https://bugs.freedesktop.org/show_bug.cgi?id=82201
--- Comment #2 from Alex Deucher agd5f@yahoo.com --- Please attach your dmesg output with radeon.dpm=1 set on the kernel command line in grub. That dumps some additional debugging output. Also please attach a copy of your vbios.
(as root) (use lspci to get the bus id) cd /sys/bus/pci/devices/<pci bus id> echo 1 > rom cat rom > /tmp/vbios.rom echo 0 > rom