Re: [PATCH v5] media: docs-rst: Document m2m stateless video decoder interface

[Hans Verkuil]

Hi Alexandre,

On 6/20/19 9:35 AM, Alexandre Courbot wrote:
> Documents the protocol that user-space should follow when
> communicating with stateless video decoders.
> 
> The stateless video decoding API makes use of the new request and tags
> APIs. While it has been implemented with the Cedrus driver so far, it
> should probably still be considered staging for a short while.

Thank you for this v5! I've updated my build of the spec to include this:

https://hverkuil.home.xs4all.nl/codec-api/uapi/v4l/dev-mem2mem.html

Some comments below:

> 
> Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxxxx>
> ---
> Changes since v4:
> 
> * Specify that G_FMT must be performed again after S_FMT on the CAPTURE queue
>   to get up-to-date buffer information,
> 
> * Add description of multi-slice decoding using the
>   V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF flag.
> 
> The very last paragraph of this document describes a situation that has not yet
> been discussed: draining the queue when a held CAPTURE buffer is still there. I
> have written a basic solution for this situation, but feel free to suggest
> something better.
> 
>  Documentation/media/uapi/v4l/dev-mem2mem.rst  |   5 +
>  .../media/uapi/v4l/dev-stateless-decoder.rst  | 415 ++++++++++++++++++
>  2 files changed, 420 insertions(+)
>  create mode 100644 Documentation/media/uapi/v4l/dev-stateless-decoder.rst
> 
> diff --git a/Documentation/media/uapi/v4l/dev-mem2mem.rst b/Documentation/media/uapi/v4l/dev-mem2mem.rst
> index 67a980818dc8..db6f4efc458d 100644
> --- a/Documentation/media/uapi/v4l/dev-mem2mem.rst
> +++ b/Documentation/media/uapi/v4l/dev-mem2mem.rst
> @@ -13,6 +13,11 @@
>  Video Memory-To-Memory Interface
>  ********************************
>  
> +.. toctree::
> +    :maxdepth: 1
> +
> +    dev-stateless-decoder
> +
>  A V4L2 memory-to-memory device can compress, decompress, transform, or
>  otherwise convert video data from one format into another format, in memory.
>  Such memory-to-memory devices set the ``V4L2_CAP_VIDEO_M2M`` or
> diff --git a/Documentation/media/uapi/v4l/dev-stateless-decoder.rst b/Documentation/media/uapi/v4l/dev-stateless-decoder.rst
> new file mode 100644
> index 000000000000..186ec39c5ed7
> --- /dev/null
> +++ b/Documentation/media/uapi/v4l/dev-stateless-decoder.rst
> @@ -0,0 +1,415 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +.. _stateless_decoder:
> +
> +**************************************************
> +Memory-to-memory Stateless Video Decoder Interface
> +**************************************************
> +
> +A stateless decoder is a decoder that works without retaining any kind of state
> +between processed frames. This means that each frame is decoded independently
> +of any previous and future frames, and that the client is responsible for
> +maintaining the decoding state and providing it to the decoder with each
> +decoding request. This is in contrast to the stateful video decoder interface,
> +where the hardware and driver maintain the decoding state and all the client
> +has to do is to provide the raw encoded stream and dequeue decoded frames in
> +display order.
> +
> +This section describes how user-space ("the client") is expected to communicate
> +with stateless decoders in order to successfully decode an encoded stream.
> +Compared to stateful codecs, the decoder/client sequence is simpler, but the
> +cost of this simplicity is extra complexity in the client which is responsible
> +for maintaining a consistent decoding state.
> +
> +Stateless decoders make use of the request API. A stateless decoder must expose

Use :ref:`media-request-api` instead of "request API": that way you get a link to
the Request API section in the spec.

