(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?