https://bugs.freedesktop.org/show_bug.cgi?id=101026
--- Comment #7 from dwagner jb5sgc1n.nya@20mm.eu --- (In reply to Alex Deucher from comment #6)
(In reply to dwagner from comment #5)
[ 133.973] (--) AMDGPU(0): HDMI max TMDS frequency 300000KHz
The message is from the xserver, not the driver. It's warning you because the clock for the mode exceeds the max TMDS clock as stated by the EDID.
Hmmm - when I "cat /usr/lib/firmware/edid/LG_EG9609_edid.bin | parse-edid", the output contains:
# Maximum pixel clock is 600MHz
Does the xserver have its own, flawed EDID parser?