Comment 98 for bug 542660

Here's how to have radeon drm working (on imac 12,2 27" screen) :
boot from latest ubuntu live CD for amd64.

Booting from CD is done in BIOS mode. There seems to be a way to set BIOS mode
for booting on hard disk, too, since bootcamp does this to start windows.

Booting in EFI mode from hard disk is triggering (at least) two bugs :
1) kernel use something called EFI virtual mode, and it's broken on that machine,
   search for "run efi in physical mode" patch. It won't boot without it, and gives a
   a black screen too.
2) https://bugs.freedesktop.org/show_bug.cgi?id=27314
   screen goes black, but everything else work, X is even successfully starting,
  and recognize radeon in DRI2 mode.