[PATCH v2] media: docs-rst: Document m2m stateless video decoder interface

[Alexandre Courbot]

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.

Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxxxx>
---
Changes since v1:

* Use timestamps instead of tags to reference frames,
* Applied Paul's suggestions to not require one frame worth of data per OUTPUT
  buffer

One of the effects of requiring sub-frame units to be submitted per request is
that the stateless decoders are not exactly "stateless" anymore: if a frame is
made of several slices, then the decoder must keep track of the buffer in which
the current frame is being decoded between requests, and all the slices for the
current frame must be submitted before we can consider decoding the next one.

Also if we decide to force clients to submit one slice per request, then doesn't
some of the H.264 controls need to change? For instance, in the current v2
there is still a v4l2_ctrl_h264_decode_param::num_slices member. It is used in
Chromium to specify the number of slices given to the
V4L2_CID_MPEG_VIDEO_H264_SLICE_PARAMS control, but is apparently ignored by the
Cedrus driver. Maxime, can you comment on this?

 Documentation/media/uapi/v4l/dev-codec.rst    |   5 +
 .../media/uapi/v4l/dev-stateless-decoder.rst  | 378 ++++++++++++++++++
 2 files changed, 383 insertions(+)
 create mode 100644 Documentation/media/uapi/v4l/dev-stateless-decoder.rst

diff --git a/Documentation/media/uapi/v4l/dev-codec.rst b/Documentation/media/uapi/v4l/dev-codec.rst
index b5e017c17834..6ce38045d3c8 100644
--- a/Documentation/media/uapi/v4l/dev-codec.rst
+++ b/Documentation/media/uapi/v4l/dev-codec.rst
@@ -13,6 +13,11 @@
 Codec Interface
 ***************
 
+.. toctree::
+    :maxdepth: 1
+
+    dev-stateless-decoder
+
 A V4L2 codec can compress, decompress, transform, or otherwise convert
 video data from one format into another format, in memory. Typically
 such devices are memory-to-memory devices (i.e. devices with the
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..148b1751dd20
--- /dev/null
+++ b/Documentation/media/uapi/v4l/dev-stateless-decoder.rst
@@ -0,0 +1,378 @@
+.. 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 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 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.
+
+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 decoder/client sequence is simpler, but the cost of
+this simplicity is extra complexity in the client which must maintain a
+consistent decoding state.
+
+Stateless decoders make use of the request API. A stateless decoder must thus
+expose the ``V4L2_BUF_CAP_SUPPORTS_REQUESTS`` capability on its ``OUTPUT`` queue
+when :c:func:`VIDIOC_REQBUFS` or :c:func:`VIDIOC_CREATE_BUFS` are invoked.
+
+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 controls (e.g. parsed format
+     headers) which are codec-dependent. 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
+      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, 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.
+
+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 a request to which the
+following is attached:
+
+* The minimum amount of encoded data that can be independently decoded must be
+  provided into a buffer submitted to the ``OUTPUT`` queue. What constitutes the
+  "minimum amount of encoded data" is codec-dependent ; for H.264 this would be
+  a slice, for VP8 a frame.
+* All the metadata needed to decode the submitted encoded data, in the form of
+  controls relevant to the format being decoded.
+
+The 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.
+
+A typical frame would thus be decoded by repeating the following sequence until
+all the units of a frame are submitted:
+
+1. Queue an ``OUTPUT`` buffer containing one unit worth 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.
+
+      ``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 buffer and can also be used to specify this
+          frame as the reference of another. If a frame is made of several
+          units, then their ``OUTPUT`` buffers must all bear the same timestamp.
+          Starting a new frame before all the encoded units of the current frame
+          are queued will result in the frame being returned partially decoded,
+          with the ``V4L2_BUF_FLAG_ERROR`` flag set.
+
+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_IOC_REQUEST_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.
+
+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`` buffer can be
+used by the client for as long as they are not queued again. "Used" here
+encompasses using the buffer for compositing, display, or as a reference frame
+to decode a subsequent frame.
+
+A frame can be used 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.
+However, a decoded buffer containing a reference frame must not be queued to the
+``CAPTURE`` queue again before all the decoded frames referencing it have been
+dequeued, since queuing a ``CAPTURE`` buffer resets its timestamp.
+
+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
+can e.g. close the DMABUF file descriptors of the reference frame buffers if it
+won't need it afterwards, as long as the V4L2 ``CAPTURE`` buffer of the
+reference frame is not re-queued before all referencing frames are decoded.
+
+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.
+
+Pause
+=====
+
+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.
+
+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.
-- 
2.20.1.321.g9e740568ce-goog