Hi Hans On Fri, 28 Jun 2019 at 15:34, 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). >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 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 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. > > 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
