When you say it 'prefers RGB 8-bit' - what happens if RGB isn't available as an option? Does it then flip to 4:4:4 YCrCb 8-bit? (I'm thinking if you did a custom EDID that said 'YCrCb only'?) I wonder if it's possible to create an EDID that says 4:2:2 and 4:2:0 only at 2160p50/60?
I don't think RGB can be removed from EDID, looks like it is mandatory.
YCbCr 4:4:4 can be forced with a driver hack but I suppose there is not much use for this mode.
4:2:2 is not even present in the driver source code, so it can't be forced.
4:2:0 12-bit can probably be forced with a driver hack but I'm not sure. Driver assume that RGB 8-bit is the highest quality mode for 2160p50/60 and use it instead of 4:2:0 12-bit.
Is this discussion here relevant to this issue [GLK] no signal - with samsung 4k TV - HDMI UHD Color (ENABLED) (#271) · Issues · drm / intel · GitLab ?
It appear to be some sort of compatibility issue with Gemini Lake's HDMI 2.0 implementation and some Samsung TVs.
I wonder if Intel don't 'get' this issue yet?
HDR on Linux with Intel hardware is not a thing yet. I'm sure things will change.