Re: [PATCHv2 1/2] media: docs-rst: Document memory-to-memory video encoder interface

[Nicolas Dufresne <nicolas@ndufresne.ca>]

Le mercredi 20 mai 2020 à 12:01 +0200, Hans Verkuil a écrit :
> From: Tomasz Figa <tfiga@chromium.org>
> 
> 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@chromium.org>
> Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
> ---
>  .../userspace-api/media/v4l/dev-encoder.rst   | 727 ++++++++++++++++++
>  .../userspace-api/media/v4l/dev-mem2mem.rst   |   1 +
>  .../userspace-api/media/v4l/pixfmt-v4l2.rst   |   5 +
>  .../userspace-api/media/v4l/v4l2.rst          |   2 +
>  .../media/v4l/vidioc-encoder-cmd.rst          |  51 +-
>  5 files changed, 766 insertions(+), 20 deletions(-)
>  create mode 100644 Documentation/userspace-api/media/v4l/dev-encoder.rst
> 
> diff --git a/Documentation/userspace-api/media/v4l/dev-encoder.rst b/Documentation/userspace-api/media/v4l/dev-encoder.rst
> new file mode 100644
> index 000000000000..b30ef9c5d970
> --- /dev/null
> +++ b/Documentation/userspace-api/media/v4l/dev-encoder.rst
> @@ -0,0 +1,727 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +.. _encoder:
> +
> +*************************************************
> +Memory-to-Memory Stateful Video Encoder Interface
> +*************************************************
> +
> +A stateful video encoder takes raw video frames in display order and encodes
> +them into a bytestream. It generates complete chunks of the bytestream, including
> +all metadata, headers, etc. The resulting bytestream does not require any
> +further post-processing by the client.
> +
> +Performing software stream processing, header generation etc. in the driver
> +in order to support this interface is strongly discouraged. In case such
> +operations are needed, use of the Stateless Video Encoder Interface (in
> +development) is strongly advised.
> +
> +Conventions and Notations Used in This Document
> +===============================================
> +
> +1. The general V4L2 API rules apply if not specified in this document
> +   otherwise.
> +
> +2. The meaning of words "must", "may", "should", etc. is as per `RFC
> +   2119 <https://tools.ietf.org/html/rfc2119>`_.
> +
> +3. All steps not marked "optional" are required.
> +
> +4. :c:func:`VIDIOC_G_EXT_CTRLS` and :c:func:`VIDIOC_S_EXT_CTRLS` may be used
> +   interchangeably with :c:func:`VIDIOC_G_CTRL` and :c:func:`VIDIOC_S_CTRL`,
> +   unless specified otherwise.
> +
> +5. Single-planar API (see :ref:`planar-apis`) and applicable structures may be
> +   used interchangeably with multi-planar API, unless specified otherwise,
> +   depending on encoder capabilities and following the general V4L2 guidelines.
> +
> +6. i = [a..b]: sequence of integers from a to b, inclusive, i.e. i =
> +   [0..2]: i = 0, 1, 2.
> +
> +7. Given an ``OUTPUT`` buffer A, then A' represents a buffer on the ``CAPTURE``
> +   queue containing data that resulted from processing buffer A.
> +
> +Glossary
> +========
> +
> +Refer to :ref:`decoder-glossary`.
> +
> +State Machine
> +=============
> +
> +.. kernel-render:: DOT
> +   :alt: DOT digraph of encoder state machine
> +   :caption: Encoder State Machine
> +
> +   digraph encoder_state_machine {
> +       node [shape = doublecircle, label="Encoding"] Encoding;
> +
> +       node [shape = circle, label="Initialization"] Initialization;
> +       node [shape = circle, label="Stopped"] Stopped;
> +       node [shape = circle, label="Drain"] Drain;
> +       node [shape = circle, label="Reset"] Reset;
> +
> +       node [shape = point]; qi
> +       qi -> Initialization [ label = "open()" ];
> +
> +       Initialization -> Encoding [ label = "Both queues streaming" ];
> +
> +       Encoding -> Drain [ label = "V4L2_ENC_CMD_STOP" ];
> +       Encoding -> Reset [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
> +       Encoding -> Stopped [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
> +       Encoding -> Encoding;
> +
> +       Drain -> Stopped [ label = "All CAPTURE\nbuffers dequeued\nor\nVIDIOC_STREAMOFF(OUTPUT)" ];
> +       Drain -> Reset [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
> +
> +       Reset -> Encoding [ label = "VIDIOC_STREAMON(CAPTURE)" ];
> +       Reset -> Initialization [ label = "VIDIOC_REQBUFS(OUTPUT, 0)" ];
> +
> +       Stopped -> Encoding [ label = "V4L2_ENC_CMD_START\nor\nVIDIOC_STREAMON(OUTPUT)" ];
> +       Stopped -> Reset [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
> +   }
> +
> +Querying Capabilities
> +=====================
> +
> +1. To enumerate the set of coded formats supported by the encoder, the
> +   client may call :c:func:`VIDIOC_ENUM_FMT` on ``CAPTURE``.
