https://bugs.freedesktop.org/show_bug.cgi?id=29444
--- Comment #6 from Simon Munton simon@munton.demon.co.uk 2010-08-12 14:27:00 PDT --- It seems that I need the pci=nomsi option on the kernel command line. Without it, the radeon card uses MSI, but the kernel doesn't get any interrupts, and waiting for video blanking never ends.
I tried pci=nomsi after seeing this bug report: https://bugs.freedesktop.org/show_bug.cgi?id=29327