linux-kernel - Re: [PATCH v2] media: docs-rst: Document m2m stateless video decoder interface

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Date:   Tue, 22 Jan 2019 17:19:28 +0900
From:   Alexandre Courbot <acourbot@...omium.org>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Tomasz Figa <tfiga@...omium.org>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Pawel Osciak <posciak@...omium.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] media: docs-rst: Document m2m stateless video decoder interface

On Tue, Jan 22, 2019 at 5:06 PM Hans Verkuil <hverkuil@...all.nl> wrote:
>
> On 01/22/2019 07:26 AM, Alexandre Courbot wrote:
> > 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@...omium.org>
> > ---
> > Changes since v1:
> >
> > * Use timestamps instead of tags to reference frames,
> > * Applied Paul's suggestions to not require one frame worth of data per OUTPUT
> >   buffer
> >
> > One of the effects of requiring sub-frame units to be submitted per request is
> > that the stateless decoders are not exactly "stateless" anymore: if a frame is
> > made of several slices, then the decoder must keep track of the buffer in which
> > the current frame is being decoded between requests, and all the slices for the
> > current frame must be submitted before we can consider decoding the next one.
> >
> > Also if we decide to force clients to submit one slice per request, then doesn't
> > some of the H.264 controls need to change? For instance, in the current v2
> > there is still a v4l2_ctrl_h264_decode_param::num_slices member. It is used in
> > Chromium to specify the number of slices given to the
> > V4L2_CID_MPEG_VIDEO_H264_SLICE_PARAMS control, but is apparently ignored by the
> > Cedrus driver. Maxime, can you comment on this?
> >
> >  Documentation/media/uapi/v4l/dev-codec.rst    |   5 +
> >  .../media/uapi/v4l/dev-stateless-decoder.rst  | 378 ++++++++++++++++++
> >  2 files changed, 383 insertions(+)
> >  create mode 100644 Documentation/media/uapi/v4l/dev-stateless-decoder.rst
> >
>
> Thank you! I have uploaded a version of the V4L2 spec with this and the two older
> stateful codec patches applied:
>
> https://hverkuil.home.xs4all.nl/codec-api/uapi/v4l/dev-codec.html

Thanks! A v3 will likely be necessary (and I'll likely be more
reactive producing it) because of that one-slice-per-request
requirement. After discussing with Tomasz we think it would be
safer/simpler to require one frame per request in a first time, as we
initially agreed.

Anyway, we can discuss the details once Tomasz chimes in.