Comment # 33 on bug 62959 from Alex Deucher

(In reply to comment #32)
> Attachment 77608 [details] fixes the lockups, which suggests the DRM driver
> doesn't actually flush caches when it should.

radeon_ring_vm_flush() doesn't actually flush caches per se, it writes the new
VM page table base address, so presumably we are not handling last_flush
properly somewhere which results in a stale VM page table pointer.