https://bugs.freedesktop.org/show_bug.cgi?id=30227
--- Comment #5 from Michel Dänzer michel@daenzer.net 2010-09-17 03:11:18 PDT --- (In reply to comment #4)
This happens with radeon.agpmode=-1 on the kernel boot command line. Without it, the kernel doesn't boot (it freeze, without eth connection and monitor freeze on some openfirmware specification - I have no serial - when switching from openfirmware to kernel itself, I think).
How about agpmode=1? There's an issue with higher transfer rates that Ben Herrenschmidt is working on fixing.