Created attachment 113783 [details]
description of my system

DPM is not enabled:
the command "dmesg | grep dpm" has no input
sudo cat /sys/kernel/debug/dri/64/radeon_pm_info
default engine clock: 750000 kHz
current engine clock: 749250 kHz
default memory clock: 500000 kHz
current memory clock: 495000 kHz
voltage: 1200 mV
PCIE lanes: 16

while my driver supports this :
http://xorg.freedesktop.org/wiki/RadeonFeature/

if I force dpm in grub. Sometimes, my computer boots (and dpm works) but sometimes, it does not boot, there is a black screen. The only solution is to switch off.

further information: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1360393