Message-ID: <CAAFQd5APTQNdH0wbAJpkBO+spXT=dK3e=egv3648bGNPw5-bJA@mail.gmail.com>
Date:   Mon, 8 Apr 2019 15:59:50 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Pawel Osciak <posciak@...omium.org>,
        Alexandre Courbot <acourbot@...omium.org>,
        Kamil Debski <kamil@...as.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Jeongtae Park <jtp.park@...sung.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Tiffany Lin (林慧珊) 
        <tiffany.lin@...iatek.com>,
        Andrew-CT Chen (陳智迪) 
        <andrew-ct.chen@...iatek.com>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Todor Tomov <todor.tomov@...aro.org>,
        Nicolas Dufresne <nicolas@...fresne.ca>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        dave.stevenson@...pberrypi.org,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Maxime Jourdan <maxi.jourdan@...adoo.fr>
Subject: Re: [PATCH v3 2/2] media: docs-rst: Document memory-to-memory video
 encoder interface

On Thu, Mar 21, 2019 at 7:11 PM Hans Verkuil <hverkuil@...all.nl> wrote:
>
> Hi Tomasz,
>
> A few more comments:
>
> On 1/24/19 11:04 AM, Tomasz Figa wrote:
> > Due to complexity of the video encoding process, the V4L2 drivers of
> > stateful encoder hardware require specific sequences of V4L2 API calls
> > to be followed. These include capability enumeration, initialization,
> > encoding, encode parameters change, drain and reset.
> >
> > 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 coda.
> >
> > The only thing missing was the real specification included as a part of
> > Linux Media documentation. Fix it now and document the encoder part of
> > the Codec API.
> >
> > Signed-off-by: Tomasz Figa <tfiga@...omium.org>
> > ---
> >  Documentation/media/uapi/v4l/dev-encoder.rst  | 586 ++++++++++++++++++
> >  Documentation/media/uapi/v4l/dev-mem2mem.rst  |   1 +
> >  Documentation/media/uapi/v4l/pixfmt-v4l2.rst  |   5 +
> >  Documentation/media/uapi/v4l/v4l2.rst         |   2 +
> >  .../media/uapi/v4l/vidioc-encoder-cmd.rst     |  38 +-
> >  5 files changed, 617 insertions(+), 15 deletions(-)
> >  create mode 100644 Documentation/media/uapi/v4l/dev-encoder.rst
> >
> > diff --git a/Documentation/media/uapi/v4l/dev-encoder.rst b/Documentation/media/uapi/v4l/dev-encoder.rst
> > new file mode 100644
> > index 000000000000..fb8b05a132ee
> > --- /dev/null
> > +++ b/Documentation/media/uapi/v4l/dev-encoder.rst
> > @@ -0,0 +1,586 @@
> > +.. -*- coding: utf-8; mode: rst -*-
> > +
> > +.. _encoder:
> > +
> > +*************************************************
> > +Memory-to-memory Stateful Video Encoder Interface
> > +*************************************************
> > +
> > +A stateful video encoder takes raw video frames in display order and encodes
> > +them into a bitstream. It generates complete chunks of the bitstream, including
> > +all metadata, headers, etc. The resulting bitstream does not require any
> > +further post-processing by the client.
> > +
> > +Performing software stream processing, header generation etc. in the driver
> > +in order to support this interface is strongly discouraged. In case such
> > +operations are needed, use of the Stateless Video Encoder Interface (in
> > +development) is strongly advised.
> > +
> > +Conventions and 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.
>
> decoder -> encoder
>

Ack.

