> > +
> > +       No decoded frames are produced during this phase.
> > +
> > +7.  This step only applies for coded formats that contain resolution
> > +    information in the stream.
> > +    Receive and handle ``V4L2_EVENT_SOURCE_CHANGE`` from the driver
> > +    via :c:func:`VIDIOC_DQEVENT`. The driver must send this event once
> > +    enough data is obtained from the stream to allocate CAPTURE
> > +    buffers and to begin producing decoded frames.
> > +
> > +    a. Required fields:
> > +
> > +       i. type = ``V4L2_EVENT_SOURCE_CHANGE``
> > +
> > +    b. Return values: as per spec.
> > +
> > +    c. The driver must return u.src_change.changes =
> > +       ``V4L2_EVENT_SRC_CH_RESOLUTION``.
> > +
> > +8.  This step only applies for coded formats that contain resolution
> > +    information in the stream.
> > +    Call :c:func:`VIDIOC_G_FMT` for CAPTURE queue to get format for the
> > +    destination buffers parsed/decoded from the bitstream.
> > +
> > +    a. Required fields:
> > +
> > +       i. type = CAPTURE
> > +
> > +    b. Return values: as per spec.
> > +
> > +    c. Return fields:
> > +
> > +       i.   fmt.pix_mp.width, fmt.pix_mp.height: coded resolution
> > +            for the decoded frames
> > +
> > +       ii.  fmt.pix_mp.pixelformat: default/required/preferred by
> > +            driver pixelformat for decoded frames.
> > +
> > +       iii. num_planes: set to number of planes for pixelformat.
> > +
> > +       iv.  For each plane p = [0, num_planes-1]:
> > +            plane_fmt[p].sizeimage, plane_fmt[p].bytesperline as
> > +            per spec for coded resolution.
> > +
> > +    .. note::
> > +
> > +       Te value of pixelformat may be any pixel format supported,
> > +       and must
> > +       be supported for current stream, based on the information
> > +       parsed from the stream and hardware capabilities. It is
> > +       suggested that driver chooses the preferred/optimal format
> > +       for given configuration. For example, a YUV format may be
> > +       preferred over an RGB format, if additional conversion step
> > +       would be required.
> > +
> > +9.  (optional) Enumerate CAPTURE formats via :c:func:`VIDIOC_ENUM_FMT` on
> > +    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 on of them via :c:func:`VIDIOC_S_FMT`.
> > +
> > +    a. Fields/return values as per spec.
> > +
> > +    .. note::
> > +
> > +       The driver must return only formats supported for the
> > +       current stream parsed in this initialization sequence, even
> > +       if more formats may be supported by the driver in general.
> > +       For example, a driver/hardware may support YUV and RGB
> > +       formats for resolutions 1920x1088 and lower, but only YUV for
> > +       higher resolutions (e.g. due to memory bandwidth
> > +       limitations). After parsing a resolution of 1920x1088 or
> > +       lower, :c:func:`VIDIOC_ENUM_FMT` may return a set of YUV and RGB
> > +       pixelformats, but after parsing resolution higher than
> > +       1920x1088, the driver must not return (unsupported for this
> > +       resolution) RGB.
> > +
> > +       However, subsequent resolution change event
> > +       triggered after discovering a resolution change within the
> > +       same stream may switch the stream into a lower resolution;
> > +       :c:func:`VIDIOC_ENUM_FMT` must return RGB formats again in that case.
> > +
> > +10.  (optional) Choose a different CAPTURE format than suggested via
> > +     :c:func:`VIDIOC_S_FMT` on CAPTURE queue. It is possible for the client
> > +     to choose a different format than selected/suggested by the
> > +     driver in :c:func:`VIDIOC_G_FMT`.
> > +
> > +     a. Required fields:
> > +
> > +        i.  type = CAPTURE
> > +
> > +        ii. fmt.pix_mp.pixelformat set to a coded format
> > +
> > +     b. Return values:
> > +
> > +        i. EINVAL: unsupported format.
> > +
> > +     c. Calling :c:func:`VIDIOC_ENUM_FMT` to discover currently available formats
> > +        after receiving ``V4L2_EVENT_SOURCE_CHANGE`` is useful to find
> > +        out a set of allowed pixelformats for given configuration,
> > +        but not required.
> > +
> > +11.  (optional) Acquire visible resolution via :c:func:`VIDIOC_G_SELECTION`.
> > +
> > +    a. Required fields:
> > +
> > +       i.  type = CAPTURE
> > +
> > +       ii. target = ``V4L2_SEL_TGT_CROP``
> > +
> > +    b. Return values: per spec.
> > +
> > +    c. Return fields
> > +
> > +       i. r.left, r.top, r.width, r.height: visible rectangle; this must
> > +          fit within coded resolution returned from :c:func:`VIDIOC_G_FMT`.