> +
> +   * The full set of supported formats will be returned, regardless of the
> +     format set on ``OUTPUT``.
> +
> +2. To enumerate the set of supported raw formats, the client may call
> +   :c:func:`VIDIOC_ENUM_FMT` on ``OUTPUT``.
> +
> +   * Only the formats supported for the format currently active on ``CAPTURE``
> +     will be returned.
> +
> +   * In order to enumerate raw formats supported by a given coded format,
> +     the client must first set that coded format on ``CAPTURE`` and then
> +     enumerate the formats on ``OUTPUT``.
> +
> +3. The client may use :c:func:`VIDIOC_ENUM_FRAMESIZES` to detect supported
> +   resolutions for a given format, passing the desired pixel format in
> +   :c:type:`v4l2_frmsizeenum` ``pixel_format``.
> +
> +   * Values returned by :c:func:`VIDIOC_ENUM_FRAMESIZES` for a coded pixel
> +     format will include all possible coded resolutions supported by the
> +     encoder for the given coded pixel format.
> +
> +   * Values returned by :c:func:`VIDIOC_ENUM_FRAMESIZES` for a raw pixel format
> +     will include all possible frame buffer resolutions supported by the
> +     encoder for the given raw pixel format and coded format currently set on
> +     ``CAPTURE``.
> +
> +4. The client may use :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` to detect supported
> +   frame intervals for a given format and resolution, passing the desired pixel
> +   format in :c:type:`v4l2_frmsizeenum` ``pixel_format`` and the resolution
> +   in :c:type:`v4l2_frmsizeenum` ``width`` and :c:type:`v4l2_frmsizeenum`
> +   ``height``.
> +
> +   * Values returned by :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` for a coded pixel
> +     format and coded resolution will include all possible frame intervals
> +     supported by the encoder for the given coded pixel format and resolution.
> +
> +   * Values returned by :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` for a raw pixel
> +     format and resolution will include all possible frame intervals supported
> +     by the encoder for the given raw pixel format and resolution and for the
> +     coded format, coded resolution and coded frame interval currently set on
> +     ``CAPTURE``.
> +
> +   * Support for :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` is optional. If it is
> +     not implemented, then there are no special restrictions other than the
> +     limits of the codec itself.
> +
> +5. Supported profiles and levels for the coded format currently set on
> +   ``CAPTURE``, if applicable, may be queried using their respective controls
> +   via :c:func:`VIDIOC_QUERYCTRL`.
> +
> +6. Any additional encoder capabilities may be discovered by querying
> +   their respective controls.
> +
> +Initialization
> +==============
> +
> +1. Set the coded format on the ``CAPTURE`` queue via :c:func:`VIDIOC_S_FMT`
> +
> +   * **Required fields:**
> +
> +     ``type``
> +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
> +
> +     ``pixelformat``
> +         the coded format to be produced.
> +
> +     ``sizeimage``
> +         desired size of ``CAPTURE`` buffers; the encoder may adjust it to
> +         match hardware requirements.
> +
> +     ``width``, ``height``
> +         ignored (read-only).
> +
> +     other fields
> +         follow standard semantics.
> +
> +   * **Return fields:**
> +
> +     ``sizeimage``
> +         adjusted size of ``CAPTURE`` buffers.
> +
> +     ``width``, ``height``
> +         the coded size selected by the encoder based on current state, e.g.
> +         ``OUTPUT`` format, selection rectangles, etc. (read-only).
> +
> +   .. important::
> +
> +      Changing the ``CAPTURE`` format may change the currently set ``OUTPUT``
> +      format. How the new ``OUTPUT`` format is determined is up to the encoder
> +      and the client must ensure it matches its needs afterwards.
> +
> +2. **Optional.** Enumerate supported ``OUTPUT`` formats (raw formats for
> +   source) for the selected coded format via :c:func:`VIDIOC_ENUM_FMT`.
> +
> +   * **Required fields:**
> +
> +     ``type``
> +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
> +
> +     other fields
> +         follow standard semantics.
