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) Best regards, Tomasz