> +the ``V4L2_BUF_CAP_SUPPORTS_REQUESTS`` capability on its ``OUTPUT`` queue when
> +:c:func:`VIDIOC_REQBUFS` or :c:func:`VIDIOC_CREATE_BUFS` are invoked.
> +
> +Depending on the encoded formats supported by the decoder, a single decoded
> +frame may be the result of several decode requests (for instance, H.264 streams
> +with multiple slices per frame). Decoders that support such formats must also
> +expose the ``V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF`` capability on their
> +``OUTPUT`` queue.
> +
> +Querying capabilities
> +=====================
> +
> +1. To enumerate the set of coded formats supported by the decoder, the client
> +   calls :c:func:`VIDIOC_ENUM_FMT` on the ``OUTPUT`` queue.
> +
> +   * The driver must always return the full set of supported ``OUTPUT`` formats,
> +     irrespective of the format currently set on the ``CAPTURE`` queue.
> +
> +   * Simultaneously, the driver must restrain the set of values returned by
> +     codec-specific capability controls (such as H.264 profiles) to the set
> +     actually supported by the hardware.
> +
> +2. To enumerate the set of supported raw formats, the client calls
> +   :c:func:`VIDIOC_ENUM_FMT` on the ``CAPTURE`` queue.
> +
> +   * The driver must return only the formats supported for the format currently
> +     active on the ``OUTPUT`` queue.
> +
> +   * Depending on the currently set ``OUTPUT`` format, the set of supported raw
> +     formats may depend on the value of some codec-dependent controls. The

Drop the spurious 'The' at the end.

> +     The client is responsible for making sure that these controls are set
> +     before querying the ``CAPTURE`` queue. Failure to do so will result in the
> +     default values for these controls being used, and a returned set of formats
> +     that may not be usable for the media the client is trying to decode.
> +
> +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`'s ``pixel_format``.
> +
> +4. Supported profiles and levels for the current ``OUTPUT`` format, if
> +   applicable, may be queried using their respective controls via
> +   :c:func:`VIDIOC_QUERYCTRL`.
> +
> +Initialization
> +==============
> +
> +1. Set the coded format on the ``OUTPUT`` queue via :c:func:`VIDIOC_S_FMT`.
> +
> +   * **Required fields:**
> +
> +     ``type``
> +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
> +
> +     ``pixelformat``
> +         a coded pixel format.
> +
> +     ``width``, ``height``
> +         coded width and height parsed from the stream.
> +
> +     other fields
> +         follow standard semantics.
> +
> +   .. note::
> +
> +      Changing the ``OUTPUT`` format may change the currently set ``CAPTURE``
> +      format. The driver will derive a new ``CAPTURE`` format from the
> +      ``OUTPUT`` format being set, including resolution, colorimetry
> +      parameters, etc. If the client needs a specific ``CAPTURE`` format,
> +      it must adjust it afterwards.
> +
> +2. Call :c:func:`VIDIOC_S_EXT_CTRLS` to set all the controls (parsed headers,
> +   etc.) required by the ``OUTPUT`` format to enumerate the ``CAPTURE`` formats.
> +
> +3. Call :c:func:`VIDIOC_G_FMT` for ``CAPTURE`` queue to get the format for the
> +   destination buffers parsed/decoded from the bitstream.
> +
> +   * **Required fields:**
> +
> +     ``type``
> +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
> +
> +   * **Returned fields:**
> +
> +     ``width``, ``height``
> +         frame buffer resolution for the decoded frames.
> +
> +     ``pixelformat``
> +         pixel format for decoded frames.
> +
> +     ``num_planes`` (for _MPLANE ``type`` only)
> +         number of planes for pixelformat.
> +
> +     ``sizeimage``, ``bytesperline``
> +         as per standard semantics; matching frame buffer format.
> +
> +   .. note::
> +
> +      The value of ``pixelformat`` may be any pixel format supported for the
> +      ``OUTPUT`` format, based on the hardware capabilities. It is suggested
> +      that driver chooses the preferred/optimal format for the current

driver -> the driver

> +      configuration. For example, a YUV format may be preferred over an RGB
> +      format, if an additional conversion step would be required for RGB.
> +
> +4. *[optional]* Enumerate ``CAPTURE`` formats via :c:func:`VIDIOC_ENUM_FMT` on
> +   the ``CAPTURE`` queue. The client may use this ioctl to discover which
> +   alternative raw formats are supported for the current ``OUTPUT`` format and
> +   select one of them via :c:func:`VIDIOC_S_FMT`.
> +
> +   .. note::
> +
> +      The driver will return only formats supported for the currently selected
> +      ``OUTPUT`` format and currently set controls, even if more formats may be
> +      supported by the decoder in general.
> +
> +      For example, a decoder may support YUV and RGB formats for
> +      resolutions 1920x1088 and lower, but only YUV for higher resolutions (due
> +      to hardware limitations). After setting a resolution of 1920x1088 or lower
> +      as the ``OUTPUT`` format, :c:func:`VIDIOC_ENUM_FMT` may return a set of
> +      YUV and RGB pixel formats, but after setting a resolution higher than
> +      1920x1088, the driver will not return RGB pixel formats, since they are
> +      unsupported for this resolution.
> +
> +5. *[optional]* Choose a different ``CAPTURE`` format than suggested via
> +   :c:func:`VIDIOC_S_FMT` on ``CAPTURE`` queue. It is possible for the client to
> +   choose a different format than selected/suggested by the driver in
> +   :c:func:`VIDIOC_G_FMT`.
> +
> +    * **Required fields:**
> +
> +      ``type``
> +          a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
> +
> +      ``pixelformat``
> +          a raw pixel format.

