On Thu, Jul 11, 2019 at 10:42 AM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> wrote: > > 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@xxxxxxxxx> 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) > > Interesting, do I read correctly that what the encoder does not produce > what the decoder needs ? Apparently. :) But given all the diversity that was mentioned in this thread, one can't expect to be able to feed a decoder with the exact buffers from an encoder, although first of all I'm not sure why one would even want to do that. Best regards, Tomasz