> > +
> > +6. i = [a..b]: sequence of integers from a to b, inclusive, i.e. i =
> > +   [0..2]: i = 0, 1, 2.
> > +
> > +7. Given an ``OUTPUT`` buffer A, then A’ represents a buffer on the ``CAPTURE``
> > +   queue containing data that resulted from processing buffer A.
> > +
> > +Glossary
> > +========
> > +
> > +Refer to :ref:`decoder-glossary`.
> > +
> > +State machine
> > +=============
> > +
> > +.. kernel-render:: DOT
> > +   :alt: DOT digraph of encoder state machine
> > +   :caption: Encoder state machine
> > +
> > +   digraph encoder_state_machine {
> > +       node [shape = doublecircle, label="Encoding"] Encoding;
> > +
> > +       node [shape = circle, label="Initialization"] Initialization;
> > +       node [shape = circle, label="Stopped"] Stopped;
> > +       node [shape = circle, label="Drain"] Drain;
> > +       node [shape = circle, label="Reset"] Reset;
> > +
> > +       node [shape = point]; qi
> > +       qi -> Initialization [ label = "open()" ];
> > +
> > +       Initialization -> Encoding [ label = "Both queues streaming" ];
> > +
> > +       Encoding -> Drain [ label = "V4L2_DEC_CMD_STOP" ];
> > +       Encoding -> Reset [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
> > +       Encoding -> Stopped [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
> > +       Encoding -> Encoding;
> > +
> > +       Drain -> Stopped [ label = "All CAPTURE\nbuffers dequeued\nor\nVIDIOC_STREAMOFF(CAPTURE)" ];
> > +       Drain -> Reset [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
> > +
> > +       Reset -> Encoding [ label = "VIDIOC_STREAMON(CAPTURE)" ];
> > +       Reset -> Initialization [ label = "VIDIOC_REQBUFS(OUTPUT, 0)" ];
> > +
> > +       Stopped -> Encoding [ label = "V4L2_DEC_CMD_START\nor\nVIDIOC_STREAMON(OUTPUT)" ];
> > +       Stopped -> Reset [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
> > +   }
> > +
> > +Querying capabilities
> > +=====================
> > +
> > +1. To enumerate the set of coded formats supported by the encoder, the
> > +   client may call :c:func:`VIDIOC_ENUM_FMT` on ``CAPTURE``.
> > +
> > +   * The full set of supported formats will be returned, regardless of the
> > +     format set on ``OUTPUT``.
> > +
> > +2. To enumerate the set of supported raw formats, the client may call
> > +   :c:func:`VIDIOC_ENUM_FMT` on ``OUTPUT``.
> > +
> > +   * Only the formats supported for the format currently active on ``CAPTURE``
> > +     will be returned.
> > +
> > +   * In order to enumerate raw formats supported by a given coded format,
> > +     the client must first set that coded format on ``CAPTURE`` and then
> > +     enumerate the formats on ``OUTPUT``.
> > +
> > +3. The client may use :c:func:`VIDIOC_ENUM_FRAMESIZES` to detect supported
> > +   resolutions for a given format, passing desired pixel format in
> > +   :c:type:`v4l2_frmsizeenum` ``pixel_format``.
> > +
> > +   * Values returned by :c:func:`VIDIOC_ENUM_FRAMESIZES` for a coded pixel
> > +     format will include all possible coded resolutions supported by the
> > +     encoder for given coded pixel format.
> > +
> > +   * Values returned by :c:func:`VIDIOC_ENUM_FRAMESIZES` for a raw pixel format
> > +     will include all possible frame buffer resolutions supported by the
> > +     encoder for given raw pixel format and coded format currently set on
> > +     ``CAPTURE``.
> > +
> > +4. Supported profiles and levels for the coded format currently set on
> > +   ``CAPTURE``, if applicable, may be queried using their respective controls
> > +   via :c:func:`VIDIOC_QUERYCTRL`.
> > +
> > +5. Any additional encoder capabilities may be discovered by querying
> > +   their respective controls.
> > +
> > +Initialization
> > +==============
> > +
> > +1. Set the coded format on the ``CAPTURE`` queue via :c:func:`VIDIOC_S_FMT`
> > +
> > +   * **Required fields:**
> > +
> > +     ``type``
> > +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``
> > +
> > +     ``pixelformat``
> > +         the coded format to be produced
> > +
> > +     ``sizeimage``
> > +         desired size of ``CAPTURE`` buffers; the encoder may adjust it to
> > +         match hardware requirements
> > +
> > +     ``width``, ``height``
> > +         ignored (always zero)
> > +
> > +     other fields
> > +         follow standard semantics
> > +
> > +   * **Return fields:**
> > +
> > +     ``sizeimage``
> > +         adjusted size of ``CAPTURE`` buffers
> > +
> > +   .. important::
> > +
> > +      Changing the ``CAPTURE`` format may change the currently set ``OUTPUT``
> > +      format. The encoder will derive a new ``OUTPUT`` format from the
