I see this happening on Radeon HD 7950, kernel 3.13, which has the radeon dpm
enabled by dafault. It is intermittent, happens on ~30% boots, and causes hang
followed by reboot (no panic or oops msgs). Unfortunately, I could only catch
the dmesg output via serial, when my machine hangs, kernel logs are not even
saved to the disk.

Now, I see "[drm:si_dpm_set_power_state] *ERROR* si_disable_ulv failed" on
serial only when ignore_loglevel kernel parameter is unset. Machine will hang
ad reboot each time I spot it.
I attached here the dmesg with ignore_loglevel and drm.debug=1 params, both
failure and ok cases, for comparison. When failing, the machine just hangs
breefly, and reboots, right after "[drm]    pitch is 7680" message.

With radeon.dpm=0 parameter, this problem NEVER happns!