Thank you. There’s a small tweak to Kodi that lets you select 422 at 4k60Hz but it hasn’t made it into the repos yet. But you should be able to select that from the commandline.
If you have a minute, you could try first forcing 8 bits with
echo 8bitnow | sudo tee /sys/class/amhdmitx/amhdmitx0/attr
Then set a 4k60Hz GUI the usual way
Then try to set 4k60Hz 444 with
echo 2160p60hz | sudo tee /sys/class/display/mode
this may well blankscreen. If it does,
echo 2160p60hz420 | sudo tee /sys/class/display/mode
… and reset the GUI resolution using the GUI
if it doesn’t blankscreen,
echo 42210bitnow | sudo tee /sys/class/amhdmitx/amhdmitx0/attr
if you still have a picture (GUI), that will be interesting. please cat config to confirm.
Playing a 4k60Hz vid now will still blankscreen so no need to try that.