> > +
> > +12. (optional) Get minimum number of buffers required for CAPTURE queue
> > +    via :c:func:`VIDIOC_G_CTRL`. This is useful if client intends to use
> > +    more buffers than minimum required by hardware/format (see
> > +    allocation).
> > +
> > +    a. Required fields:
> > +
> > +       i. id = ``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE``
> > +
> > +    b. Return values: per spec.
> > +
> > +    c. Return fields:
> > +
> > +       i. value: minimum number of buffers required to decode the stream
> > +          parsed in this initialization sequence.
> > +
> > +    .. note::
> > +
> > +       Note that the minimum number of buffers must be at least the
> > +       number required to successfully decode the current stream.
> > +       This may for example be the required DPB size for an H.264
> > +       stream given the parsed stream configuration (resolution,
> > +       level).
> > +
> > +13. Allocate destination (raw format) buffers via :c:func:`VIDIOC_REQBUFS` on the
> > +    CAPTURE queue.
> > +
> > +    a. Required fields:
> > +
> > +       i.   count = n, where n > 0.
> > +
> > +       ii.  type = CAPTURE
> > +
> > +       iii. memory = as per spec
> > +
> > +    b. Return values: Per spec.
> > +
> > +    c. Return fields:
> > +
> > +       i. count: adjusted to allocated number of buffers.
> > +
> > +    d. The driver must adjust count to minimum of required number of
> > +       destination buffers for given format and stream configuration
> > +       and the count passed. The client must check this value after
> > +       the ioctl returns to get the number of buffers allocated.
> > +
> > +    .. note::
> > +
> > +       Passing count = 1 is useful for letting the driver choose
> > +       the minimum.
> > +
> > +    .. note::
> > +
> > +       To allocate more than minimum number of buffers (for pipeline
> > +       depth), use G_CTRL(``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE)`` to
> > +       get minimum number of buffers required, and pass the obtained
> > +       value plus the number of additional buffers needed in count
> > +       to :c:func:`VIDIOC_REQBUFS`.
> > +
> > +14. Call :c:func:`VIDIOC_STREAMON` to initiate decoding frames.
> > +
> > +    a. Required fields: as per spec.
> > +
> > +    b. Return values: as per spec.
> > +
> > +Decoding
> > +--------
> > +
> > +This state is reached after a successful initialization sequence. In
> > +this state, client queues and dequeues buffers to both queues via
> > +:c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`, as per spec.
> > +
> > +Both queues operate independently. The client may queue and dequeue
> > +buffers to queues in any order and at any rate, also at a rate different
> > +for each queue. The client may queue buffers within the same queue in
> > +any order (V4L2 index-wise). It is recommended for the client to operate
> > +the queues independently for best performance.
> > +
> > +Source OUTPUT buffers must contain:
> > +
> > +-  H.264/AVC: one or more complete NALUs of an Annex B elementary
> > +   stream; one buffer does not have to contain enough data to decode
> > +   a frame;
> > +
> > +-  VP8/VP9: one or more complete frames.
> > +
> > +No direct relationship between source and destination buffers and the
> > +timing of buffers becoming available to dequeue should be assumed in the
> > +Stream API. Specifically:
> > +
> > +-  a buffer queued to OUTPUT queue may result in no buffers being
> > +   produced on the CAPTURE queue (e.g. if it does not contain
> > +   encoded data, or if only metadata syntax structures are present
> > +   in it), or one or more buffers produced on the CAPTURE queue (if
> > +   the encoded data contained more than one frame, or if returning a
> > +   decoded frame allowed the driver to return a frame that preceded
> > +   it in decode, but succeeded it in display order)
> > +
> > +-  a buffer queued to OUTPUT may result in a buffer being produced on
> > +   the CAPTURE queue 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 flush or EOS)
> > +
> > +Seek
> > +----
> > +
> > +Seek is controlled by the OUTPUT queue, as it is the source of bitstream
> > +data. CAPTURE queue remains unchanged/unaffected.
> > +
> > +1. Stop the OUTPUT queue to begin the seek sequence via
> > +   :c:func:`VIDIOC_STREAMOFF`.
> > +
> > +   a. Required fields:
> > +
> > +      i. type = OUTPUT
> > +
> > +   b. The driver must drop all the pending OUTPUT buffers and they are
> > +      treated as returned to the client (as per spec).
> > +
> > +2. Restart the OUTPUT queue via :c:func:`VIDIOC_STREAMON`
> > +
> > +   a. Required fields:
> > +
> > +      i. type = OUTPUT
> > +
> > +   b. The driver must be put in a state after seek and be ready to
> > +      accept new source bitstream buffers.
> > +
> > +3. Start queuing buffers to OUTPUT queue containing stream data after
> > +   the seek until a suitable resume point is found.
