(In reply to Jonas from comment #14)
> I get the same behaviour here with my XFX R9 390, latest Arch Linux and
> amdgpu driver. Forcing "high" power profile makes it work, but it also
> raises the temps (and probably power draw) too much to my liking.
> 
> If we could only up the vram frequency but not the core, maybe it would be a
> more acceptable situation of temps/features.

You should be able to do this eg. on my R9 285

echo manual > /sys/class/drm/card0/device/power_dpm_force_performance_level

cat /sys/class/drm/card0/device/pp_dpm_mclk 
0: 150Mhz *
1: 1375Mhz 

echo 1 > /sys/class/drm/card0/device/pp_dpm_mclk

cat /sys/class/drm/card0/device/pp_dpm_mclk 
0: 150Mhz 
1: 1375Mhz *

Though it will undo if you change modes or go into DPMS.