See Tomasz' comment. I've added this for the stateful decoder text, you can probably
reuse it here:

      ``width``, ``height``
         frame buffer resolution of the decoded stream; typically unchanged from
         what was returned with :c:func:`VIDIOC_G_FMT`, but it may be different
         if the hardware supports composition and/or scaling.

> +
> +   After performing this step, the client must perform step 3 again in order
> +   to obtain up-to-date information about the buffers size and layout.
> +
> +6. Allocate source (bitstream) buffers via :c:func:`VIDIOC_REQBUFS` on
> +   ``OUTPUT`` queue.
> +
> +    * **Required fields:**
> +
> +      ``count``
> +          requested number of buffers to allocate; greater than zero.
> +
> +      ``type``
> +          a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
> +
> +      ``memory``
> +          follows standard semantics.
> +
> +    * **Return fields:**
> +
> +      ``count``
> +          actual number of buffers allocated.
> +
> +    * If required, the driver will adjust ``count`` to be equal or bigger to the
> +      minimum of required number of ``OUTPUT`` buffers for the given format and
> +      requested count. The client must check this value after the ioctl returns
> +      to get the actual number of buffers allocated.
> +
> +7. Allocate destination (raw format) buffers via :c:func:`VIDIOC_REQBUFS` on the
> +   ``CAPTURE`` queue.
> +
> +    * **Required fields:**
> +
> +      ``count``
> +          requested number of buffers to allocate; greater than zero. The client
> +          is responsible for deducing the minimum number of buffers required
> +          for the stream to be properly decoded (taking e.g. reference frames
> +          into account) and pass an equal or bigger number.
> +
> +      ``type``
> +          a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
> +
> +      ``memory``
> +          follows standard semantics. ``V4L2_MEMORY_USERPTR`` is not supported
> +          for ``CAPTURE`` buffers.
> +
> +    * **Return fields:**
> +
> +      ``count``
> +          adjusted to allocated number of buffers, in case the codec requires
> +          more buffers than requested.
> +
> +    * The driver must adjust count to the minimum of required number of
> +      ``CAPTURE`` buffers for the current format, stream configuration and
> +      requested count. The client must check this value after the ioctl
> +      returns to get the number of buffers allocated.
> +
> +8. Allocate requests (likely one per ``OUTPUT`` buffer) via
> +    :c:func:`MEDIA_IOC_REQUEST_ALLOC` on the media device.
> +
> +9. Start streaming on both ``OUTPUT`` and ``CAPTURE`` queues via
> +    :c:func:`VIDIOC_STREAMON`.
> +
> +Decoding
> +========
> +
> +For each frame, the client is responsible for submitting at least one request to
> +which the following is attached:
> +
> +* The amount of encoded data expected by the codec for its current
> +  configuration, as a buffer submitted to the ``OUTPUT`` queue. Typically, this
> +  corresponds to one frame worth of encoded data, but some formats may allow (or
> +  require) different amounts per unit.
> +* All the metadata needed to decode the submitted encoded data, in the form of
> +  controls relevant to the format being decoded.
> +
> +The amount of data and contents of the source ``OUTPUT`` buffer, as well as the
> +controls that must be set on the request, depend on the active coded pixel
> +format and might be affected by codec-specific extended controls, as stated in
> +documentation of each format.
> +
> +If there is a possibility that the decoded frame will require one or more
> +decode requests after the current one in order to be produced, then the client
> +must set the ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF`` flag on the ``OUTPUT``
