[Mesa-dev] hardware xvmc video decoding with nouveau

[Younes Manton]

Hi,

I tried to give some thought to how this interface would work for
decoders that can't or would prefer not to support multiple decode
buffers, and most of the scenarios I came up with seem to work out, so
overall I'm not opposed to it. Even though I still think this can be
done by each driver without being in the interface, I agree it
simplifies things and keeps most of the problems in XvMC rather than
in every state tracker.

Anyway, here are some specific comments:

+   for (; num_macroblocks > 0; --num_macroblocks) {
+      mb->base.codec = PIPE_VIDEO_CODEC_MPEG12;
+      mb->macroblock_address = xvmc_mb->x +
context->width_in_macroblocks * xvmc_mb->y;
+      mb->macroblock_type = xvmc_mb->macroblock_type;

Why macroblock_address? A decoder that prefers x/y will have to do a
div/mod to get it back, so it's better to just pass x/y along and let
each decoder do what it wants with it.

+   void (*begin_frame)(struct pipe_video_decoder *decoder);
+   void (*end_frame)(struct pipe_video_decoder *decoder);

The Nvidia HW decoder doesn't need these, but I think
begin_frame/end_frame are not as useful as they can be because you
don't always know which video buffers they will apply to. When you
call this in RecursiveEndFrame() it can result in 0, 1, 2, or 3
end_frame() calls, but which video buffers does it apply to? The 0 and
3 case are obvious, but 1 or 2 is ambigious if the current video
buffer has 1 or 2 reference frames.

Also, based on the above, I think there is a bug in
vl_mpeg12_end_frame(struct pipe_video_decoder *decoder). It will unmap
and destroy the tex_transfers attached to the current buf, but what
about the reference frames? It has the same problem, it doesn't know
about the reference frames when end_frame is called so it operates on
the same buf multiple times in RecursiveEndFrame().

+      vldecoder->cur_buffer %= vldecoder->num_buffers;
+
+      vldecoder->decoder->set_decode_buffer(vldecoder->decoder,
vldecoder->buffers[vldecoder->cur_buffer]);
+      vldecoder->decoder->set_decode_target(vldecoder->decoder,
vlsurf->video_buffer);
+
+      return vlVdpDecoderRenderMpeg12(vldecoder->decoder,
(VdpPictureInfoMPEG1Or2 *)picture_info,
+                                      bitstream_buffer_count,
bitstream_buffers);

Not sure I understand why this is necessary. Why do you need to use a
different decode buffer for each frame? In the XvMC case you want to
keep data for each target video buffer seperate, because the XvMC API
allows you to submit macroblocks to any surface at any time. Why is
this necessary for VDPAU if it requires the user to submit the entire
frame at once?

Those are the issues that I noticed at first. Martin may have some
comments specific to his work with VPE. What do you think?