[libre-riscv-dev] Video planning

[Lauri Kasanen]

Hi,

Here's some preliminary planning for the video part. Top-level headings.
Any actual work will need to know the ISA.

Decoding is prioritized over encoding. In general, faster decoding
speeds up a part of encoding.

Codecs to target:
Audio: MP3, AC3, Vorbis, Opus
Video: MPEG1/2, MPEG4 ASP (xvid), H.264, H.265, VP8, VP9, AV1

YUV-RGB conversion for the most common formats:
rgb/bgr24, rgbx/bgrx/xrgb/xbgr32, nv12, nv21

Most of these are DCT-based, meaning the instructions will likely
benefit image formats like JPEG as well. However image formats are not
a part of this project, though MJPEG could be considered a video codec?
Should we include it and thus speed up JPEG decoding?

Audio was not explicitly mentioned in the proposal, but fast audio
decoding is necessary for smooth video playback. Otherwise a 300MHz
chip may not even play Opus in realtime.

The entropy part of codecs similarly has overlap with general
compression.

Any speedups should really be compared to C (what gcc generates), not a
hand-made simple asm implementation. Both for no use in trying to match
the compiler for simple loops, and for potential new instructions
possibly needing entirely different approaches. We'll also need some
guideline on what parallelism to target in the simulator (how many ops
we can assume will be completed at once).

The software part will be ffmpeg and binutils forks until the standards
are set and  hw exists; then submission; then other projects
(gstreamer, lib*).

- Lauri