> +
> +   * **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 to match encoder minimums, maximums and alignment
> +         requirements, as required by the currently selected formats, as
> +         reported by :c:func:`VIDIOC_ENUM_FRAMESIZES`.
> +
> +     other fields
> +         follow standard semantics.
> +
> +   * Setting the ``OUTPUT`` format will reset the selection rectangles to their
> +     default values, based on the new resolution, as described in the next
> +     step.
> +
> +4. Set the raw and coded frame interval on the ``OUTPUT`` queue via
> +   :c:func:`VIDIOC_S_PARM`
> +
> +   * ** Required fields:**
> +
> +     ``type``
> +	 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
> +
> +     ``parm.output``
> +	 set all fields except ``parm.output.timeperframe`` to 0.
> +
> +     ``parm.output.timeperframe``
> +	 the desired frame interval; the encoder may adjust it to
> +	 match hardware requirements.
> +
> +   * **Return fields:**
> +
> +     ``parm.output.timeperframe``
> +	 the adjusted frame interval.
> +
> +   .. important::
> +
> +      Changing the ``OUTPUT`` frame interval *also* sets the framerate that
> +      the encoder uses to encode the video. So setting the frame interval
> +      to 1/24 (or 24 frames per second) will produce a coded video stream
> +      that can be played back at that speed. The frame interval for the
> +      ``OUTPUT`` queue is just a hint, the application may provide raw
> +      frames at a different rate. It can be used by the driver to help
> +      schedule multiple encoders running in parallel.
> +
> +      In the next step the ``CAPTURE`` frame interval can optionally be
> +      changed to a different value. This is useful for off-line encoding
> +      were the coded frame interval can be different from the rate at
> +      which raw frames are supplied.
> +
> +   .. important::
> +
> +      ``timeperframe`` deals with *frames*, not fields. So for interlaced
> +      formats this is the time per two fields, since a frame consists of
> +      a top and a bottom field.
> +
> +   .. note::
> +
> +      It is due to historical reasons that changing the ``OUTPUT`` frame
> +      interval also changes the coded frame interval on the ``CAPTURE``
> +      queue. Ideally these would be independent settings, but that would
> +      break the existing API.
> +
> +5. **Optional** Set the coded frame interval on the ``CAPTURE`` queue via
> +   :c:func:`VIDIOC_S_PARM`. This is only necessary if the coded frame
> +   interval is different from the raw frame interval, which is typically
> +   the case for off-line encoding.
> +
> +   * ** Required fields:**
> +
> +     ``type``
> +	 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
> +
> +     ``parm.capture``
> +	 set all fields except ``parm.capture.timeperframe`` to 0.
> +
> +     ``parm.capture.timeperframe``
> +	 the desired coded frame interval; the encoder may adjust it to
> +	 match hardware requirements.
> +
> +   * **Return fields:**
> +
> +     ``parm.capture.timeperframe``
> +	 the adjusted frame interval.
> +
> +   .. important::
> +
> +      Changing the ``CAPTURE`` frame interval sets the framerate for the
> +      coded video. It does *not* set the rate at which buffers arrive on the
> +      ``CAPTURE`` queue, that depends on how fast the encoder is and how
> +      fast raw frames are queued on the ``OUTPUT`` queue.
> +
> +   .. important::
> +
> +      ``timeperframe`` deals with *frames*, not fields. So for interlaced
> +      formats this is the time per two fields, since a frame consists of
> +      a top and a bottom field.
> +
> +   .. note::
> +
> +      Not all drivers support this functionality, in that case just set
> +      the desired coded frame interval for the ``OUTPUT`` queue.

There is a slight contorsion in the resulting user-space API. When I
read this, the logical thing to do for live streams would be to just
set the OUTPUT and the driver will take care of CAPTURE for me.

While if I want to do offline, I don't know if this is supported or
not. So the flow would be a bit special:

  S_PARM(OUTPUT) with coded video frame rate
  S_PARM(CAPTURE) width coded video
  if ^ worked:
     S_PARM(OUTPUT) with fastest rate possible

Ideally I would have preferred if there was a more straight forward way
to configure offline encoding for fastest performance with specific
coded framerate. I don't think it's a blocker though, performance is
not critical at all here. Maybe I'm missing something that allow to
check if this is supported or not without trying it ?

> +
> +      However, drivers that can schedule multiple encoders based on the
> +      ``OUTPUT`` frame interval must support this optional feature.
