On Mon, Aug 12, 2019 at 8:07 PM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote:
>
> From: Alexandre Courbot <acourbot@xxxxxxxxxxxx>
>
> 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>
> ---
> Documentation/media/uapi/v4l/dev-mem2mem.rst | 1 +
> .../media/uapi/v4l/dev-stateless-decoder.rst | 424 ++++++++++++++++++
> 2 files changed, 425 insertions(+)
> create mode 100644 Documentation/media/uapi/v4l/dev-stateless-decoder.rst
>
> diff --git a/Documentation/media/uapi/v4l/dev-mem2mem.rst b/Documentation/media/uapi/v4l/dev-mem2mem.rst
> index caa05f5f6380..70953958cee6 100644
> --- a/Documentation/media/uapi/v4l/dev-mem2mem.rst
> +++ b/Documentation/media/uapi/v4l/dev-mem2mem.rst
> @@ -46,3 +46,4 @@ devices are given in the following sections.
> :maxdepth: 1
>
> dev-decoder
> + dev-stateless-decoder
> 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..4a26646eeec5
> --- /dev/null
> +++ b/Documentation/media/uapi/v4l/dev-stateless-decoder.rst
> @@ -0,0 +1,424 @@
> +.. 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`.
Re-reading this I think we should add a step describing how to check
the capabilities on the OUTPUT queue, e.g.
2. To check that the drivers supports requests and (optionally) holding capture
buffers, the client calls :c:func:`VIDIOC_REQBUFS` on the ``OUTPUT``
queue:
* **Required fields:**
``count``
must be 0, so no buffers are allocated by this operation.
``type``
a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
``memory``
must be ``V4L2_MEMORY_MMAP``.
* **Return fields:**
``capabilities``
capabilities of the queue. The client can confirm that the
V4L2_BUF_CAP_SUPPORTS_REQUESTS and
V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF flags are
set.
> +
> +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.
As discussed on another patch, can we specify that V4L2_DEC_CMD_FLUSH
is valid (or maybe even must be used) even if
V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF is not used? This would make
clients simpler.
> +
> +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.
> --
> 2.20.1
>
