Just had a hang using 5.4.0-rc3, mesa 19.3~git1910171930.4b458b~oibaf~d,
AMD_DEBUG="nodma nongg" while using firefox:

Oct 24 16:31:26 oipo-X570-AORUS-ELITE kernel: [27386.467009] broken atomic
modeset userspace detected, disabling atomic
Oct 24 21:04:58 oipo-X570-AORUS-ELITE kernel: [43796.470041]
[drm:amdgpu_dm_atomic_commit_tail [amdgpu]] *ERROR* Waiting for fences timed
out!
Oct 24 21:04:58 oipo-X570-AORUS-ELITE kernel: [43798.773602]
[drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring sdma0 timeout, signaled
seq=1756792, emitted seq=1756794
Oct 24 21:04:58 oipo-X570-AORUS-ELITE kernel: [43798.773683]
[drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process GPU
Process pid 17048 thread firefox:cs0 pid 17134
Oct 24 21:04:58 oipo-X570-AORUS-ELITE kernel: [43798.773685] [drm] GPU recovery
disabled.