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

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Date:   Tue, 9 Apr 2019 19:28:09 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Philipp Zabel <p.zabel@...gutronix.de>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <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>,
        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 Fri, Apr 5, 2019 at 8:21 PM Hans Verkuil <hverkuil@...all.nl> wrote:
>
> On 4/5/19 12:59 PM, Philipp Zabel wrote:
> > 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?
>
> Yes. Basically in this case the STOP is still in progress, so returning
> EBUSY makes sense and should be documented.
>

Done. Also for the encoder.

> >
> > 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?
>
> Hmm, the decoder spec says that flags should be set to 0, and current codec
> drivers explicitly disallow a non-0 flags value.
>
> I'm not sure if there is a practical use-case for V4L2_DEC_CMD_STOP_IMMEDIATELY
> in the context of a decoder. If you want to stop immediately, then just call
> streamoff on the OUTPUT queue.
>

Agreed.

> >
> >> +   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?
>
> No.

Ack.

Best regards,
Tomasz

>
> Regards,
>
>         Hans
>
> >
> > regards
> > Philipp
> >
>