> +
> +6. **Optional.** Set the visible resolution for the stream metadata via
> +   :c:func:`VIDIOC_S_SELECTION` on the ``OUTPUT`` queue if it is desired
> +   to be different than the full OUTPUT resolution.
> +
> +   * **Required fields:**
> +
> +     ``type``
> +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
> +
> +     ``target``
> +         set to ``V4L2_SEL_TGT_CROP``.
> +
> +     ``r.left``, ``r.top``, ``r.width``, ``r.height``
> +         visible rectangle; this must fit within the `V4L2_SEL_TGT_CROP_BOUNDS`
> +         rectangle and may be subject to adjustment to match codec and
> +         hardware constraints.
> +
> +   * **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.
> +
> +   .. warning::
> +
> +      The encoder may adjust the crop/compose rectangles to the nearest
> +      supported ones to meet codec and hardware requirements. The client needs
> +      to check the adjusted rectangle returned by :c:func:`VIDIOC_S_SELECTION`.
> +
> +7. Allocate buffers for both ``OUTPUT`` and ``CAPTURE`` via
> +   :c:func:`VIDIOC_REQBUFS`. This may be performed in any order.
> +
> +   * **Required fields:**
> +
> +     ``count``
> +         requested number of buffers to allocate; greater than zero.
> +
> +     ``type``
> +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT`` or
> +         ``CAPTURE``.
> +
> +     other fields
> +         follow standard semantics.
> +
> +   * **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.
> +
> +8. Begin streaming on both ``OUTPUT`` and ``CAPTURE`` queues via
> +   :c:func:`VIDIOC_STREAMON`. This may be performed in any order. The actual
> +   encoding process starts when both queues start streaming.
> +
> +.. note::
> +
> +   If the client stops the ``CAPTURE`` queue during the encode process and then
> +   restarts it again, the encoder will begin generating a stream independent
> +   from the stream generated before the stop. The exact constraints depend
> +   on the coded format, but may include the following implications:
> +
> +   * encoded frames produced after the restart must not reference any
> +     frames produced before the stop, e.g. no long term references for
> +     H.264/HEVC,
> +
> +   * any headers that must be included in a standalone stream must be
> +     produced again, e.g. SPS and PPS for H.264/HEVC.
> +
> +Encoding
> +========
> +
> +This state is reached after the `Initialization` sequence finishes
> +successfully.  In this state, the client queues and dequeues buffers to both
> +queues via :c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`, following the
> +standard semantics.
> +
> +The content of encoded ``CAPTURE`` buffers depends on the active coded pixel
> +format and may be affected by codec-specific extended controls, as stated
> +in the documentation of each format.
> +
> +Both queues operate independently, following standard behavior of V4L2 buffer
> +queues and memory-to-memory devices. In addition, the order of encoded frames
> +dequeued from the ``CAPTURE`` queue may differ from the order of queuing raw
> +frames to the ``OUTPUT`` queue, due to properties of the selected coded format,
> +e.g. frame reordering.
> +
> +The client must not assume any direct relationship between ``CAPTURE`` and
> +``OUTPUT`` buffers and any specific timing of buffers becoming
> +available to dequeue. Specifically:
> +
> +* a buffer queued to ``OUTPUT`` may result in more than one buffer produced on
> +  ``CAPTURE`` (for example, if returning an encoded frame allowed the encoder
> +  to return a frame that preceded it in display, but succeeded it in the decode
> +  order; however, there may be other reasons for this as well),
> +
> +* a buffer queued to ``OUTPUT`` may result in a buffer being produced on
> +  ``CAPTURE`` later into encode process, and/or after processing further
> +  ``OUTPUT`` buffers, or be returned out of order, e.g. if display
> +  reordering is used,
> +
> +* buffers may become available on the ``CAPTURE`` queue without additional
> +  buffers queued to ``OUTPUT`` (e.g. during drain or ``EOS``), because of the
> +  ``OUTPUT`` buffers queued in the past whose 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 ``CAPTURE`` buffer, e.g. if the
> +  encoder needs to use the frame as a reference for encoding further frames.
> +
> +.. note::
> +
> +   To allow matching encoded ``CAPTURE`` buffers with ``OUTPUT`` buffers they
> +   originated from, the client can set the ``timestamp`` field of the
> +   :c:type:`v4l2_buffer` struct when queuing an ``OUTPUT`` buffer. The
> +   ``CAPTURE`` buffer(s), which resulted from encoding that ``OUTPUT`` buffer
> +   will have their ``timestamp`` field set to the same value when dequeued.
