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

[Hans Verkuil]

Hi Alexandre,

Thank you very much for working on this, much appreciated!

On 08/31/2018 09:47 AM, Alexandre Courbot wrote:
> This patch documents the protocol that user-space should follow when
> communicating with stateless video decoders. It is based on the
> following references:
> 
> * The current protocol used by Chromium (converted from config store to
>   request API)
> 
> * The submitted Cedrus VPU driver
> 
> As such, some things may not be entirely consistent with the current
> state of drivers, so it would be great if all stakeholders could point
> out these inconsistencies. :)
> 
> This patch is supposed to be applied on top of the Request API V18 as
> well as the memory-to-memory video decoder interface series by Tomasz
> Figa.
> 
> It should be considered an early RFC.
> 
> Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxxxx>
> ---
>  .../media/uapi/v4l/dev-stateless-decoder.rst  | 413 ++++++++++++++++++
>  Documentation/media/uapi/v4l/devices.rst      |   1 +
>  .../media/uapi/v4l/extended-controls.rst      |  23 +
>  3 files changed, 437 insertions(+)
>  create mode 100644 Documentation/media/uapi/v4l/dev-stateless-decoder.rst
> 
> 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..bf7b13a8ee16
> --- /dev/null
> +++ b/Documentation/media/uapi/v4l/dev-stateless-decoder.rst
> @@ -0,0 +1,413 @@
> +.. -*- coding: utf-8; mode: rst -*-
> +
> +.. _stateless_decoder:
> +
> +**************************************************
> +Memory-to-memory Stateless Video Decoder Interface
> +**************************************************
> +
> +A stateless decoder is a decoder that works without retaining any kind of state
> +between processing 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 driver. This is in
> +contrast to the stateful video decoder interface, where the hardware maintains
> +the decoding state and all the client has to do is to provide the raw encoded
> +stream.
> +
> +This section describes how user-space ("the client") is expected to communicate
> +with such decoders in order to successfully decode an encoded stream. Compared
> +to stateful codecs, the driver/client protocol is simpler, but cost of this
> +simplicity is extra complexity in the client which must maintain the decoding
> +state.
> +
> +Querying capabilities
> +=====================
> +
> +1. To enumerate the set of coded formats supported by the driver, the client
> +   calls :c:func:`VIDIOC_ENUM_FMT` on the ``OUTPUT`` queue.
> +
> +   * The driver must always return the full set of supported formats for the
> +     currently set ``OUTPUT`` format, irrespective of the format currently set
> +     on the ``CAPTURE`` queue.
> +
> +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.
> +
> +   * In order to enumerate raw formats supported by a given coded format, the
> +     client must thus set that coded format on the ``OUTPUT`` queue first, and
> +     then enumerate the ``CAPTURE`` queue.
> +
> +3. The client may use :c:func:`VIDIOC_ENUM_FRAMESIZES` to detect supported
> +   resolutions for a given format, passing desired pixel format in
> +   :c:type:`v4l2_frmsizeenum` ``pixel_format``.
> +
> +   * Values returned by :c:func:`VIDIOC_ENUM_FRAMESIZES` on ``OUTPUT`` queue
> +     must include all possible coded resolutions supported by the decoder
> +     for given coded pixel format.
> +
> +   * Values returned by :c:func:`VIDIOC_ENUM_FRAMESIZES` on ``CAPTURE`` queue
> +     must include all possible frame buffer resolutions supported by the
> +     decoder for given raw pixel format and coded format currently set on
> +     ``OUTPUT`` queue.
> +
> +    .. note::
> +
> +       The client may derive the supported resolution range for a
> +       combination of coded and raw format by setting width and height of
> +       ``OUTPUT`` format to 0 and calculating the intersection of
> +       resolutions returned from calls to :c:func:`VIDIOC_ENUM_FRAMESIZES`
> +       for the given coded and raw formats.
> +
> +4. Supported profiles and levels for given format, if applicable, may be
> +   queried using their respective controls via :c:func:`VIDIOC_QUERYCTRL`.
> +
> +Initialization
> +==============
> +
> +1. *[optional]* Enumerate supported ``OUTPUT`` formats and resolutions. See
> +   capability enumeration.
> +
> +2. 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``
> +         parsed width and height of the coded format
> +
> +     other fields
> +         follow standard semantics
> +
> +   .. note::
> +
> +      Changing ``OUTPUT`` format may change currently set ``CAPTURE``
> +      format. The driver will derive a new ``CAPTURE`` format from
> +      ``OUTPUT`` format being set, including resolution, colorimetry
> +      parameters, etc. If the client needs a specific ``CAPTURE`` format,
> +      it must adjust it afterwards.
