--- /dev/null
+.. 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.
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