https://bugs.freedesktop.org/show_bug.cgi?id=27314

--- Comment #70 from Alex Deucher agd5f@yahoo.com --- (In reply to comment #69)

I have an iMac 11,1 and I cannot get X to work. Documentation is scarce, so I don't know how to discover what is the problem. When I start X, it doesn't show any errors, but I get a blank screen. Same for when I insert the radeon module with kms on. I suspect that the EDID is not being handled correctly. Does any of you know how I can create an external EDID for a 2560x1440 monitor? My video card is an ATI Radeon HD 4850, by the way.

If xrandr --verbose shows the appropriate modes for your display, there is no problem with the EDID. You might try a 3.11 kernel.