Le vendredi 28 juin 2019 à 16:21 +0100, Dave Stevenson a écrit : > Hi Hans > > On Fri, 28 Jun 2019 at 15:34, Hans Verkuil <hverk...@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). > > From the discussion that started this thread, with H264 and similar, > does the V4L2 buffer contain just the frame data, or the SPS/PPS > headers as well.
In existing mainline encoder driver the SPS/PPS is included in the first frame produced. Decoders expect them to be in the first frame queued. For decoder, this is being relaxed now that we have a mechanism to notify the state change after the header has been processed. > > > 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? > > > > > > 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 18.104.22.168 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. > > > > Specifically, the venus decoder needs to know the resolution of the coded > > video > > beforehand 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. > > On BCM283x: > > The underlying decoder will accept anything, but giving it a single > frame per buffer reduces latency as the bitstream parser gets kicked > earlier. Based on previous discussions I am setting the flag so that > it expects one compressed frame per buffer, but I don't believe it > goes wrong should that not be the case (it'll just waste a bit of > processing effort). > It'll parse the headers and produce a V4L2_EVENT_SOURCE_CHANGE event > should the capture queue format not match the stream parameters. > Interlacing isn't supported yet (it's on the list), but I believe the > hardware produces the equivalent to V4L2_FIELD_INTERLACED_[TB|BT]. > > The encoder currently spits out the H264 SPS/PPS headers as a separate > V4L2 buffer, and then one compressed frame per V4L2 buffer (provided > the buffer is big enough). Should > V4L2_CID_MPEG_VIDEO_REPEAT_SEQ_HEADER be set, then it will repeat the > headers in an independent V4L2 buffer before each I frame. > I'm quite happy to amend this should we have a decent spec of what is > required. As I've never found a spec it's been trial and error until > now. > There is no interlaced support available. > > Dave
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