Re: [PATCH v2 1/2] media: docs-rst: Document memory-to-memory video decoder interface

[Hans Verkuil]

On 10/22/2018 04:48 PM, Tomasz Figa wrote:
> Due to complexity of the video decoding process, the V4L2 drivers of
> stateful decoder hardware require specific sequences of V4L2 API calls
> to be followed. These include capability enumeration, initialization,
> decoding, seek, pause, dynamic resolution change, drain and end of
> stream.
> 
> 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 decoder part of
> the Codec API.
> 
> Signed-off-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> ---
>  Documentation/media/uapi/v4l/dev-decoder.rst  | 1082 +++++++++++++++++
>  Documentation/media/uapi/v4l/devices.rst      |    1 +
>  Documentation/media/uapi/v4l/pixfmt-v4l2.rst  |    5 +
>  Documentation/media/uapi/v4l/v4l2.rst         |   10 +-
>  .../media/uapi/v4l/vidioc-decoder-cmd.rst     |   40 +-
>  Documentation/media/uapi/v4l/vidioc-g-fmt.rst |   14 +
>  6 files changed, 1137 insertions(+), 15 deletions(-)
>  create mode 100644 Documentation/media/uapi/v4l/dev-decoder.rst
> 
> diff --git a/Documentation/media/uapi/v4l/dev-decoder.rst b/Documentation/media/uapi/v4l/dev-decoder.rst
> new file mode 100644
> index 000000000000..09c7a6621b8e
> --- /dev/null
> +++ b/Documentation/media/uapi/v4l/dev-decoder.rst
> @@ -0,0 +1,1082 @@
> +.. -*- coding: utf-8; mode: rst -*-
> +
> +.. _decoder:
> +
> +*************************************************
> +Memory-to-memory Stateful Video Decoder Interface
> +*************************************************
> +
> +A stateful video decoder takes complete chunks of the bitstream (e.g. Annex-B
> +H.264/HEVC stream, raw VP8/9 stream) and decodes them into raw video frames in
> +display order. The decoder is expected not to require any additional information
> +from the client to process these buffers.
> +
> +Performing software parsing, processing etc. of the stream in the driver in
> +order to support this interface is strongly discouraged. In case such
> +operations are needed, use of the Stateless Video Decoder 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 âmustâ, âmayâ, âshouldâ, etc. is as per RFC
> +   2119.
> +
> +3. All steps not marked âoptionalâ are required.
> +
> +4. :c:func:`VIDIOC_G_EXT_CTRLS`, :c:func:`VIDIOC_S_EXT_CTRLS` may be used
> +   interchangeably with :c:func:`VIDIOC_G_CTRL`, :c:func:`VIDIOC_S_CTRL`,
> +   unless specified otherwise.
> +
> +5. Single-plane API (see spec) and applicable structures may be used
> +   interchangeably with Multi-plane API, unless specified otherwise,
> +   depending on decoder 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, Aâ represents a buffer on the ``CAPTURE``
> +   queue containing data (decoded frame/stream) that resulted from processing
> +   buffer A.
> +
> +.. _decoder-glossary:
> +
> +Glossary
> +========
> +
> +CAPTURE
> +   the destination buffer queue; for decoder, the queue of buffers containing
> +   decoded frames; for encoder, the queue of buffers containing encoded
> +   bitstream; ``V4L2_BUF_TYPE_VIDEO_CAPTURE```` or
> +   ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``; data are captured from the hardware
> +   into ``CAPTURE`` buffers
> +
> +client
> +   application client communicating with the decoder or encoder implementing
> +   this interface
> +
> +coded format
> +   encoded/compressed video bitstream format (e.g. H.264, VP8, etc.); see
> +   also: raw format
> +
> +coded height
> +   height for given coded resolution
> +
> +coded resolution
> +   stream resolution in pixels aligned to codec and hardware requirements;
> +   typically visible resolution rounded up to full macroblocks;
> +   see also: visible resolution
> +
> +coded width
> +   width for given coded resolution
> +
> +decode order
> +   the order in which frames are decoded; may differ from display order if the
> +   coded format includes a feature of frame reordering; for decoders,
> +   ``OUTPUT`` buffers must be queued by the client in decode order; for
> +   encoders ``CAPTURE`` buffers must be returned by the encoder in decode order
> +
> +destination
> +   data resulting from the decode process; ``CAPTURE``
> +
> +display order
> +   the order in which frames must be displayed; for encoders, ``OUTPUT``
> +   buffers must be queued by the client in display order; for decoders,
> +   ``CAPTURE`` buffers must be returned by the decoder in display order
> +
> +DPB
> +   Decoded Picture Buffer; an H.264 term for a buffer that stores a decoded
> +   raw frame available for reference in further decoding steps.
> +
> +EOS