> > +      ``CAPTURE`` format being set, including resolution, colorimetry
> > +      parameters, etc. If the client needs a specific ``OUTPUT`` format, it
> > +      must adjust it afterwards.
> > +
> > +2. **Optional.** Enumerate supported ``OUTPUT`` formats (raw formats for
> > +   source) for the selected coded format via :c:func:`VIDIOC_ENUM_FMT`.
> > +
> > +   * **Required fields:**
> > +
> > +     ``type``
> > +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``
> > +
> > +     other fields
> > +         follow standard semantics
> > +
> > +   * **Return fields:**
> > +
> > +     ``pixelformat``
> > +         raw format supported for the coded format currently selected on
> > +         the ``CAPTURE`` queue.
> > +
> > +     other fields
> > +         follow standard semantics
> > +
> > +3. Set the raw source format on the ``OUTPUT`` queue via
> > +   :c:func:`VIDIOC_S_FMT`.
> > +
> > +   * **Required fields:**
> > +
> > +     ``type``
> > +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``
> > +
> > +     ``pixelformat``
> > +         raw format of the source
> > +
> > +     ``width``, ``height``
> > +         source resolution
> > +
> > +     other fields
> > +         follow standard semantics
> > +
> > +   * **Return fields:**
> > +
> > +     ``width``, ``height``
> > +         may be adjusted by encoder to match alignment requirements, as
> > +         required by the currently selected formats
> > +
> > +     other fields
> > +         follow standard semantics
> > +
> > +   * Setting the source resolution will reset the selection rectangles to their
> > +     default values, based on the new resolution, as described in the step 5
> > +     below.
> > +
> > +4. **Optional.** Set the visible resolution for the stream metadata via
> > +   :c:func:`VIDIOC_S_SELECTION` on the ``OUTPUT`` queue.
> > +
> > +   * **Required fields:**
> > +
> > +     ``type``
> > +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``
> > +
> > +     ``target``
> > +         set to ``V4L2_SEL_TGT_CROP``
> > +
> > +     ``r.left``, ``r.top``, ``r.width``, ``r.height``
> > +         visible rectangle; this must fit within the `V4L2_SEL_TGT_CROP_BOUNDS`
> > +         rectangle and may be subject to adjustment to match codec and
> > +         hardware constraints
> > +
> > +   * **Return fields:**
> > +
> > +     ``r.left``, ``r.top``, ``r.width``, ``r.height``
> > +         visible rectangle adjusted by the encoder
> > +
> > +   * The following selection targets are supported on ``OUTPUT``:
> > +
> > +     ``V4L2_SEL_TGT_CROP_BOUNDS``
> > +         equal to the full source frame, matching the active ``OUTPUT``
> > +         format
> > +
> > +     ``V4L2_SEL_TGT_CROP_DEFAULT``
> > +         equal to ``V4L2_SEL_TGT_CROP_BOUNDS``
> > +
> > +     ``V4L2_SEL_TGT_CROP``
> > +         rectangle within the source buffer to be encoded into the
> > +         ``CAPTURE`` stream; defaults to ``V4L2_SEL_TGT_CROP_DEFAULT``
> > +
> > +         .. note::
> > +
> > +            A common use case for this selection target is encoding a source
> > +            video with a resolution that is not a multiple of a macroblock,
> > +            e.g.  the common 1920x1080 resolution may require the source
> > +            buffers to be aligned to 1920x1088 for codecs with 16x16 macroblock
> > +            size. To avoid encoding the padding, the client needs to explicitly
> > +            configure this selection target to 1920x1080.
> > +
> > +     ``V4L2_SEL_TGT_COMPOSE_BOUNDS``
> > +         maximum rectangle within the coded resolution, which the cropped
> > +         source frame can be composed into; if the hardware does not support
> > +         composition or scaling, then this is always equal to the rectangle of
> > +         width and height matching ``V4L2_SEL_TGT_CROP`` and located at (0, 0)
> > +
> > +     ``V4L2_SEL_TGT_COMPOSE_DEFAULT``
> > +         equal to a rectangle of width and height matching
> > +         ``V4L2_SEL_TGT_CROP`` and located at (0, 0)
> > +
> > +     ``V4L2_SEL_TGT_COMPOSE``
> > +         rectangle within the coded frame, which the cropped source frame
> > +         is to be composed into; defaults to
> > +         ``V4L2_SEL_TGT_COMPOSE_DEFAULT``; read-only on hardware without
> > +         additional compose/scaling capabilities; resulting stream will
> > +         have this rectangle encoded as the visible rectangle in its
> > +         metadata
>
> I would only support the COMPOSE targets if the hardware can actually do
> scaling and/or composing. That is conform standard V4L2 behavior where
> cropping/composing is only implemented if the hardware can actually do
> this.
>