> +
> +3. *[optional]* Get minimum number of buffers required for ``OUTPUT``
> +   queue via :c:func:`VIDIOC_G_CTRL`. This is useful if client intends to
> +   use more buffers than minimum required by hardware/format.
> +
> +   * **Required fields:**
> +
> +     ``id``
> +         set to ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT``
> +
> +   * **Return fields:**
> +
> +     ``value``
> +         required number of ``OUTPUT`` buffers for the currently set
> +         format
> +
> +4. Call :c:func:`VIDIOC_G_FMT` for ``CAPTURE`` queue to get format for the
> +   destination buffers parsed/decoded from the bitstream.
> +
> +   * **Required fields:**
> +
> +     ``type``
> +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``
> +
> +   * **Return 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 given configuration.
> +      For example, a YUV format may be preferred over an RGB format, if
> +      additional conversion step would be required.
> +
> +5. *[optional]* Enumerate ``CAPTURE`` formats via :c:func:`VIDIOC_ENUM_FMT` on
> +   ``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, even if more formats may be supported by the driver in
> +      general.
> +
> +      For example, a driver/hardware may support YUV and RGB formats for
> +      resolutions 1920x1088 and lower, but only YUV for higher resolutions (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, unsupported for this
> +      resolution.
> +
> +6. *[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
> +
> +    .. note::
> +
> +       Calling :c:func:`VIDIOC_ENUM_FMT` to discover currently available
> +       formats after receiving ``V4L2_EVENT_SOURCE_CHANGE`` is useful to find
> +       out a set of allowed formats for given configuration, but not required,
> +       if the client can accept the defaults.
> +
> +7. *[optional]* Get minimum number of buffers required for ``CAPTURE`` queue via
> +   :c:func:`VIDIOC_G_CTRL`. This is useful if client intends to use more buffers
> +   than minimum required by hardware/format.
> +
> +    * **Required fields:**
> +
> +      ``id``
> +          set to ``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE``
> +
> +    * **Return fields:**
> +
> +      ``value``
> +          minimum number of buffers required to decode the stream parsed in this
> +          initialization sequence.
> +
> +    .. note::
> +
> +       Note that the minimum number of buffers must be at least the number
> +       required to successfully decode the current stream. This may for example
> +       be the required DPB size for an H.264 stream given the parsed stream
> +       configuration (resolution, level).
> +
> +8. 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
> +
> +      ``sizeimage``
> +          follows standard semantics; the client is free to choose any
> +          suitable size, however, it may be subject to change by the
> +          driver
> +
> +    * **Return fields:**
> +
> +      ``count``
> +          actual number of buffers allocated
> +
> +    * The driver must adjust count to minimum of required number of ``OUTPUT``
> +      buffers for given format and count passed. The client must check this
> +      value after the ioctl returns to get the number of buffers allocated.
> +
> +    .. note::
> +
> +       To allocate more than minimum number of buffers (for pipeline depth), use
> +       G_CTRL(``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT``) to get minimum number of
> +       buffers required by the driver/format, and pass the obtained value plus
> +       the number of additional buffers needed in count to
> +       :c:func:`VIDIOC_REQBUFS`.
> +
> +9. 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
> +
> +      ``type``
> +          a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``
> +
> +      ``memory``
> +          follows standard semantics
> +
> +    * **Return fields:**
> +
> +      ``count``
> +          adjusted to allocated number of buffers
> +
> +    * The driver must adjust count to minimum of required number of
> +      destination buffers for given format and stream configuration and the
> +      count passed. The client must check this value after the ioctl
> +      returns to get the number of buffers allocated.
> +
> +    .. note::
> +
> +       To allocate more than minimum number of buffers (for pipeline
> +       depth), use G_CTRL(``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE``) to
> +       get minimum number of buffers required, and pass the obtained value
> +       plus the number of additional buffers needed in count to
> +       :c:func:`VIDIOC_REQBUFS`.
> +
> +10. Allocate requests (likely one per ``OUTPUT`` buffer) via
> +    :c:func:`MEDIA_IOC_REQUEST_ALLOC` on the media device.
> +
> +11. Start streaming on both ``OUTPUT`` and ``CAPTURE`` queues via
> +    :c:func:`VIDIOC_STREAMON`.