> +   end of stream
> +
> +IDR
> +   Instantaneous Decoder Refresh; a type of a keyframe in H.264-encoded stream,
> +   which clears the list of earlier reference frames (DPBs)
> +
> +keyframe
> +   an encoded frame that does not reference frames decoded earlier, i.e.
> +   can be decoded fully on its own.
> +
> +macroblock
> +   a processing unit in image and video compression formats based on linear
> +   block transforms (e.g. H.264, VP8, VP9); codec-specific, but for most of
> +   popular codecs the size is 16x16 samples (pixels)
> +
> +OUTPUT
> +   the source buffer queue; for decoders, the queue of buffers containing
> +   encoded bitstream; for encoders, the queue of buffers containing raw frames;
> +   ``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``; the
> +   hardware is fed with data from ``OUTPUT`` buffers
> +
> +PPS
> +   Picture Parameter Set; a type of metadata entity in H.264 bitstream
> +
> +raw format
> +   uncompressed format containing raw pixel data (e.g. YUV, RGB formats)
> +
> +resume point
> +   a point in the bitstream from which decoding may start/continue, without
> +   any previous state/data present, e.g.: a keyframe (VP8/VP9) or
> +   SPS/PPS/IDR sequence (H.264); a resume point is required to start decode
> +   of a new stream, or to resume decoding after a seek
> +
> +source
> +   data fed to the decoder or encoder; ``OUTPUT``
> +
> +source height
> +   height in pixels for given source resolution; relevant to encoders only
> +
> +source resolution
> +   resolution in pixels of source frames being source to the encoder and
> +   subject to further cropping to the bounds of visible resolution; relevant to
> +   encoders only
> +
> +source width
> +   width in pixels for given source resolution; relevant to encoders only
> +
> +SPS
> +   Sequence Parameter Set; a type of metadata entity in H.264 bitstream
> +
> +stream metadata
> +   additional (non-visual) information contained inside encoded bitstream;
> +   for example: coded resolution, visible resolution, codec profile
> +
> +visible height
> +   height for given visible resolution; display height
> +
> +visible resolution
> +   stream resolution of the visible picture, in pixels, to be used for
> +   display purposes; must be smaller or equal to coded resolution;
> +   display resolution
> +
> +visible width
> +   width for given visible resolution; display width
> +
> +State machine
> +=============
> +
> +.. kernel-render:: DOT
> +   :alt: DOT digraph of decoder state machine
> +   :caption: Decoder state machine
> +
> +   digraph decoder_state_machine {
> +       node [shape = doublecircle, label="Decoding"] Decoding;
> +
> +       node [shape = circle, label="Initialization"] Initialization;
> +       node [shape = circle, label="Capture\nsetup"] CaptureSetup;
> +       node [shape = circle, label="Dynamic\nresolution\nchange"] ResChange;
> +       node [shape = circle, label="Stopped"] Stopped;
> +       node [shape = circle, label="Drain"] Drain;
> +       node [shape = circle, label="Seek"] Seek;
> +       node [shape = circle, label="End of stream"] EoS;
> +
> +       node [shape = point]; qi
> +       qi -> Initialization [ label = "open()" ];
> +
> +       Initialization -> CaptureSetup [ label = "CAPTURE\nformat\nestablished" ];
> +
> +       CaptureSetup -> Stopped [ label = "CAPTURE\nbuffers\nready" ];
> +
> +       Decoding -> ResChange [ label = "Stream\nresolution\nchange" ];
> +       Decoding -> Drain [ label = "V4L2_DEC_CMD_STOP" ];
> +       Decoding -> EoS [ label = "EoS mark\nin the stream" ];
> +       Decoding -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
> +       Decoding -> Stopped [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
> +       Decoding -> Decoding;
> +
> +       ResChange -> CaptureSetup [ label = "CAPTURE\nformat\nestablished" ];
> +       ResChange -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
> +
> +       EoS -> Drain [ label = "Implicit\ndrain" ];
> +
> +       Drain -> Stopped [ label = "All CAPTURE\nbuffers dequeued\nor\nVIDIOC_STREAMOFF(CAPTURE)" ];
> +       Drain -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
> +
> +       Seek -> Decoding [ label = "VIDIOC_STREAMON(OUTPUT)" ];
> +       Seek -> Initialization [ label = "VIDIOC_REQBUFS(OUTPUT, 0)" ];
> +
> +       Stopped -> Decoding [ label = "V4L2_DEC_CMD_START\nor\nVIDIOC_STREAMON(CAPTURE)" ];
> +       Stopped -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
> +   }
> +
> +Querying capabilities
> +=====================
> +
> +1. To enumerate the set of coded formats supported by the decoder, the