> +
> +   In addition to the straightforward case of one ``OUTPUT`` buffer producing
> +   one ``CAPTURE`` buffer, the following cases are defined:
> +
> +   * one ``OUTPUT`` buffer generates multiple ``CAPTURE`` buffers: the same
> +     ``OUTPUT`` timestamp will be copied to multiple ``CAPTURE`` buffers,
> +
> +   * the encoding order differs from the presentation order (i.e. the
> +     ``CAPTURE`` buffers are out-of-order compared to the ``OUTPUT`` buffers):
> +     ``CAPTURE`` timestamps will not retain the order of ``OUTPUT`` timestamps.
> +
> +.. note::
> +
> +   To let the client distinguish between frame types (keyframes, intermediate
> +   frames; the exact list of types depends on the coded format), the
> +   ``CAPTURE`` buffers will have corresponding flag bits set in their
> +   :c:type:`v4l2_buffer` struct when dequeued. See the documentation of
> +   :c:type:`v4l2_buffer` and each coded pixel format for exact list of flags
> +   and their meanings.
> +
> +Should an encoding error occur, it will be reported to the client with the level
> +of details depending on the encoder capabilities. Specifically:
> +
> +* the ``CAPTURE`` buffer (if any) that contains the results of the failed encode
> +  operation will be returned with the ``V4L2_BUF_FLAG_ERROR`` flag set,
> +
> +* if the encoder is able to precisely report the ``OUTPUT`` buffer(s) that triggered
> +  the error, such buffer(s) will be returned with the ``V4L2_BUF_FLAG_ERROR`` flag
> +  set.
> +
> +.. note::
> +
> +   If a ``CAPTURE`` buffer is too small then it is just returned with the
> +   ``V4L2_BUF_FLAG_ERROR`` flag set. More work is needed to detect that this
> +   error occurred because the buffer was too small, and to provide support to
> +   free existing buffers that were too small.
> +
> +In case of a fatal failure that does not allow the encoding to continue, any
> +further operations on corresponding encoder file handle will return the -EIO
> +error code. The client may close the file handle and open a new one, or
> +alternatively reinitialize the instance by stopping streaming on both queues,
> +releasing all buffers and performing the Initialization sequence again.
> +
> +Encoding Parameter Changes
> +==========================
> +
> +The client is allowed to use :c:func:`VIDIOC_S_CTRL` to change encoder
> +parameters at any time. The availability of parameters is encoder-specific
> +and the client must query the encoder to find the set of available controls.
> +
> +The ability to change each parameter during encoding is encoder-specific, as
> +per the standard semantics of the V4L2 control interface. The client may
> +attempt to set a control during encoding and if the operation fails with the
> +-EBUSY error code, the ``CAPTURE`` queue needs to be stopped for the
> +configuration change to be allowed. To do this, it may follow the `Drain`
> +sequence to avoid losing the already queued/encoded frames.
> +
> +The timing of parameter updates is encoder-specific, as per the standard
> +semantics of the V4L2 control interface. If the client needs to apply the
> +parameters exactly at specific frame, using the Request API
> +(:ref:`media-request-api`) should be considered, if supported by the encoder.
> +
> +Drain
> +=====
> +
> +To ensure that all the queued ``OUTPUT`` buffers have been processed and the
> +related ``CAPTURE`` buffers are given to the client, the client must follow the
> +drain sequence described below. After the drain sequence ends, the client has
> +received all encoded frames for all ``OUTPUT`` buffers queued before the
> +sequence was started.
> +
> +1. Begin the drain sequence by issuing :c:func:`VIDIOC_ENCODER_CMD`.
> +
> +   * **Required fields:**
> +
> +     ``cmd``
> +         set to ``V4L2_ENC_CMD_STOP``.
> +
> +     ``flags``
> +         set to 0.
> +
> +     ``pts``
> +         set to 0.
> +
> +   .. warning::
> +
> +      The sequence can be only initiated if both ``OUTPUT`` and ``CAPTURE``
> +      queues are streaming. For compatibility reasons, the call to
> +      :c:func:`VIDIOC_ENCODER_CMD` will not fail even if any of the queues is
> +      not streaming, but at the same time it will not initiate the `Drain`
> +      sequence and so the steps described below would not be applicable.