If I am not mistaken, steps 1-11 are the same as for stateful codecs. It is better
to just refer to that instead of copying them.

> +
> +Decoding
> +========
> +
> +For each frame, the client is responsible for submitting a request to which the
> +following is attached:
> +
> +* Exactly one frame worth of encoded data in a buffer submitted to the
> +  ``OUTPUT`` queue,
> +* All the controls relevant to the format being decoded (see below for details).
> +
> +``CAPTURE`` buffers must not be part of the request, but must be queued
> +independently. The driver will pick one of the queued ``CAPTURE`` buffers and
> +decode the frame into it. Although the client has no control over which
> +``CAPTURE`` buffer will be used with a given ``OUTPUT`` buffer, it is guaranteed
> +that ``CAPTURE`` buffers will be returned in decode order (i.e. the same order
> +as ``OUTPUT`` buffers were submitted), so it is trivial to associate a dequeued
> +``CAPTURE`` buffer to its originating request and ``OUTPUT`` buffer.
> +
> +If the request is submitted without an ``OUTPUT`` buffer or if one of the
> +required controls are missing, then :c:func:`MEDIA_REQUEST_IOC_QUEUE` will return
> +``-EINVAL``.

Not entirely true: if buffers are missing, then ENOENT is returned. Missing required
controls or more than one OUTPUT buffer will result in EINVAL. This per the latest
Request API changes.

 Decoding errors are signaled by the ``CAPTURE`` buffers being
> +dequeued carrying the ``V4L2_BUF_FLAG_ERROR`` flag.

Add here that if the reference frame had an error, then all other frames that refer
to it should also set the ERROR flag. It is up to userspace to decide whether or
not to drop them (part of the frame might still be valid).

I am not sure whether this should be documented, but there are some additional
restrictions w.r.t. reference frames:

Since decoders need access to the decoded reference frames there are some corner
cases that need to be checked:

1) V4L2_MEMORY_USERPTR cannot be used for the capture queue: the driver does not
   know when a malloced but dequeued buffer is freed, so the reference frame
   could suddenly be gone.

2) V4L2_MEMORY_DMABUF can be used, but drivers should check that the dma buffer is
   still available AND increase the dmabuf refcount while it is used by the HW.

3) What to do if userspace has requeued a buffer containing a reference frame,
   and you want to decode a B/P-frame that refers to that buffer? We need to
   check against that: I think that when you queue a capture buffer whose index
   is used in a pending request as a reference frame, than that should fail with
   an error. And trying to queue a request referring to a buffer that has been
   requeued should also fail.

We might need to add some support for this in v4l2-mem2mem.c or vb2.

We will have similar (but not quite identical) issues with stateless encoders.