> +   client may call :c:func:`VIDIOC_ENUM_FMT` on ``OUTPUT``.
> +
> +   * The full set of supported formats will be returned, regardless of the
> +     format set on ``CAPTURE``.
> +
> +2. To enumerate the set of supported raw formats, the client may call
> +   :c:func:`VIDIOC_ENUM_FMT` on ``CAPTURE``.
> +
> +   * Only the formats supported for the format currently active on ``OUTPUT``
> +     will be returned.
> +
> +   * In order to enumerate raw formats supported by a given coded format,
> +     the client must first set that coded format on ``OUTPUT`` and then
> +     enumerate formats on ``CAPTURE``.
> +
> +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
> +     formats will include all possible coded resolutions supported by the
> +     decoder 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
> +     decoder for given raw pixel format and the coded format currently set on
> +     ``OUTPUT``.
> +
> +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
> +   `Querying capabilities` above.
> +
> +2. Set the coded format on ``OUTPUT`` via :c:func:`VIDIOC_S_FMT`
> +
> +   * **Required fields:**
> +
> +     ``type``
> +         a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``
> +
> +     ``pixelformat``
> +         a coded pixel format
> +
> +     ``width``, ``height``
> +         required only if cannot be parsed from the stream for the given
> +         coded format; optional otherwise - set to zero to ignore
> +
> +     ``sizeimage``
> +         desired size of ``OUTPUT`` buffers; the decoder may adjust it to
> +         match hardware requirements
> +
> +     other fields
> +         follow standard semantics
> +
> +   * **Return fields:**
> +
> +     ``sizeimage``
> +         adjusted size of ``CAPTURE`` buffers
> +
> +   * If width and height are set to non-zero values, the ``CAPTURE`` format
> +     will be updated with an appropriate frame buffer resolution instantly.
> +     However, for coded formats that include stream resolution information,
> +     after the decoder is done parsing the information from the stream, it will
> +     update the ``CAPTURE`` format with new values and signal a source change
> +     event.
> +
> +   .. warning::
> +
> +      Changing the ``OUTPUT`` format may change the currently set ``CAPTURE``
> +      format. The decoder 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.
> +
> +3.  **Optional.** Query the minimum number of buffers required for ``OUTPUT``
> +    queue via :c:func:`VIDIOC_G_CTRL`. This is useful if the client intends to
> +    use more buffers than the minimum required by hardware/format.
> +
> +    * **Required fields:**
> +
> +      ``id``
> +          set to ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT``
> +
> +    * **Return fields:**
> +
> +      ``value``
> +          the minimum number of ``OUTPUT`` buffers required for the currently
> +          set format
> +
> +4.  Allocate source (bitstream) buffers via :c:func:`VIDIOC_REQBUFS` on
> +    ``OUTPUT``.
> +
> +    * **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``
> +          the 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 buffers (for pipeline
> +       depth), the client may query the ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT``
> +       control to get the minimum number of buffers required by the
> +       decoder/format, 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` on the ``OUTPUT`` queue 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``
> +
> +      ``memory``
> +          follows standard semantics
> +
> +      ``format``
> +          follows standard semantics
> +
> +    * **Return fields:**
> +
> +      ``count``
> +          adjusted to the number of allocated buffers
> +
> +    .. 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.
> +
> +5.  Start streaming on the ``OUTPUT`` queue via :c:func:`VIDIOC_STREAMON`.
> +
> +6.  **This step only applies to coded formats that contain resolution information
> +    in the stream.**

As far as I know all codecs have resolution/metadata in the stream.

As discussed in the "[PATCH vicodec v4 0/3] Add support to more pixel formats in
vicodec" thread, it is easiest to assume that there is always metadata.

Perhaps there should be a single mention somewhere that such codecs are not
supported at the moment, but to be frank how can you decode a stream without
it containing such essential information? You are much more likely to implement
such a codec as a stateless codec.

So I would just drop this sentence here (and perhaps at other places in this
document or the encoder document as well).

Regards,

	Hans