(In reply to Julian from comment #63)
> At first I thought it was an issue with the writeback feature that caches
> certain register values because it also caches an rptr value that is used in
> the driver's gpu_lockup_check and is, to my knowledge, never actually
> written to.

More likely it never gets written because the GPU has hung due to something
else.

> 
> Buuut using radeon.no_wb=1 doesn't help. So if I've found a bug it is not
> the culprit of the lockups.

The no_wb option isn't really applicable on newer chips and most likely won't
work.  Newer hw does not support the necessary features to not support wb. 
It's mainly a leftover from the early radeons.