(In reply to Alex Deucher from comment #3)
> Please attach your xorg log and dmesg output.

Upstream Linux kernel does not support variable mclk on R9 390. I enabled
variable mclk by patching some code in drivers/gpu/drm/amd/amdgpu/ (mostly
ci_dpm.c).

dmesg output contains some logging messages I added to my copy of Linux kernel
source code.

> What resolution and refresh rate are you using on your monitor?

1920x1080 60Hz

> Also are you using radeon or amdgpu?

Currently amdgpu.ko, radeon.ko in the past.