.. SPDX-License-Identifier: GPL-2.0
.. c:namespace:: V4L

.. _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.

    * **Returned 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.

    * **Returned 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.
