(In reply to Bastian from comment #73)
> (In reply to Bastian from comment #69)
> > Distro/Kernel: Ubuntu 18.04.2/5.0.2
> > GPU AMD:       Readon RX570 - Mesa 19.0
> > Display:       Iiyama G23530HSU  75hz
> > Desktop:       Gnome
> > 
> > video: https://www.youtube.com/watch?v=gNJ2kJ8hsHA
> > 
> > Have the same problem as the op. The problems started with the 4.20 Kernels.
> > Had not this problem with 4.19 LTS Kernels. The flickering doesn't appear on
> > wayland if I choose only 60hz. On xorg it is irrelevant how much hz I use,
> > it flickers regardless. 
> > 
> > sudo -i
> > echo "manual" >
> > /sys/class/drm/card0/device/power_dpm_force_performance_level 
> > echo "2" > /sys/class/drm/card0/device/pp_dpm_mclk
> > 
> > Fixes the flickering.
> 
> Just a quick update. After switching from HDMI to DisplayPort the issue
> resolved itself for me.

That's interesting. It's the opposite for me - only DP flickers, not HDMI.
Lots of people on reddit have been discovering this as they're coming from
older kernels, only this week I've helped 4 different posters asking about it.
We'd like to hear anything from devs at this point so I'll CC this to AMD's DC
dev Nicholas Kazlauskas.