Please see my other reply to your earlier similar comment in this thread.

> > +
> > +   .. warning::
> > +
> > +      The encoder may adjust the crop/compose rectangles to the nearest
> > +      supported ones to meet codec and hardware requirements. The client needs
> > +      to check the adjusted rectangle returned by :c:func:`VIDIOC_S_SELECTION`.
> > +
> > +5. Allocate buffers for both ``OUTPUT`` and ``CAPTURE`` via
> > +   :c:func:`VIDIOC_REQBUFS`. This may be performed in any order.
> > +
> > +   * **Required fields:**
> > +
> > +     ``count``
> > +         requested number of buffers to allocate; greater than zero
> > +
> > +     ``type``
> > +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT`` or
> > +         ``CAPTURE``
> > +
> > +     other fields
> > +         follow standard semantics
> > +
> > +   * **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 OUTPUT buffers (for pipeline
> > +      depth), the client may query the ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT``
> > +      control to get the minimum number of buffers required, and pass the
> > +      obtained value plus the number of additional buffers needed in the
> > +      ``count`` field to :c:func:`VIDIOC_REQBUFS`.
> > +
> > +   Alternatively, :c:func:`VIDIOC_CREATE_BUFS` can be used to have more
> > +   control over buffer allocation.
> > +
> > +   * **Required fields:**
> > +
> > +     ``count``
> > +         requested number of buffers to allocate; greater than zero
> > +
> > +     ``type``
> > +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``
> > +
> > +     other fields
> > +         follow standard semantics
> > +
> > +   * **Return fields:**
> > +
> > +     ``count``
> > +         adjusted to the number of allocated buffers
> > +
> > +6. Begin streaming on both ``OUTPUT`` and ``CAPTURE`` queues via
> > +   :c:func:`VIDIOC_STREAMON`. This may be performed in any order. The actual
> > +   encoding process starts when both queues start streaming.
> > +
> > +.. note::
> > +
> > +   If the client stops the ``CAPTURE`` queue during the encode process and then
> > +   restarts it again, the encoder will begin generating a stream independent
> > +   from the stream generated before the stop. The exact constraints depend
> > +   on the coded format, but may include the following implications:
> > +
> > +   * encoded frames produced after the restart must not reference any
> > +     frames produced before the stop, e.g. no long term references for
> > +     H.264,
> > +
> > +   * any headers that must be included in a standalone stream must be
> > +     produced again, e.g. SPS and PPS for H.264.
> > +
> > +Encoding
> > +========
> > +
> > +This state is reached after the `Initialization` sequence finishes
> > +successfully.  In this state, the client queues and dequeues buffers to both
> > +queues via :c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`, following the
> > +standard semantics.
> > +
> > +The contents of encoded ``CAPTURE`` buffers depend on the active coded pixel
> > +format and may be affected by codec-specific extended controls, as stated
> > +in the documentation of each format.
> > +
> > +Both queues operate independently, following standard behavior of V4L2 buffer