> +
> +2. Any ``OUTPUT`` buffers queued by the client before the
> +   :c:func:`VIDIOC_ENCODER_CMD` was issued will be processed and encoded as
> +   normal. The client must continue to handle both queues independently,
> +   similarly to normal encode operation. This includes:
> +
> +   * queuing and dequeuing ``CAPTURE`` buffers, until a buffer marked with the
> +     ``V4L2_BUF_FLAG_LAST`` flag is dequeued,
> +
> +     .. warning::
> +
> +        The last buffer may be empty (with :c:type:`v4l2_buffer`
> +        ``bytesused`` = 0) and in that case it must be ignored by the client,
> +        as it does not contain an encoded frame.
> +
> +     .. note::
> +
> +        Any attempt to dequeue more ``CAPTURE`` buffers beyond the buffer
> +        marked with ``V4L2_BUF_FLAG_LAST`` will result in a -EPIPE error from
> +        :c:func:`VIDIOC_DQBUF`.
> +
> +   * dequeuing processed ``OUTPUT`` buffers, until all the buffers queued
> +     before the ``V4L2_ENC_CMD_STOP`` command are dequeued,
> +
> +   * dequeuing the ``V4L2_EVENT_EOS`` event, if the client subscribes to it.
> +
> +   .. note::
> +
> +      For backwards compatibility, the encoder will signal a ``V4L2_EVENT_EOS``
> +      event when the last frame has been encoded and all frames are ready to be
> +      dequeued. It is deprecated behavior and the client must not rely on it.
> +      The ``V4L2_BUF_FLAG_LAST`` buffer flag should be used instead.
> +
> +3. Once all ``OUTPUT`` buffers queued before the ``V4L2_ENC_CMD_STOP`` call are
> +   dequeued and the last ``CAPTURE`` buffer is dequeued, the encoder is stopped
> +   and it will accept, but not process any newly queued ``OUTPUT`` buffers
> +   until the client issues any of the following operations:
> +
> +   * ``V4L2_ENC_CMD_START`` - the encoder will not be reset and will resume
> +     operation normally, with all the state from before the drain,
> +
> +   * a pair of :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the
> +     ``CAPTURE`` queue - the encoder will be reset (see the `Reset` sequence)
> +     and then resume encoding,
> +
> +   * a pair of :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the
> +     ``OUTPUT`` queue - the encoder will resume operation normally, however any
> +     source frames queued to the ``OUTPUT`` queue between ``V4L2_ENC_CMD_STOP``
> +     and :c:func:`VIDIOC_STREAMOFF` will be discarded.
> +
> +.. note::
> +
> +   Once the drain sequence is initiated, the client needs to drive it to
> +   completion, as described by the steps above, unless it aborts the process by
> +   issuing :c:func:`VIDIOC_STREAMOFF` on any of the ``OUTPUT`` or ``CAPTURE``
> +   queues.  The client is not allowed to issue ``V4L2_ENC_CMD_START`` or
> +   ``V4L2_ENC_CMD_STOP`` again while the drain sequence is in progress and they
> +   will fail with -EBUSY error code if attempted.
> +
> +   For reference, handling of various corner cases is described below:
> +
> +   * In case of no buffer in the ``OUTPUT`` queue at the time the
> +     ``V4L2_ENC_CMD_STOP`` command was issued, the drain sequence completes
> +     immediately and the encoder returns an empty ``CAPTURE`` buffer with the
> +     ``V4L2_BUF_FLAG_LAST`` flag set.
> +
> +   * In case of no buffer in the ``CAPTURE`` queue at the time the drain
> +     sequence completes, the next time the client queues a ``CAPTURE`` buffer
> +     it is returned at once as an empty buffer with the ``V4L2_BUF_FLAG_LAST``
> +     flag set.
> +
> +   * If :c:func:`VIDIOC_STREAMOFF` is called on the ``CAPTURE`` queue in the
> +     middle of the drain sequence, the drain sequence is canceled and all
> +     ``CAPTURE`` buffers are implicitly returned to the client.
> +
> +   * If :c:func:`VIDIOC_STREAMOFF` is called on the ``OUTPUT`` queue in the
> +     middle of the drain sequence, the drain sequence completes immediately and
> +     next ``CAPTURE`` buffer will be returned empty with the
> +     ``V4L2_BUF_FLAG_LAST`` flag set.
> +
> +   Although mandatory, the availability of encoder commands may be queried
> +   using :c:func:`VIDIOC_TRY_ENCODER_CMD`.
> +
> +Reset
> +=====
> +
> +The client may want to request the encoder to reinitialize the encoding, so
> +that the following stream data becomes independent from the stream data
> +generated before. Depending on the coded format, that may imply that:
> +
> +* encoded frames produced after the restart must not reference any frames
> +  produced before the stop, e.g. no long term references for H.264/HEVC,
> +
> +* any headers that must be included in a standalone stream must be produced
> +  again, e.g. SPS and PPS for H.264/HEVC.