> +buffer. This will result in the (potentially partially) decoded ``CAPTURE``
> +buffer not being made available for dequeueing, and reused for the next decode
> +request if the timestamp of the next ``OUTPUT`` buffer has not changed.
> +
> +A typical frame would thus be decoded using the following sequence:
> +
> +1. Queue an ``OUTPUT`` buffer containing one unit of encoded bitstream data for
> +   the decoding request, using :c:func:`VIDIOC_QBUF`.
> +
> +    * **Required fields:**
> +
> +      ``index``
> +          index of the buffer being queued.
> +
> +      ``type``
> +          type of the buffer.
> +
> +      ``bytesused``
> +          number of bytes taken by the encoded data frame in the buffer.
> +
> +      ``flags``
> +          the ``V4L2_BUF_FLAG_REQUEST_FD`` flag must be set. Additionally, if
> +          we are not sure that the current decode request is the last one needed
> +          to produce a fully decoded frame, then
> +          ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF`` must also be set.
> +
> +      ``request_fd``
> +          must be set to the file descriptor of the decoding request.
> +
> +      ``timestamp``
> +          must be set to a unique value per frame. This value will be propagated
> +          into the decoded frame's buffer and can also be used to use this frame
> +          as the reference of another. If using multiple decode requests per
> +          frame, then the timestamps of all the ``OUTPUT`` buffers for a given
> +          frame must be identical. If the timestamp changes, then the currently
> +          held ``CAPTURE`` buffer will be made available for dequeuing and the
> +          current request will work on a new ``CAPTURE`` buffer.
> +
> +2. Set the codec-specific controls for the decoding request, using
> +   :c:func:`VIDIOC_S_EXT_CTRLS`.
> +
> +    * **Required fields:**
> +
> +      ``which``
> +          must be ``V4L2_CTRL_WHICH_REQUEST_VAL``.
> +
> +      ``request_fd``
> +          must be set to the file descriptor of the decoding request.
> +
> +      other fields
> +          other fields are set as usual when setting controls. The ``controls``
> +          array must contain all the codec-specific controls required to decode
> +          a frame.
> +
> +   .. note::
> +
> +      It is possible to specify the controls in different invocations of
> +      :c:func:`VIDIOC_S_EXT_CTRLS`, or to overwrite a previously set control, as
> +      long as ``request_fd`` and ``which`` are properly set. The controls state
> +      at the moment of request submission is the one that will be considered.
> +
> +   .. note::
> +
> +      The order in which steps 1 and 2 take place is interchangeable.
> +
> +3. Submit the request by invoking :c:func:`MEDIA_REQUEST_IOC_QUEUE` on the
> +   request FD.
> +
> +    If the request is submitted without an ``OUTPUT`` buffer, or if some of the
> +    required controls are missing from the request, then
> +    :c:func:`MEDIA_REQUEST_IOC_QUEUE` will return ``-ENOENT``. If more than one
> +    ``OUTPUT`` buffer is queued, then it will return ``-EINVAL``.
> +    :c:func:`MEDIA_REQUEST_IOC_QUEUE` returning non-zero means that no
> +    ``CAPTURE`` buffer will be produced for this request.
> +
> +``CAPTURE`` buffers must not be part of the request, and are queued
> +independently. They are returned in decode order (i.e. the same order as coded
> +frames were submitted to the ``OUTPUT`` queue).
> +
> +Runtime decoding errors are signaled by the dequeued ``CAPTURE`` buffers
> +carrying the ``V4L2_BUF_FLAG_ERROR`` flag. If a decoded reference frame has an
> +error, then all following decoded frames that refer to it also have the
> +``V4L2_BUF_FLAG_ERROR`` flag set, although the decoder will still try to
> +produce a (likely corrupted) frame.