> > +queues and memory-to-memory devices. In addition, the order of encoded frames
> > +dequeued from the ``CAPTURE`` queue may differ from the order of queuing raw
> > +frames to the ``OUTPUT`` queue, due to properties of the selected coded format,
> > +e.g. frame reordering.
> > +
> > +The client must not assume any direct relationship between ``CAPTURE`` and
> > +``OUTPUT`` buffers and any specific timing of buffers becoming
> > +available to dequeue. Specifically:
> > +
> > +* a buffer queued to ``OUTPUT`` may result in more than 1 buffer produced on
> > +  ``CAPTURE`` (if returning an encoded frame allowed the encoder to return a
> > +  frame that preceded it in display, but succeeded it in the decode order),
> > +
> > +* a buffer queued to ``OUTPUT`` may result in a buffer being produced on
> > +  ``CAPTURE`` later into encode process, and/or after processing further
> > +  ``OUTPUT`` buffers, or be returned out of order, e.g. if display
> > +  reordering is used,
> > +
> > +* buffers may become available on the ``CAPTURE`` queue without additional
> > +  buffers queued to ``OUTPUT`` (e.g. during drain or ``EOS``), because of the
> > +  ``OUTPUT`` buffers queued in the past whose decoding results are only
> > +  available at later time, due to specifics of the decoding process,
> > +
> > +* buffers queued to ``OUTPUT`` may not become available to dequeue instantly
> > +  after being encoded into a corresponding ``CATPURE`` buffer, e.g. if the
> > +  encoder needs to use the frame as a reference for encoding further frames.
> > +
> > +.. note::
> > +
> > +   To allow matching encoded ``CAPTURE`` buffers with ``OUTPUT`` buffers they
> > +   originated from, the client can set the ``timestamp`` field of the
> > +   :c:type:`v4l2_buffer` struct when queuing an ``OUTPUT`` buffer. The
> > +   ``CAPTURE`` buffer(s), which resulted from encoding that ``OUTPUT`` buffer
> > +   will have their ``timestamp`` field set to the same value when dequeued.
> > +
> > +   In addition to the straightforward case of one ``OUTPUT`` buffer producing
> > +   one ``CAPTURE`` buffer, the following cases are defined:
> > +
> > +   * one ``OUTPUT`` buffer generates multiple ``CAPTURE`` buffers: the same
> > +     ``OUTPUT`` timestamp will be copied to multiple ``CAPTURE`` buffers,
> > +
> > +   * the encoding order differs from the presentation order (i.e. the
> > +     ``CAPTURE`` buffers are out-of-order compared to the ``OUTPUT`` buffers):
> > +     ``CAPTURE`` timestamps will not retain the order of ``OUTPUT`` timestamps
> > +     and thus monotonicity of the timestamps cannot be guaranteed.
> > +
> > +.. note::
> > +
> > +   To let the client distinguish between frame types (keyframes, intermediate
> > +   frames; the exact list of types depends on the coded format), the
> > +   ``CAPTURE`` buffers will have corresponding flag bits set in their
> > +   :c:type:`v4l2_buffer` struct when dequeued. See the documentation of
> > +   :c:type:`v4l2_buffer` and each coded pixel format for exact list of flags
> > +   and their meanings.
>
> I don't think we can require this since a capture buffer may contain multiple
> encoded frames.
>

