I have a Radeon HD 4350/4550 (RV770) card in my notebook. After resuming from
suspend, there is a significant 3D performance drop (a bit less than 1/4 of
original performance remains). The performance drop is exactly at the same
level as it is after "echo low >
/sys/class/drm/card0/device/power_dpm_force_performance_level", albeit
power_dpm_force_performance_level still contains "auto".

Bug is found in Ubuntu 14.04, but it was there at least since 13.10 (I have
just found the workaround). The bug is there with kernel 3.13 (official), with
3.14 mainline and 3.15-RC8 which was build for Ubuntu 14.10.

Workaround: when resume, execute "echo auto >
/sys/class/drm/card0/device/power_dpm_force_performance_level".