> +
> +The contents of source ``OUTPUT`` buffers, as well as the controls that must be
> +set on the request, depend on active coded pixel format and might be affected by
> +codec-specific extended controls, as stated in documentation of each format
> +individually.
> +
> +MPEG-2 buffer content and controls
> +----------------------------------
> +The following information is valid when the ``OUTPUT`` queue is set to the
> +``V4L2_PIX_FMT_MPEG2_SLICE`` format.
> +
> +The ``OUTPUT`` buffer must contain all the macroblock slices of a given frame,
> +i.e. if a frame requires several macroblock slices to be entirely decoded, then
> +all these slices must be provided. In addition, the following controls must be
> +set on the request:
> +
> +V4L2_CID_MPEG_VIDEO_MPEG2_SLICE_PARAMS
> +   Slice parameters (one per slice) for the current frame.
> +
> +Optional controls:
> +
> +V4L2_CID_MPEG_VIDEO_MPEG2_QUANTIZATION
> +   Quantization matrices for the current frame.
> +
> +H.264 buffer content and controls
> +---------------------------------
> +The following information is valid when the ``OUTPUT`` queue is set to the
> +``V4L2_PIX_FMT_H264_SLICE`` format.
> +
> +The ``OUTPUT`` buffer must contain all the macroblock slices of a given frame,
> +i.e. if a frame requires several macroblock slices to be entirely decoded, then
> +all these slices must be provided. In addition, the following controls must be
> +set on the request:
> +
> +V4L2_CID_MPEG_VIDEO_H264_SPS
> +   Instance of struct v4l2_ctrl_h264_sps, containing the SPS of to use with the
> +   frame.
> +
> +V4L2_CID_MPEG_VIDEO_H264_PPS
> +   Instance of struct v4l2_ctrl_h264_pps, containing the PPS of to use with the
> +   frame.
> +
> +V4L2_CID_MPEG_VIDEO_H264_SCALING_MATRIX
> +   Instance of struct v4l2_ctrl_h264_scaling_matrix, containing the scaling
> +   matrix to use when decoding the frame.
> +
> +V4L2_CID_MPEG_VIDEO_H264_SLICE_PARAM
> +   Array of struct v4l2_ctrl_h264_slice_param, containing at least as many
> +   entries as there are slices in the corresponding ``OUTPUT`` buffer.
> +
> +V4L2_CID_MPEG_VIDEO_H264_DECODE_PARAM
> +   Instance of struct v4l2_ctrl_h264_decode_param, containing the high-level
> +   decoding parameters for a H.264 frame.
> +
> +Seek
> +====
> +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 protocol 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 kernel.
> +
> +The client is then free to ignore any returned ``CAPTURE`` buffer that comes
> +from the pre-seek position.
> +
> +Pause
> +=====
> +
> +In order to pause, the client should just cease queuing buffers onto the
> +``OUTPUT`` queue. This is different from the general V4L2 API definition of
> +pause, which involves calling :c:func:`VIDIOC_STREAMOFF` on the queue.
> +Without source bitstream data, there is no data to process and the hardware
> +remains idle.
> +
> +Dynamic resolution change
> +=========================
> +
> +If the client detects a resolution change in the stream, it may need to
> +reallocate the ``CAPTURE`` buffers to fit the new size.
> +
> +1. Wait until all submitted requests have completed and dequeue the
> +   corresponding output buffers.
> +
> +2. Call :c:func:`VIDIOC_STREAMOFF` on the ``CAPTURE`` queue.
> +
> +3. Free all ``CAPTURE`` buffers by calling :c:func:`VIDIOC_REQBUFS` on the
> +   ``CAPTURE`` queue with a buffer count of zero.
> +
> +4. Set the new format and resolution on the ``CAPTURE`` queue.
> +
> +5. Allocate new ``CAPTURE`` buffers for the new resolution.
> +
> +6. Call :c:func:`VIDIOC_STREAMON` on the ``CAPTURE`` queue to resume the stream.
> +
> +The client can then start queueing new ``CAPTURE`` buffers and submit requests
> +to decode the next buffers at the new resolution.
> +
> +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
> +driver.
> +
> +End of stream
> +=============
> +
> +If the decoder encounters an end of stream marking in the stream, the
> +driver must send a ``V4L2_EVENT_EOS`` event to the client after all frames
> +are decoded and ready to be dequeued on the ``CAPTURE`` queue, with the
> +:c:type:`v4l2_buffer` ``flags`` set to ``V4L2_BUF_FLAG_LAST``. This
> +behavior is identical to the drain sequence triggered by the client via
> +``V4L2_DEC_CMD_STOP``.
> diff --git a/Documentation/media/uapi/v4l/devices.rst b/Documentation/media/uapi/v4l/devices.rst
> index 1822c66c2154..a8e568eda7d8 100644
> --- a/Documentation/media/uapi/v4l/devices.rst
> +++ b/Documentation/media/uapi/v4l/devices.rst
> @@ -16,6 +16,7 @@ Interfaces
>      dev-osd
>      dev-codec
>      dev-decoder
> +    dev-stateless-decoder
>      dev-encoder
>      dev-effect
>      dev-raw-vbi
> diff --git a/Documentation/media/uapi/v4l/extended-controls.rst b/Documentation/media/uapi/v4l/extended-controls.rst
> index a9252225b63e..c0411ebf4c12 100644
> --- a/Documentation/media/uapi/v4l/extended-controls.rst
> +++ b/Documentation/media/uapi/v4l/extended-controls.rst
> @@ -810,6 +810,29 @@ enum v4l2_mpeg_video_bitrate_mode -
>      otherwise the decoder expects a single frame in per buffer.
>      Applicable to the decoder, all codecs.
>  
> +``V4L2_CID_MPEG_VIDEO_H264_SPS``
> +    Instance of struct v4l2_ctrl_h264_sps, containing the SPS of to use with
> +    the next queued frame. Applicable to the H.264 stateless decoder.
> +
> +``V4L2_CID_MPEG_VIDEO_H264_PPS``
> +    Instance of struct v4l2_ctrl_h264_pps, containing the PPS of to use with
> +    the next queued frame. Applicable to the H.264 stateless decoder.
> +
> +``V4L2_CID_MPEG_VIDEO_H264_SCALING_MATRIX``
> +    Instance of struct v4l2_ctrl_h264_scaling_matrix, containing the scaling
> +    matrix to use when decoding the next queued frame. Applicable to the H.264
> +    stateless decoder.
> +
> +``V4L2_CID_MPEG_VIDEO_H264_SLICE_PARAM``
> +    Array of struct v4l2_ctrl_h264_slice_param, containing at least as many
> +    entries as there are slices in the corresponding ``OUTPUT`` buffer.
> +    Applicable to the H.264 stateless decoder.
> +
> +``V4L2_CID_MPEG_VIDEO_H264_DECODE_PARAM``
> +    Instance of struct v4l2_ctrl_h264_decode_param, containing the high-level
> +    decoding parameters for a H.264 frame. Applicable to the H.264 stateless
> +    decoder.
> +
>  ``V4L2_CID_MPEG_VIDEO_H264_VUI_SAR_ENABLE (boolean)``
>      Enable writing sample aspect ratio in the Video Usability
>      Information. Applicable to the H264 encoder.
> 

Regards,

	Hans