media: docs-rst: Document m2m stateless video decoder interface

    :maxdepth: 1

    dev-decoder

    dev-stateless-decoder

.. 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 :ref:`media-request-api`. A stateless

decoder must 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.

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 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 bytestream.

   * **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 the 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 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.

      ``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 (bytestream) 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 bytestream 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 (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

can e.g. close the DMABUF file descriptors of reference frame buffers if it

won't need them afterwards.

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 bytestream 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. 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, then the

   last frame is not available on the ``CAPTURE`` queue. In this case, a

   ``V4L2_DEC_CMD_FLUSH`` command shall be sent. This will make the driver

   dequeue the held ``CAPTURE`` buffer.

2. Wait until all submitted requests have completed and dequeue the

   corresponding output buffers.

3. Call :c:func:`VIDIOC_STREAMOFF` on both the ``OUTPUT`` and ``CAPTURE``

   queues.

4. Free all ``CAPTURE`` buffers by calling :c:func:`VIDIOC_REQBUFS` on the

   ``CAPTURE`` queue with a buffer count of zero.

5. 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

=====

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, then the

last frame is not available on the ``CAPTURE`` queue. In this case, a

``V4L2_DEC_CMD_FLUSH`` command shall be sent. This will make the driver

dequeue the held ``CAPTURE`` buffer.

After that, in order to drain the stream on a stateless decoder, the client

just needs to wait until all the submitted requests are completed.