Comment 12 for bug 1024507

Necrobumping this thing as I've been battling the same issue on an iMac 10,1 (Radeon Mobility 4670) with the most recent LTS releases. Booting normally on 14.04 OR the most recent 16.04 nightly results in a black screen when using the radeon driver. Interestingly, I can get display if I plug in an external screen on the external miniDP connector (but only on one display at a time. It will NOT display to both monitors simultaneously).

Looking at Xorg.0.log, it can detect both DisplayPort-0 (assuming this to be the internal DP connector for the built in panel) and eDP (assuming this to be external DP connector), retrieves EDID, DISCONNECTS DisplayPort-0 and then loads up using eDP. No error state reported by X, just blank screen on the built-in panel. xrandr -d :0 -q results in DisplayPort-0 disconnected, edP as the primary and a long list of modes retrieved from the internal panel's EDID. It almost seems like X's wires are crossed with these two outputs, trying to load on the external displayport using EDID gained by initially connecting to DP0.

System is otherwise running and usable - can SSH in and fool around with things while it's running / restart LightDM / play with xrandr to try and fail to make the thing work.

Listed workaround booting with nomodeset allows the system to load graphics (slowly) over LLVMPipe on the internal display.

Other fixes tried: wily-lts kernel (4.2) , latest 4.5rc kernel available in kernel.ubuntu.com/~kernel-ppa/mainline, updated driver/X stack for 14.04 . No effect.