> +
> +This can be achieved by performing the reset sequence.
> +
> +1. Perform the `Drain` sequence to ensure all the in-flight encoding finishes
> +   and respective buffers are dequeued.
> +
> +2. Stop streaming on the ``CAPTURE`` queue via :c:func:`VIDIOC_STREAMOFF`. This
> +   will return all currently queued ``CAPTURE`` buffers to the client, without
> +   valid frame data.
> +
> +3. Start streaming on the ``CAPTURE`` queue via :c:func:`VIDIOC_STREAMON` and
> +   continue with regular encoding sequence. The encoded frames produced into
> +   ``CAPTURE`` buffers from now on will contain a standalone stream that can be
> +   decoded without the need for frames encoded before the reset sequence,
> +   starting at the first ``OUTPUT`` buffer queued after issuing the
> +   `V4L2_ENC_CMD_STOP` of the `Drain` sequence.
> +
> +This sequence may be also used to change encoding parameters for encoders
> +without the ability to change the parameters on the fly.
> +
> +Commit Points
> +=============
> +
> +Setting formats and allocating buffers triggers changes in the behavior of the
> +encoder.
> +
> +1. Setting the format on the ``CAPTURE`` queue may change the set of formats
> +   supported/advertised on the ``OUTPUT`` queue. In particular, it also means
> +   that the ``OUTPUT`` format may be reset and the client must not rely on the
> +   previously set format being preserved.
> +
> +2. Enumerating formats on the ``OUTPUT`` queue always returns only formats
> +   supported for the current ``CAPTURE`` format.
> +
> +3. Setting the format on the ``OUTPUT`` queue does not change the list of
> +   formats available on the ``CAPTURE`` queue. An attempt to set the ``OUTPUT``
> +   format that is not supported for the currently selected ``CAPTURE`` format
> +   will result in the encoder adjusting the requested ``OUTPUT`` format to a
> +   supported one.
> +
> +4. Enumerating formats on the ``CAPTURE`` queue always returns the full set of
> +   supported coded formats, irrespective of the current ``OUTPUT`` format.
> +
> +5. While buffers are allocated on any of the ``OUTPUT`` or ``CAPTURE`` queues,
> +   the client must not change the format on the ``CAPTURE`` queue. Drivers will
> +   return the -EBUSY error code for any such format change attempt.
> +
> +To summarize, setting formats and allocation must always start with the
> +``CAPTURE`` queue and the ``CAPTURE`` queue is the master that governs the
> +set of supported formats for the ``OUTPUT`` queue.
> diff --git a/Documentation/userspace-api/media/v4l/dev-mem2mem.rst b/Documentation/userspace-api/media/v4l/dev-mem2mem.rst
> index 9279d87c08a1..40aff9c95267 100644
> --- a/Documentation/userspace-api/media/v4l/dev-mem2mem.rst
> +++ b/Documentation/userspace-api/media/v4l/dev-mem2mem.rst
> @@ -46,4 +46,5 @@ devices are given in the following sections.
>      :maxdepth: 1
>  
>      dev-decoder
> +    dev-encoder
>      dev-stateless-decoder
> diff --git a/Documentation/userspace-api/media/v4l/pixfmt-v4l2.rst b/Documentation/userspace-api/media/v4l/pixfmt-v4l2.rst
> index 759420a872d6..e0ee2823ab1f 100644
> --- a/Documentation/userspace-api/media/v4l/pixfmt-v4l2.rst
> +++ b/Documentation/userspace-api/media/v4l/pixfmt-v4l2.rst
> @@ -44,6 +44,11 @@ Single-planar format structure
>  	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.
> +
> +	For compressed formats on the CAPTURE side of a stateful mem2mem
> +	encoder, the fields must be zero, since the coded size is expected to
> +	be calculated internally by the encoder itself, based on the OUTPUT
> +	side. For more details see :ref:`encoder` and format descriptions.
>      * - __u32
>        - ``pixelformat``
>        - The pixel format or type of compression, set by the application.
> diff --git a/Documentation/userspace-api/media/v4l/v4l2.rst b/Documentation/userspace-api/media/v4l/v4l2.rst
> index ab7c97c39b97..35796c4fbe52 100644
> --- a/Documentation/userspace-api/media/v4l/v4l2.rst
> +++ b/Documentation/userspace-api/media/v4l/v4l2.rst
> @@ -63,6 +63,7 @@ Authors, in alphabetical order:
>  - Figa, Tomasz <tfiga@chromium.org>
>  
>    - Documented the memory-to-memory decoder interface.