a (likely corrupted) frame -> (likely corrupted) frames

> +
> +Buffer management while decoding
> +================================
> +Contrary to stateful decoders, a stateless decoder does not perform any kind of
> +buffer management: it only guarantees that dequeued ``CAPTURE`` buffers can be
> +used by the client for as long as they are not queued again. "Used" here
> +encompasses using the buffer for compositing or display.
> +
> +A dequeued capture buffer can also be used as the reference frame of another
> +buffer.
> +
> +A frame is specified as reference by converting its timestamp into nanoseconds,
> +and storing it into the relevant member of a codec-dependent control structure.
> +The :c:func:`v4l2_timeval_to_ns` function must be used to perform that
> +conversion. The timestamp of a frame can be used to reference it as soon as all
> +its units of encoded data are successfully submitted to the ``OUTPUT`` queue.
> +
> +A decoded buffer containing a reference frame must not be reused as a decoding
> +target until all the frames referencing it have been decoded. The safest way to
> +achieve this is to refrain from queueing a reference buffer until all the
> +decoded frames referencing it have been dequeued. However, if the driver can
> +guarantee that buffers queued to the ``CAPTURE`` queue are processed in queued
> +order, then user-space can take advantage of this guarantee and queue a
> +reference buffer when the following conditions are met:
> +
> +1. All the requests for frames affected by the reference frame have been
> +   queued, and
> +
> +2. A sufficient number of ``CAPTURE`` buffers to cover all the decoded
> +   referencing frames have been queued.
> +
> +When queuing a decoding request, the driver will increase the reference count of
> +all the resources associated with reference frames. This means that the client

Does this really happen? I don't think we've implemented support for incrementing
the ref count yet. It *should* happen like this, that I agree with.

> +can e.g. close the DMABUF file descriptors of reference frame buffers if it
> +won't need them afterwards.

I'm fairly certain that this will fail with the current code.

We're missing vb2 helpers that ensure that the memory of reference frames sticks
around: the memory refcount should be incremented when validating the request, and
decremented when the request is completed.

If the reference frame memory that is requested has already been deleted (i.e. the
CAPTURE buffer dmabuf has been closed before the new request was queued), then
the request validation code will fail.

I know this has been discussed before, but we never actually implemented this.

Unfortunately, this will require new vb2 memops, so it is a bit more work.

> +
> +Seeking
> +=======
> +In order to seek, the client just needs to submit requests using input buffers
> +corresponding to the new stream position. It must however be aware that
> +resolution may have changed and follow the dynamic resolution change sequence in
> +that case. Also depending on the codec used, picture parameters (e.g. SPS/PPS
> +for H.264) may have changed and the client is responsible for making sure that a
> +valid state is sent to the decoder.
> +
> +The client is then free to ignore any returned ``CAPTURE`` buffer that comes
> +from the pre-seek position.
> +
> +Pausing
> +=======
> +
> +In order to pause, the client can just cease queuing buffers onto the ``OUTPUT``
> +queue. Without source bitstream data, there is no data to process and the codec
> +will remain idle.
> +
> +Dynamic resolution change
> +=========================
> +
> +If the client detects a resolution change in the stream, it will need to perform
> +the initialization sequence again with the new resolution:
> +
> +1. Wait until all submitted requests have completed and dequeue the
> +   corresponding output buffers.
> +
> +2. Call :c:func:`VIDIOC_STREAMOFF` on both the ``OUTPUT`` and ``CAPTURE``
> +   queues.

Note: you have the same problem with HOLD_CAPTURE_BUF here as in the Drain
situation. I think this should be mentioned here as well.

> +
> +3. Free all ``CAPTURE`` buffers by calling :c:func:`VIDIOC_REQBUFS` on the
> +   ``CAPTURE`` queue with a buffer count of zero.
> +
> +4. Perform the initialization sequence again (minus the allocation of
> +   ``OUTPUT`` buffers), with the new resolution set on the ``OUTPUT`` queue.
> +   Note that due to resolution constraints, a different format may need to be
> +   picked on the ``CAPTURE`` queue.
> +
> +Drain
> +=====
> +
> +In order to drain the stream on a stateless decoder, the client just needs to
> +wait until all the submitted requests are completed. There is no need to send a
> +``V4L2_DEC_CMD_STOP`` command since requests are processed sequentially by the
> +decoder.
> +
> +If the last submitted request resulted in a ``CAPTURE`` buffer being held by the
> +use of the ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF`` flag, the last frame may not
> +be available on the ``CAPTURE`` queue. In this case, an extra request with an
> +empty ``OUTPUT`` buffer (e.g. which ``bytesused`` is set to 0) and a different

which -> where

> +timestamp can be queued. This will make the driver release the held ``CAPTURE``
> +buffer without doing any extra processing.
> 

I don't like this. It's a special request which would require special handling in
drivers. It just makes things more complicated.

How about this: add a new command to VIDIOC_DECODER_CMD: V4L2_DEC_CMD_END_OF_FRAME.

A stateless decoder that supports HOLD_CAPTURE_BUF would implement support for this
command to flush the last frame, if any.

What do you think?

Regards,

	Hans