> > +
> > +   .. note::
> > +
> > +      There is no requirement to begin queuing stream
> > +      starting exactly from a resume point (e.g. SPS or a keyframe).
> > +      The driver must handle any data queued and must keep processing
> > +      the queued buffers until it finds a suitable resume point.
> > +      While looking for a resume point, the driver processes OUTPUT
> > +      buffers and returns them to the client without producing any
> > +      decoded frames.
> > +
> > +4. After a resume point is found, the driver will start returning
> > +   CAPTURE buffers with decoded frames.
> > +
> > +   .. note::
> > +
> > +      There is no precise specification for CAPTURE queue of when it
> > +      will start producing buffers containing decoded data from
> > +      buffers queued after the seek, as it operates independently
> > +      from OUTPUT queue.
> > +
> > +      -  The driver is allowed to and may return a number of remaining CAPTURE
> > +         buffers containing decoded frames from before the seek after the
> > +         seek sequence (STREAMOFF-STREAMON) is performed.
> > +
> > +      -  The driver is also allowed to and may not return all decoded frames
> > +         queued but not decode before the seek sequence was initiated.
> > +         E.g. for an OUTPUT queue sequence: QBUF(A), QBUF(B),
> > +         STREAMOFF(OUT), STREAMON(OUT), QBUF(G), QBUF(H), any of the
> > +         following results on the CAPTURE queue is allowed: {A’, B’, G’,
> > +         H’}, {A’, G’, H’}, {G’, H’}.
> > +
> > +Pause
> > +-----
> > +
> > +In order to pause, the client should just cease queuing buffers onto the
> > +OUTPUT queue. This is different from the general V4L2 API definition of
> > +pause, which involves calling :c:func:`VIDIOC_STREAMOFF` on the queue. Without
> > +source bitstream data, there is not data to process and the hardware
> > +remains idle. Conversely, using :c:func:`VIDIOC_STREAMOFF` on OUTPUT queue
> > +indicates a seek, which 1) drops all buffers in flight and 2) after a
> > +subsequent :c:func:`VIDIOC_STREAMON` will look for and only continue from a
> > +resume point. This is usually undesirable for pause. The
> > +STREAMOFF-STREAMON sequence is intended for seeking.
> > +
> > +Similarly, CAPTURE queue should remain streaming as well, as the
> > +STREAMOFF-STREAMON sequence on it is intended solely for changing buffer
> > +sets
> > +
> > +Dynamic resolution change
> > +-------------------------
> > +
> > +When driver encounters a resolution change in the stream, the dynamic
> > +resolution change sequence is started.
> > +
> > +1.  On encountering a resolution change in the stream. The driver must
> > +    first process and decode all remaining buffers from before the
> > +    resolution change point.
> > +
> > +2.  After all buffers containing decoded frames from before the
> > +    resolution change point are ready to be dequeued on the
> > +    CAPTURE queue, the driver sends a ``V4L2_EVENT_SOURCE_CHANGE``
> > +    event for source change type ``V4L2_EVENT_SRC_CH_RESOLUTION``.
> > +    The last buffer from before the change must be marked with
> > +    :c:type:`v4l2_buffer` ``flags`` flag ``V4L2_BUF_FLAG_LAST`` as in the flush
> > +    sequence.
> > +
> > +    .. note::
> > +
> > +       Any attempts to dequeue more buffers beyond the buffer marked
> > +       with ``V4L2_BUF_FLAG_LAST`` will result in a -EPIPE error from
> > +       :c:func:`VIDIOC_DQBUF`.
> > +
> > +3.  After dequeuing all remaining buffers from the CAPTURE queue, the
> > +    client must call :c:func:`VIDIOC_STREAMOFF` on the CAPTURE queue. The
> > +    OUTPUT queue remains streaming (calling STREAMOFF on it would
> > +    trigger a seek).
> > +    Until STREAMOFF is called on the CAPTURE queue (acknowledging
> > +    the event), the driver operates as if the resolution hasn’t
> > +    changed yet, i.e. :c:func:`VIDIOC_G_FMT`, etc. return previous
> > +    resolution.
> > +
> > +4.  The client frees the buffers on the CAPTURE queue using
> > +    :c:func:`VIDIOC_REQBUFS`.
> > +
> > +    a. Required fields:
> > +
> > +       i.   count = 0
> > +
> > +       ii.  type = CAPTURE
> > +
> > +       iii. memory = as per spec
> > +
> > +5.  The client calls :c:func:`VIDIOC_G_FMT` for CAPTURE to get the new format
> > +    information.
> > +    This is identical to calling :c:func:`VIDIOC_G_FMT` after
> > +    ``V4L2_EVENT_SRC_CH_RESOLUTION`` in the initialization
> > +    sequence and should be handled similarly.
