linux-kernel - Re: [PATCH 0/2] Document memory-to-memory video codec interfaces

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Date:   Wed, 25 Jul 2018 22:35:34 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Philipp Zabel <p.zabel@...gutronix.de>
Cc:     Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil@...all.nl>,
        Pawel Osciak <posciak@...omium.org>,
        Alexandre Courbot <acourbot@...omium.org>, kamil@...as.org,
        a.hajda@...sung.com, Kyungmin Park <kyungmin.park@...sung.com>,
        jtp.park@...sung.com,
        Tiffany Lin (林慧珊) 
        <tiffany.lin@...iatek.com>,
        Andrew-CT Chen (陳智迪) 
        <andrew-ct.chen@...iatek.com>, todor.tomov@...aro.org,
        nicolas@...fresne.ca,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        dave.stevenson@...pberrypi.org,
        Ezequiel Garcia <ezequiel@...labora.com>
Subject: Re: [PATCH 0/2] Document memory-to-memory video codec interfaces

Hi Philipp,

On Wed, Jul 25, 2018 at 10:28 PM Philipp Zabel <p.zabel@...gutronix.de> wrote:
>
> Hi Tomasz,
>
> On Tue, 2018-07-24 at 23:06 +0900, Tomasz Figa wrote:
> > This series attempts to add the documentation of what was discussed
> > during Media Workshops at LinuxCon Europe 2012 in Barcelona and then
> > later Embedded Linux Conference Europe 2014 in Düsseldorf and then
> > eventually written down by Pawel Osciak and tweaked a bit by Chrome OS
> > video team (but mostly in a cosmetic way or making the document more
> > precise), during the several years of Chrome OS using the APIs in
> > production.
> >
> > Note that most, if not all, of the API is already implemented in
> > existing mainline drivers, such as s5p-mfc or mtk-vcodec. Intention of
> > this series is just to formalize what we already have.
> >
> > It is an initial conversion from Google Docs to RST, so formatting is
> > likely to need some further polishing. It is also the first time for me
> > to create such long RST documention. I could not find any other instance
> > of similar userspace sequence specifications among our Media documents,
> > so I mostly followed what was there in the source. Feel free to suggest
> > a better format.
> >
> > Much of credits should go to Pawel Osciak, for writing most of the
> > original text of the initial RFC.
> >
> > Changes since RFC:
> > (https://lore.kernel.org/patchwork/project/lkml/list/?series=348588)
> >  - The number of changes is too big to list them all here. Thanks to
> >    a huge number of very useful comments from everyone (Philipp, Hans,
> >    Nicolas, Dave, Stanimir, Alexandre) we should have the interfaces much
> >    more specified now. The issues collected since previous revisions and
> >    answers leading to this revision are listed below.
>
> Thanks a lot for the update, and especially for the nice Q&A summary of
> the discussions so far.
>
> [...]
> > Decoder issues
> >
> [...]
> >   How should ENUM_FRAMESIZES be affected by profiles and levels?
> >
> >   Answer: Not in current specification - the logic is too complicated and
> >   it might make more sense to actually handle this in user space.  (In
> >   theory, level implies supported frame sizes + other factors.)
>
> For decoding I think it makes more sense to let the hardware decode them
> from the stream and present them as read-only controls, such as:
>
> 42a68012e67c ("media: coda: add read-only h.264 decoder profile/level
> controls")

To clarify, this point is only about the effect on ENUM_FRAMESIZES.
Profile and level controls are mentioned in capabilities enumeration,
but it may make sense to add optional steps of querying them in
Initialization sequence.

>
> if possible. For encoding, the coda firmware determines level from
> bitrate and coded resolution, itself, so I agree not making this part of
> the spec is a good idea for now.

Encoder controls are driver-specific in general, since the encoding
capabilities vary a lot, so I decided to just briefly mention the
general idea of encoding parameters in "Encoding parameter changes"
section. It could be a good idea to add a reference to the MPEG
control documentation there, though.

Best regards,
Tomasz