Oh no, @robertoj I made an error. I did not mean to mislead. I meant to say that it works on my rk322x-box, with that version. I neglected that datum. I don't use windowing environments on my box, so it was the
mpv-armhf --vo=gpu --hwdec=drm --gpu-hwdec-interop=drmprime-drm --drm-draw-plane=overlay --drm-drmprime-video-plane=primary <video.mp4>
command. Actually, it needs to be modified because that command gives the warning
[vo/gpu] gpu-hwdec-interop was selected with the legacy name 'drmprime-drm'. Please change it to 'drmprime-overlay' in your config file or command line.
Fortunately, it tells you exactly what to do, and I can affirm that drmprime-overlay works without the warning. H264 1080p60 over HDMI works fine, even coupled to streamlink. H265 did not work fine. I am not equipped to test more highly.
Thank you for the detail, @jock. Trying to get things working naively, I managed to get the FFMPEG build with rkmpp support from https://github.com/hbiyik/FFmpeg linked against librga and libmpp, but that requires the old kernel, apparently. It is very difficult to find any exposition as to how these various iterations of the various subsystems work!