On Sat, Jun 29, 2019 at 3:09 AM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> wrote: > > Le vendredi 28 juin 2019 à 16:34 +0200, Hans Verkuil a écrit : > > 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? > > > > > > 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. > > > > Let's see what the results are. > > Hans though a summary of what existing userspace expects / assumes > would be nice. > > GStreamer: > ========== > Encodes: > fwht, h263, h264, hevc, jpeg, mpeg4, vp8, vp9 > Decodes: > fwht, h263, h264, hevc, jpeg, mpeg2, mpeg4, vc1, vp8, vp9 > > It assumes that each encoded v4l2_buffer contains exactly one frame > (any format, two fields for interlaced content). It may still work > otherwise, but some issues will appear, timestamp shift, lost of > metadata (e.g. timecode, cc, etc.). > > FFMpeg: > ======= > Encodes: > h263, h264, hevc, mpeg4, vp8 > Decodes: > h263, h264, hevc, mpeg2, mpeg4, vc1, vp8, vp9 > > Similarly to GStreamer, it assumes that one AVPacket will fit one > v4l2_buffer. On the encoding side, it seems less of a problem, but they > don't fully implement the FFMPEG CODEC API for frame matching, which I > suspect would create some ambiguity if it was. > > Chromium: > ========= > Decodes: > H264, VP8, VP9 > Encodes: > H264 VP8 too. It can in theory handle any format V4L2 could expose, but these 2 seem to be the only commonly used codecs used in practice and supported by hardware. > > That is the code I know the less, but the encoder does not seem > affected by the nal alignment. The keyframe flag and timestamps seems > to be used and are likely expected to correlate with the input, so I > suspect that there exist some possible ambiguity if the output is not > full frame. For the decoder, I'll have to ask someone else to comment, > the code is hard to follow and I could not get to the place where > output buffers are filled. I thought the GStreamer code was tough, but > this is quite similarly a mess. Not sure what's so complicated there. There is a clearly isolated function that does the parsing: https://cs.chromium.org/chromium/src/media/gpu/v4l2/v4l2_video_decode_accelerator.cc?rcl=2880fe4f6b246809f1be72c5a5698dced4cd85d1&l=984 It puts special NALUs like SPS and PPS in separate buffers and for frames it's 1 frame (all slices of the frame) : 1 buffer. Best regards, Tomasz
