Re: [PATCH 1/2] media: docs-rst: Document memory-to-memory video decoder interface

[Hans Verkuil <hverkuil@xs4all.nl>]

On 07/24/2018 04:06 PM, Tomasz Figa wrote:
> Due to complexity of the video decoding process, the V4L2 drivers of
> stateful decoder hardware require specific sequences of V4L2 API calls
> to be followed. These include capability enumeration, initialization,
> decoding, seek, pause, dynamic resolution change, drain and end of
> stream.
> 
> Specifics of the above have been discussed during Media Workshops at
> LinuxCon Europe 2012 in Barcelona and then later Embedded Linux
> Conference Europe 2014 in Düsseldorf. The de facto Codec API that
> originated at those events was later implemented by the drivers we already
> have merged in mainline, such as s5p-mfc or coda.
> 
> The only thing missing was the real specification included as a part of
> Linux Media documentation. Fix it now and document the decoder part of
> the Codec API.
> 
> Signed-off-by: Tomasz Figa <tfiga@chromium.org>
> ---
>  Documentation/media/uapi/v4l/dev-decoder.rst | 872 +++++++++++++++++++
>  Documentation/media/uapi/v4l/devices.rst     |   1 +
>  Documentation/media/uapi/v4l/v4l2.rst        |  10 +-
>  3 files changed, 882 insertions(+), 1 deletion(-)
>  create mode 100644 Documentation/media/uapi/v4l/dev-decoder.rst
> 
> diff --git a/Documentation/media/uapi/v4l/dev-decoder.rst b/Documentation/media/uapi/v4l/dev-decoder.rst
> new file mode 100644
> index 000000000000..f55d34d2f860
> --- /dev/null
> +++ b/Documentation/media/uapi/v4l/dev-decoder.rst
> @@ -0,0 +1,872 @@

<snip>

> +6.  This step only applies to coded formats that contain resolution
> +    information in the stream. Continue queuing/dequeuing bitstream
> +    buffers to/from the ``OUTPUT`` queue via :c:func:`VIDIOC_QBUF` and
> +    :c:func:`VIDIOC_DQBUF`. The driver must keep processing and returning
> +    each buffer to the client until required metadata to configure the
> +    ``CAPTURE`` queue are found. This is indicated by the driver sending
> +    a ``V4L2_EVENT_SOURCE_CHANGE`` event with
> +    ``V4L2_EVENT_SRC_CH_RESOLUTION`` source change type. There is no
> +    requirement to pass enough data for this to occur in the first buffer
> +    and the driver must be able to process any number.
> +
> +    * If data in a buffer that triggers the event is required to decode
> +      the first frame, the driver must not return it to the client,
> +      but must retain it for further decoding.
> +
> +    * If the client set width and height of ``OUTPUT`` format to 0, calling
> +      :c:func:`VIDIOC_G_FMT` on the ``CAPTURE`` queue will return -EPERM,
> +      until the driver configures ``CAPTURE`` format according to stream
> +      metadata.

What about calling TRY/S_FMT on the capture queue: will this also return -EPERM?
I assume so.

Regards,

	Hans