I thought we required that only one encoded frame was in one CAPTURE
buffer. Real time use cases rely heavily on this frame type
information, so I can't imagine not requiring this.

> It would actually make more sense to return it in the output buffer, but I don't
> know if a hardware encoder can actually provide that information.
>

I believe all the already existing drivers provide the information
about the encoded frame type, but I don't think they provide the
information about what source frame it came from.

> Another use of these flags for an output buffer is to force a keyframe if for
> example a scene change was detected.
>
> My feeling is that we should drop this note. Forcing a keyframe by setting that
> flag for the output buffer might actually be a useful thing to do for a stateful
> encoder.
>

However, to force keyframe, one sets it in the OUTPUT buffer. Then, to
actually get the right CAPTURE buffer, one has to look for one with
this flag set.

> > +
> > +Encoding parameter changes
> > +==========================
> > +
> > +The client is allowed to use :c:func:`VIDIOC_S_CTRL` to change encoder
> > +parameters at any time. The availability of parameters is encoder-specific
> > +and the client must query the encoder to find the set of available controls.
> > +
> > +The ability to change each parameter during encoding is encoder-specific, as
> > +per the standard semantics of the V4L2 control interface. The client may
> > +attempt to set a control during encoding and if the operation fails with the
> > +-EBUSY error code, the ``CAPTURE`` queue needs to be stopped for the
> > +configuration change to be allowed. To do this, it may follow the `Drain`
> > +sequence to avoid losing the already queued/encoded frames.
> > +
> > +The timing of parameter updates is encoder-specific, as per the standard
> > +semantics of the V4L2 control interface. If the client needs to apply the
> > +parameters exactly at specific frame, using the Request API
> > +(:ref:`media-request-api`) should be considered, if supported by the encoder.
> > +
> > +Drain
> > +=====
> > +
> > +To ensure that all the queued ``OUTPUT`` buffers have been processed and the
> > +related ``CAPTURE`` buffers are given to the client, the client must follow the
> > +drain sequence described below. After the drain sequence ends, the client has
> > +received all encoded frames for all ``OUTPUT`` buffers queued before the
> > +sequence was started.
> > +
> > +1. Begin the drain sequence by issuing :c:func:`VIDIOC_ENCODER_CMD`.
> > +
> > +   * **Required fields:**
> > +
> > +     ``cmd``
> > +         set to ``V4L2_ENC_CMD_STOP``
> > +
> > +     ``flags``
> > +         set to 0
> > +
> > +     ``pts``
> > +         set to 0
> > +
> > +   .. warning::
> > +
> > +      The sequence can be only initiated if both ``OUTPUT`` and ``CAPTURE``
> > +      queues are streaming. For compatibility reasons, the call to
> > +      :c:func:`VIDIOC_ENCODER_CMD` will not fail even if any of the queues is
> > +      not streaming, but at the same time it will not initiate the `Drain`
> > +      sequence and so the steps described below would not be applicable.
> > +
> > +2. Any ``OUTPUT`` buffers queued by the client before the
> > +   :c:func:`VIDIOC_ENCODER_CMD` was issued will be processed and encoded as
> > +   normal. The client must continue to handle both queues independently,
> > +   similarly to normal encode operation. This includes:
> > +
> > +   * queuing and dequeuing ``CAPTURE`` buffers, until a buffer marked with the
> > +     ``V4L2_BUF_FLAG_LAST`` flag is dequeued,
> > +
> > +     .. warning::
> > +
> > +        The last buffer may be empty (with :c:type:`v4l2_buffer`
> > +        ``bytesused`` = 0) and in that case it must be ignored by the client,
> > +        as it does not contain an encoded frame.
> > +
> > +     .. note::
> > +
> > +        Any attempt to dequeue more buffers beyond the buffer marked with
> > +        ``V4L2_BUF_FLAG_LAST`` will result in a -EPIPE error from
> > +        :c:func:`VIDIOC_DQBUF`.
> > +
> > +   * dequeuing processed ``OUTPUT`` buffers, until all the buffers queued
> > +     before the ``V4L2_ENC_CMD_STOP`` command are dequeued,
> > +
> > +   * dequeuing the ``V4L2_EVENT_EOS`` event, if the client subscribes to it.
> > +
> > +   .. note::
> > +
> > +      For backwards compatibility, the encoder will signal a ``V4L2_EVENT_EOS``
> > +      event when the last frame has been decoded and all frames are ready to be
>
> decoded -> encoded
>

Ack.

Best regards,
Tomasz

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