With the current version of mesa (20.2.4) I cannot use hardware accelerated video decoding using my radeon gpu on riscv64 with the exact symptoms described in this upstream bug report.

I have locally updated to mesa 21.3.3 (ebuild borrowed from Gentoo) for testing, and with that version - as described by the linked bug report (>= 21) - hardware accelerated video decoding works as it's supposed to (1080p h264 video playing smoothly instead of showing only every couple of frames).