Re: [PATCH v3 2/2] media: docs-rst: Document memory-to-memory video encoder interface

From: Hans Verkuil
Date: Thu Mar 21 2019 - 06:11:05 EST

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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 DÃsseldorf. The de facto Codec API that
> originated at those events was later implemented by the drivers we already
> have merged in mainline, such as s5p-mfc or coda.
>
> The only thing missing was the real specification included as a part of
> Linux Media documentation. Fix it now and document the encoder part of
> the Codec API.
>
> Signed-off-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> ---
> 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

> +
> +6. i = [a..b]: sequence of integers from a to b, inclusive, i.e. i =
> + [0..2]: i = 0, 1, 2.
> +
> +7. Given an ``OUTPUT`` buffer A, then Aâ represents a buffer on the ``CAPTURE``
> + queue containing data that resulted from processing buffer A.
> +
> +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.

> +
> + .. 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.

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.

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.

> +
> +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

> + 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.
> +
> + 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,
> +
> +* any headers that must be included in a standalone stream must be produced
> + again, e.g. SPS and PPS for H.264.
> +
> +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, irrespectively of the current ``OUTPUT`` format.
> +
> +5. While buffers are allocated on the ``CAPTURE`` queue, the client must not
> + change the format on the 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/media/uapi/v4l/dev-mem2mem.rst b/Documentation/media/uapi/v4l/dev-mem2mem.rst
> index a0e06a88c872..82630ea89b16 100644
> --- a/Documentation/media/uapi/v4l/dev-mem2mem.rst
> +++ b/Documentation/media/uapi/v4l/dev-mem2mem.rst
> @@ -17,6 +17,7 @@ Video Memory-To-Memory Interface
> :maxdepth: 1
>
> dev-decoder
> + dev-encoder
>
> A V4L2 memory-to-memory device can compress, decompress, transform, or
> otherwise convert video data from one format into another format, in memory.
> diff --git a/Documentation/media/uapi/v4l/pixfmt-v4l2.rst b/Documentation/media/uapi/v4l/pixfmt-v4l2.rst
> index caf14e440447..593d25b935bf 100644
> --- a/Documentation/media/uapi/v4l/pixfmt-v4l2.rst
> +++ b/Documentation/media/uapi/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/media/uapi/v4l/v4l2.rst b/Documentation/media/uapi/v4l/v4l2.rst
> index 97015b9b40b8..e236ea23481b 100644
> --- a/Documentation/media/uapi/v4l/v4l2.rst
> +++ b/Documentation/media/uapi/v4l/v4l2.rst
> @@ -63,6 +63,7 @@ Authors, in alphabetical order:
> - Figa, Tomasz <tfiga@xxxxxxxxxxxx>
>
> - Documented the memory-to-memory decoder interface.
> + - Documented the memory-to-memory encoder interface.
>
> - H Schimek, Michael <mschimek@xxxxxx>
>
> @@ -75,6 +76,7 @@ Authors, in alphabetical order:
> - Osciak, Pawel <posciak@xxxxxxxxxxxx>
>
> - Documented the memory-to-memory decoder interface.
> + - Documented the memory-to-memory encoder interface.
>
> - Osciak, Pawel <pawel@xxxxxxxxxx>
>
> diff --git a/Documentation/media/uapi/v4l/vidioc-encoder-cmd.rst b/Documentation/media/uapi/v4l/vidioc-encoder-cmd.rst
> index c313ca8b8cb5..2b59a4e7f5ce 100644
> --- a/Documentation/media/uapi/v4l/vidioc-encoder-cmd.rst
> +++ b/Documentation/media/uapi/v4l/vidioc-encoder-cmd.rst
> @@ -57,19 +57,23 @@ currently only used by the STOP command and contains one bit: If the
> until the end of the current *Group Of Pictures*, otherwise it will stop
> immediately.
>
> -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}|
> @@ -114,16 +118,20 @@ introduced in Linux 2.6.21.
> - 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.
> +
> + A stateful mem2mem encoder will proceed with encoding the source
> + buffers pending before the command is issued and then stop producing
> + new frames. It 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 buffer with the
> + ``V4L2_BUF_FLAG_LAST`` flag set was dequeued, the :ref:`VIDIOC_DQBUF
> + <VIDIOC_QBUF>` ioctl will not block anymore, but return an ``EPIPE``
> + error code. No flags or other arguments are accepted in case of mem2mem
> + encoders. See :ref:`encoder` for more details.
> * - ``V4L2_ENC_CMD_PAUSE``
> - 2
> - Pause the encoder. When the encoder has not been started yet, the
>

Regards,

Hans

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