Re: [virtio-dev] Re: [RFC PATCH v6] virtio-video: Add virtio video device specification

[Alexander Gordeev <alexander.gordeev@opensynergy.com>]

Hello Cornelia,

On 20.12.22 10:51, Cornelia Huck wrote:
> On Mon, Dec 19 2022, Alexander Gordeev <alexander.gordeev@opensynergy.com> wrote:
>
> > Hello Alexandre,
> >
> > Thanks for the update. Please check my comments below.
> > I'm new to the virtio video spec development, so I may lack some
> > historic perspective. I would gladly appreciate pointing me to some
> > older emails explaining decisions, that I might not understand. I hope
> > to read through all of them later. Overall I have a lot of experience in
> > the video domain and in virtio video device development in Opsy, so I
> > hope, that my comments are relevant and useful.
> Thank you for commenting! I know the virtio spec, but I'm not familiar
> with the video part, so your feedback is highly appreciated :)
>
> Prior discussions should be in the virtio-dev archives; at least v3
> (https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2flists.oasis%2dopen.org%2farchives%2fvirtio%2ddev%2f202002%2fmsg00002.html&umid=ce4587fa-5f66-4a08-be71-c7fad0355b4c&auth=53c7c7de28b92dfd96e93d9dd61a23e634d2fbec-8ac7749933f48b779f5d37c4630ba2d37c15b0eb)
> and v4
> (https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2flists.oasis%2dopen.org%2farchives%2fvirtio%2ddev%2f202006%2fmsg00072.html&umid=ce4587fa-5f66-4a08-be71-c7fad0355b4c&auth=53c7c7de28b92dfd96e93d9dd61a23e634d2fbec-f5d8196d57b85625d3a8262d32ad87a88d5d6bee)
> seem to have gotten some feedback.
>
> [A note to Alexandre: You'll want to send your patches to virtio-comment
> as well as virtio-dev; virtio-comment is for commenting on the spec
> including proposing updates, while virtio-dev is for discussing virtio
> development. Yes, I know that we haven't always been strict about that
> in the past.]

Thank you for the references! I'll take some time to read these
discussions through.


> > On 08.12.22 08:23, Alexandre Courbot wrote:
> (...)
>
> > > +\subsubsection{Image formats}\label{sec:Device Types / Video Device / Supported formats / Image formats}
> > > +
> > > +The fourcc code of each supported image format is given, as well as its
> > > +number of planes, physical buffers, and eventual subsampling.
> > > +
> > > +\begin{description}
> > > +\item[\field{RGB3}]
> > > +one RGB plane where each component takes one byte, i.e.~3 bytes per
> > > +pixel.
> > > +\item[\field{NV12}]
> > > +one Y plane followed by interleaved U and V data, in a single buffer.
> > > +4:2:0 subsampling.
> > > +\item[\field{NV12}]
> > > +same as \field{NV12} but using two separate buffers for the Y and UV
> > > +planes.
> > s/NV12/NM12/ ?
> >
> >
> > > +\item[\field{YU12}]
> > > +one Y plane followed by one Cb plane, followed by one Cr plane, in a
> > > +single buffer. 4:2:0 subsampling.
> > > +\item[\field{YM12}]
> > > +same as \field{YU12} but using three separate buffers for the Y, U and V
> > > +planes.
> > > +\end{description}
> > This looks like V4L2 formats. Maybe add a V4L2 reference? At least the
> > V4L2 documentation has a nice description of exact plane layouts.
> > Otherwise it would be nice to have these layouts in the spec IMO.
> Ah, so this is all V4L2? It looks like we really want to refer to the
> existing V4L2 headers (like we already do for FUSE in the virtiofs
> case). Are those headers sufficient to specify the formats, or do we
> need anything else?
Yes, these fourcc code seem to come from V4L2:
https://docs.kernel.org/userspace-api/media/v4l/pixfmt-yuv-planar.html
I also think we want to have a reference to V4L2 in the spec here, because:
1. V4L2 docs have the nice component/plane layouts that help convert or
match these format definitions with other APIs.
2. These fourcc codes are not used anywhere else except in V4L2. I think
this is the only place, that distinguishes coontiguous and non
contiguous buffers with a fourcc code. So without a reference this may
look quite weird. With a reference this looks logical, because I guess
any virtio video driver for a Linux guest would naturally implement V4L2.

--
Alexander Gordeev
Senior Software Engineer

OpenSynergy GmbH
Rotherstr. 20, 10245 Berlin

Phone: +49 30 60 98 54 0 - 88
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