Re: [RFC] Stateful codecs and requirements for compressed formats

[Nicolas Dufresne <nicolas@ndufresne.ca>]

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Le mercredi 03 juillet 2019 à 17:32 +0900, Tomasz Figa a écrit :
> Hi Hans,
> 
> On Fri, Jun 28, 2019 at 11:34 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
> > Hi all,
> > 
> > I hope I Cc-ed everyone with a stake in this issue.
> > 
> > One recurring question is how a stateful encoder fills buffers and how a stateful
> > decoder consumes buffers.
> > 
> > The most generic case is that an encoder produces a bitstream and just fills each
> > CAPTURE buffer to the brim before continuing with the next buffer.
> > 
> > I don't think there are drivers that do this, I believe that all drivers just
> > output a single compressed frame. For interlaced formats I understand it is either
> > one compressed field per buffer, or two compressed fields per buffer (this is
> > what I heard, I don't know if this is true).
> > 
> > In any case, I don't think this is specified anywhere. Please correct me if I am
> > wrong.
> > 
> > The latest stateful codec spec is here:
> > 
> > https://hverkuil.home.xs4all.nl/codec-api/uapi/v4l/dev-mem2mem.html
> > 
> > Assuming what I described above is indeed the case, then I think this should
> > be documented. I don't know enough if a flag is needed somewhere to describe
> > the behavior for interlaced formats, or can we leave this open and have userspace
> > detect this?
> > 
> 
> From Chromium perspective, we don't have any use case for encoding
> interlaced contents, so we'll be okay with whatever the interested
> parties decide on. :)
> 
> > For decoders it is more complicated. The stateful decoder spec is written with
> > the assumption that userspace can just fill each OUTPUT buffer to the brim with
> > the compressed bitstream. I.e., no need to split at frame or other boundaries.
> > 
> > See section 4.5.1.7 in the spec.
> > 
> > But I understand that various HW decoders *do* have limitations. I would really
> > like to know about those, since that needs to be exposed to userspace somehow.
> 
> AFAIK mtk-vcodec needs H.264 SPS and PPS to be split into their own
> separate buffers. I believe it also needs 1 buffer to contain exactly
> 1 frame and 1 frame to be fully contained inside 1 buffer.
> 
> Venus also needed 1 buffer to contain exactly 1 frame and 1 frame to
> be fully contained inside 1 buffer. It used to have some specific
> requirements regarding SPS and PPS too, but I think that was fixed in
> the firmware.
> 
> > Specifically, the venus decoder needs to know the resolution of the coded video
> > beforehand
> 
> I don't think that's true for venus. It does parsing and can detect
> the resolution.
> 
> However that's probably the case for coda...

I'm probably the worst person to have access to the documentation, but
from the documentation I have read, it seems like this is a limitation
of the amount of the driver. Unless it's a limitation of 960,
documentation I have is 970.

> 
> > and it expects a single frame per buffer (how does that work for
> > interlaced formats?).
> > 
> > Such requirements mean that some userspace parsing is still required, so these
> > decoders are not completely stateful.
> > 
> > Can every codec author give information about their decoder/encoder?
> > 
> > I'll start off with my virtual codec driver:
> > 
> > vicodec: the decoder fully parses the bitstream. The encoder produces a single
> > compressed frame per buffer. This driver doesn't yet support interlaced formats,
> > but when that is added it will encode one field per buffer.
> > 
> > Let's see what the results are.
> 
> s5p-mfc:
>  decoder: fully parses the bitstream,
>  encoder: produces a single frame per buffer (haven't tested interlaced stuff)
> 
> mtk-vcodec:
>  decoder: expects separate buffers for SPS, PPS and full frames
> (including some random stuff like SEIMessage),
>  encoder: produces a single frame per buffer (haven't tested interlaced stuff)
> 
> Best regards,
> Tomasz

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