> > +
> > +    .. note::
> > +
> > +       It is allowed for the driver not to support the same
> > +       pixelformat as previously used (before the resolution change)
> > +       for the new resolution. The driver must select a default
> > +       supported pixelformat and return it from :c:func:`VIDIOC_G_FMT`, and
> > +       client must take note of it.
> > +
> > +6.  (optional) The client is allowed to enumerate available formats and
> > +    select a different one than currently chosen (returned via
> > +    :c:func:`VIDIOC_G_FMT)`. This is identical to a corresponding step in
> > +    the initialization sequence.
> > +
> > +7.  (optional) The client acquires visible resolution as in
> > +    initialization sequence.
> > +
> > +8.  (optional) The client acquires minimum number of buffers as in
> > +    initialization sequence.
> > +
> > +9.  The client allocates a new set of buffers for the CAPTURE queue via
> > +    :c:func:`VIDIOC_REQBUFS`. This is identical to a corresponding step in
> > +    the initialization sequence.
> > +
> > +10. The client resumes decoding by issuing :c:func:`VIDIOC_STREAMON` on the
> > +    CAPTURE queue.
> > +
> > +During the resolution change sequence, the OUTPUT queue must remain
> > +streaming. Calling :c:func:`VIDIOC_STREAMOFF` on OUTPUT queue will initiate seek.
> > +
> > +The OUTPUT queue operates separately from the CAPTURE queue for the
> > +duration of the entire resolution change sequence. It is allowed (and
> > +recommended for best performance and simplcity) for the client to keep
> > +queuing/dequeuing buffers from/to OUTPUT queue even while processing
> > +this sequence.
> > +
> > +.. note::
> > +
> > +   It is also possible for this sequence to be triggered without
> > +   change in resolution if a different number of CAPTURE buffers is
> > +   required in order to continue decoding the stream.
> > +
> > +Flush
> > +-----
> > +
> > +Flush is the process of draining the CAPTURE queue of any remaining
> > +buffers. After the flush sequence is complete, the client has received
> > +all decoded frames for all OUTPUT buffers queued before the sequence was
> > +started.
> > +
> > +1. Begin flush by issuing :c:func:`VIDIOC_DECODER_CMD`.
> > +
> > +   a. Required fields:
> > +
> > +      i. cmd = ``V4L2_DEC_CMD_STOP``
> > +
> > +2. The driver must process and decode as normal all OUTPUT buffers
> > +   queued by the client before the :c:func:`VIDIOC_DECODER_CMD` was
> > +   issued.
> > +   Any operations triggered as a result of processing these
> > +   buffers (including the initialization and resolution change
> > +   sequences) must be processed as normal by both the driver and
> > +   the client before proceeding with the flush sequence.
> > +
> > +3. Once all OUTPUT buffers queued before ``V4L2_DEC_CMD_STOP`` are
> > +   processed:
> > +
> > +   a. If the CAPTURE queue is streaming, once all decoded frames (if
> > +      any) are ready to be dequeued on the CAPTURE queue, the
> > +      driver must send a ``V4L2_EVENT_EOS``. The driver must also
> > +      set ``V4L2_BUF_FLAG_LAST`` in :c:type:`v4l2_buffer` ``flags`` field on the
> > +      buffer on the CAPTURE queue containing the last frame (if
> > +      any) produced as a result of processing the OUTPUT buffers
> > +      queued before ``V4L2_DEC_CMD_STOP``. If no more frames are
> > +      left to be returned at the point of handling
> > +      ``V4L2_DEC_CMD_STOP``, the driver must return an empty buffer
>
> Sorry to come late, I didn't notice this detail before. Why do we need
> this empty buffer special case here ? Why can't we unblock the queue
> with -EPIPE, which is an already a supported special case ? This could
> even be handled by the m2m framework.

I feel like that would be _at_least_ inconsistent, because sometimes
DQBUF would return a buffer with V4L2_BUF_FLAG_LAST and sometimes it
would fail with -EPIPE. If we want to change this to -EPIPE, then it
would probably make much more sense to just have the userspace always
detect the last buffer by -EPIPE, without caring about
V4L2_BUF_FLAG_LAST.

Still, this empty buffer IMHO simplifies both userspace and driver
implementation, since the former can just dequeue buffers until
V4L2_BUF_FLAG_LAST is found, while the latter doesn't need to do
tricky synchronization dances to mark the last in-flight buffer,
possibly already being processed by hardware, as last, since it can
just return next one empty. I wouldn't call it a special case, since
it actually unifies the handling.

Note that -EPIPE is already handled by vb2 and it's triggered by
dequeuing a buffer with V4L2_BUF_FLAG_LAST. If the driver doesn't have
any further data to return to userspace and it couldn't return an
empty buffer, the -EPIPE mechanism would never trigger.

Best regards,
Tomasz