Le mercredi 03 juillet 2019 à 17:46 +0900, Tomasz Figa a écrit : > On Sat, Jun 29, 2019 at 3:09 AM Nicolas Dufresne <nico...@ndufresne.ca> 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.
Consider this a feedback, but the mix of parsing with decoding, along with the name of the method "::AdvanceFrameFragment". Thanks for pointing to this code. Was there any HW where this split was strictly required ? > > Best regards, > Tomasz
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