https://bugs.freedesktop.org/show_bug.cgi?id=91880
--- Comment #64 from Alex Deucher alexdeucher@gmail.com --- (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.