not quite it, not on master branch yet (look for "hdr" branch for recent rebase) but for some Arch users it worked?

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With the already noted exception that mpv needs to be run once and then ran again before the monitor enters HDR mode, this seems to be working for me with the following options:

--no-config --target-trc=pq --target-peak=400 --video-output-levels=full --target-prim=dci-p3 --hdr-compute-peak=yes --gamut-clipping=no --tone-mapping=mobius --vo=gpu --gpu-api=opengl --gpu-context=drm --drm-send-hdr-meta=auto --drm-connector=HDMI-A-0

I'm on an arch based system. linux 5.10. AMDGPU. Connected over HDMI to an Acer XV272U

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