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

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Date:   Tue, 22 Jan 2019 09:05:54 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Alexandre Courbot <acourbot@...omium.org>,
        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@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] media: docs-rst: Document m2m stateless video decoder
 interface

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

Regards,

	Hans