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

[Tomasz Figa <tfiga@chromium.org>]

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...

> 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