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

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Date:   Fri, 05 Apr 2019 12:59:25 +0200
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Tomasz Figa <tfiga@...omium.org>, linux-media@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Hans Verkuil <hverkuil@...all.nl>,
        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>,
        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 Thu, 2019-01-24 at 19:04 +0900, 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
> 
> diff --git a/Documentation/media/uapi/v4l/dev-decoder.rst b/Documentation/media/uapi/v4l/dev-decoder.rst
> new file mode 100644
> index 000000000000..b7db2352ad41
> --- /dev/null
> +++ b/Documentation/media/uapi/v4l/dev-decoder.rst
> @@ -0,0 +1,1076 @@
> +.. -*- coding: utf-8; mode: rst -*-
> +
> +.. _decoder:
> +
> +*************************************************
> +Memory-to-memory Stateful Video Decoder Interface
> +*************************************************
> +
[...]
> +
> +Drain
> +=====
> +
[...]
> +
> +3. Once all the ``OUTPUT`` buffers queued before the ``V4L2_DEC_CMD_STOP`` call
> +   are dequeued and the last ``CAPTURE`` buffer is dequeued, the decoder is
> +   stopped and it will accept, but not process, any newly queued ``OUTPUT``
> +   buffers until the client issues any of the following operations:
> +
[...]
> +
> +.. note::
> +
> +   Once the drain sequence is initiated, the client needs to drive it to
> +   completion, as described by the steps above, unless it aborts the process by
> +   issuing :c:func:`VIDIOC_STREAMOFF` on any of the ``OUTPUT`` or ``CAPTURE``
> +   queues.  The client is not allowed to issue ``V4L2_DEC_CMD_START`` or
> +   ``V4L2_DEC_CMD_STOP`` again while the drain sequence is in progress and they
> +   will fail with -EBUSY error code if attempted.

Should the V4L2_DEC_CMD_STOP documentation in
Documentation/media/uapi/v4l/vidioc-decoder-cmd.rst:

  * - ``V4L2_DEC_CMD_STOP``
    - 1
    - Stop the decoder. When the decoder is already stopped, this
      command does nothing.

be updated to mention that for mem2mem decoders this not only does
nothing, but returns an -EBUSY error return value?

Also, if V4L2_DEC_CMD_STOP_IMMEDIATELY is set, should the decoder drop
all pending buffers and signal EOS immediately? Should this be mentioned
here?

> +   Although mandatory, the availability of decoder commands may be queried
> +   using :c:func:`VIDIOC_TRY_DECODER_CMD`.

Do the TRY_DECODER_CMD also return -EBUSY during the draining phase?

regards
Philipp