linux-kernel - Re: [PATCH v3 1/2] media: docs-rst: Document memory-to-memory video decoder interface

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Date:   Thu, 31 Jan 2019 11:45:11 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Tomasz Figa <tfiga@...omium.org>, linux-media@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Pawel Osciak <posciak@...omium.org>,
        Alexandre Courbot <acourbot@...omium.org>,
        Kamil Debski <kamil@...as.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Jeongtae Park <jtp.park@...sung.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Tiffany Lin (林慧珊) 
        <tiffany.lin@...iatek.com>,
        Andrew-CT Chen (陳智迪) 
        <andrew-ct.chen@...iatek.com>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Todor Tomov <todor.tomov@...aro.org>,
        Nicolas Dufresne <nicolas@...fresne.ca>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        dave.stevenson@...pberrypi.org,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Maxime Jourdan <maxi.jourdan@...adoo.fr>
Subject: Re: [PATCH v3 1/2] media: docs-rst: Document memory-to-memory video
 decoder interface

On 1/24/19 11:04 AM, 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@...omium.org>
> ---
>  Documentation/media/uapi/v4l/dev-decoder.rst  | 1076 +++++++++++++++++
>  Documentation/media/uapi/v4l/dev-mem2mem.rst  |    5 +
>  Documentation/media/uapi/v4l/pixfmt-v4l2.rst  |    5 +
>  Documentation/media/uapi/v4l/v4l2.rst         |   10 +-
>  .../media/uapi/v4l/vidioc-decoder-cmd.rst     |   40 +-
>  Documentation/media/uapi/v4l/vidioc-g-fmt.rst |   14 +
>  6 files changed, 1135 insertions(+), 15 deletions(-)
>  create mode 100644 Documentation/media/uapi/v4l/dev-decoder.rst
> 

<snip>

> +4.  **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
> +    buffers will be processed and returned to the client in order, until
> +    required metadata to configure the ``CAPTURE`` queue are found. This is
> +    indicated by the decoder sending a ``V4L2_EVENT_SOURCE_CHANGE`` event with
> +    ``V4L2_EVENT_SRC_CH_RESOLUTION`` source change type.
> +
> +    * It is not an error if the first buffer does not contain enough data for
> +      this to occur. Processing of the buffers will continue as long as more
> +      data is needed.
> +
> +    * If data in a buffer that triggers the event is required to decode the
> +      first frame, it will not be returned to the client, until the
> +      initialization sequence completes and the frame is decoded.
> +
> +    * If the client sets width and height of the ``OUTPUT`` format to 0,
> +      calling :c:func:`VIDIOC_G_FMT`, :c:func:`VIDIOC_S_FMT`,
> +      :c:func:`VIDIOC_TRY_FMT` or :c:func:`VIDIOC_REQBUFS` on the ``CAPTURE``
> +      queue will return the ``-EACCES`` error code, until the decoder
> +      configures ``CAPTURE`` format according to stream metadata.

I think this should also include the G/S_SELECTION ioctls, right?

Regards,

	Hans