https://bugs.freedesktop.org/show_bug.cgi?id=83792
--- Comment #4 from Alex Deucher agd5f@yahoo.com --- Can you also try blacklisting radeon and loading it after the box has booted and you have remote access? E.g., append modprobe.blacklist=radeon 3 to the kernel command line in grub (the 3 is to boot into runlevel 3 so X doesn't start). Then once the system is booted and you have remote access, as root, run: modprobe radeon and see if you can get the dmesg output.