> +  - Documented the memory-to-memory encoder interface.
>  
>  - H Schimek, Michael <mschimek@gmx.at>
>  
> @@ -75,6 +76,7 @@ Authors, in alphabetical order:
>  - Osciak, Pawel <posciak@chromium.org>
>  
>    - Documented the memory-to-memory decoder interface.
> +  - Documented the memory-to-memory encoder interface.
>  
>  - Osciak, Pawel <pawel@osciak.com>
>  
> diff --git a/Documentation/userspace-api/media/v4l/vidioc-encoder-cmd.rst b/Documentation/userspace-api/media/v4l/vidioc-encoder-cmd.rst
> index 16269b3b1715..d0eacce5485e 100644
> --- a/Documentation/userspace-api/media/v4l/vidioc-encoder-cmd.rst
> +++ b/Documentation/userspace-api/media/v4l/vidioc-encoder-cmd.rst
> @@ -51,25 +51,26 @@ To send a command applications must initialize all fields of a struct
>  ``VIDIOC_ENCODER_CMD`` or ``VIDIOC_TRY_ENCODER_CMD`` with a pointer to
>  this structure.
>  
> -The ``cmd`` field must contain the command code. The ``flags`` field is
> -currently only used by the STOP command and contains one bit: If the
> -``V4L2_ENC_CMD_STOP_AT_GOP_END`` flag is set, encoding will continue
> -until the end of the current *Group Of Pictures*, otherwise it will stop
> -immediately.
> +The ``cmd`` field must contain the command code. Some commands use the
> +``flags`` field for additional information.
>  
> -A :ref:`read() <func-read>` or :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`
> -call sends an implicit START command to the encoder if it has not been
> -started yet. After a STOP command, :ref:`read() <func-read>` calls will read
> +After a STOP command, :ref:`read() <func-read>` calls will read
>  the remaining data buffered by the driver. When the buffer is empty,
>  :ref:`read() <func-read>` will return zero and the next :ref:`read() <func-read>`
>  call will restart the encoder.
>  
> +A :ref:`read() <func-read>` or :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`
> +call sends an implicit START command to the encoder if it has not been
> +started yet. Applies to both queues of mem2mem encoders.
> +
>  A :ref:`close() <func-close>` or :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
>  call of a streaming file descriptor sends an implicit immediate STOP to
> -the encoder, and all buffered data is discarded.
> +the encoder, and all buffered data is discarded. Applies to both queues of
> +mem2mem encoders.
>  
>  These ioctls are optional, not all drivers may support them. They were
> -introduced in Linux 2.6.21.
> +introduced in Linux 2.6.21. They are, however, mandatory for stateful mem2mem
> +encoders (as further documented in :ref:`encoder`).
>  
>  
>  .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
> @@ -109,21 +110,24 @@ introduced in Linux 2.6.21.
>        - 0
>        - Start the encoder. When the encoder is already running or paused,
>  	this command does nothing. No flags are defined for this command.
> +
> +	For a device implementing the :ref:`encoder`, once the drain sequence
> +	is initiated with the ``V4L2_ENC_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. See :ref:`encoder` for more details.
>      * - ``V4L2_ENC_CMD_STOP``
>        - 1
>        - Stop the encoder. When the ``V4L2_ENC_CMD_STOP_AT_GOP_END`` flag
>  	is set, encoding will continue until the end of the current *Group
>  	Of Pictures*, otherwise encoding will stop immediately. When the
> -	encoder is already stopped, this command does nothing. mem2mem
> -	encoders will send a ``V4L2_EVENT_EOS`` event when the last frame
> -	has been encoded 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.
> +	encoder is already stopped, this command does nothing.
> +
> +	For a device implementing the :ref:`encoder`, the command will initiate
> +	the drain sequence as documented in :ref:`encoder`. 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_ENC_CMD_PAUSE``
>        - 2
>        - Pause the encoder. When the encoder has not been started yet, the
> @@ -152,6 +156,8 @@ introduced in Linux 2.6.21.
>        - Stop encoding at the end of the current *Group Of Pictures*,
>  	rather than immediately.
>  
> +        Does not apply to :ref:`encoder`.
> +
>  
>  Return Value
>  ============
> @@ -160,6 +166,11 @@ 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:`encoder` is still in
> +    progress. It is not allowed to issue another encoder command until it
> +    completes.
> +
>  EINVAL
>      The ``cmd`` field is invalid.
>