(In reply to comment #3)
> > Workaround: when resume, execute "echo auto >
> > /sys/class/drm/card0/device/power_dpm_force_performance_level".
> 
> How did you do that? It gives me "echo: write error: Invalid argument" on
> kernel 3.15

It simply works without any trick. It also works with stock kernel 3.13. Are
you sure that you have the same ATI card?

What I have found since I have posted the bug is that sometimes (very rarely)
this simple workaround is not enough. Now, I use the following:

echo low > /sys/class/drm/card0/device/power_dpm_force_performance_level
echo battery > /sys/class/drm/card0/device/power_dpm_state
echo auto > /sys/class/drm/card0/device/power_dpm_force_performance_level
echo balanced > /sys/class/drm/card0/device/power_dpm_state