Created attachment 141494 [details]
Relevant part of kernel log
When using dual monitors (I'm not sure is correlated but first error shows on turning second monitor with xrandr) GPU locks up with:
[ 451.840071] radeon 0000:01:00.0: ring 0 stalled for more than 32210msec
[ 451.840077] radeon 0000:01:00.0: ring 3 stalled for more than 32210msec
[ 451.840084] radeon 0000:01:00.0: GPU lockup (current fence id 0x000000000003d501 last fence id 0x000000000003d768 on ring 3)
[ 451.840088] radeon 0000:01:00.0: GPU lockup (current fence id 0x000000000001e67a last fence id 0x000000000001e74a on ring 0)
When turning on second monitor this shows up:
[drm:radeon_cs_parser_relocs [radeon]] *ERROR* gem object lookup failed 0xa
[drm:radeon_cs_ioctl [radeon]] *ERROR* Failed to parse relocation -2!
Old driver 17.2.0-0.20170513 (some git snapshot from 2017-05-13, don't know exact commit) works fine even when used with Mesa-18.1.6 package (I've reverted Radeon SI driver only).
Created attachment 141495 [details]
Created attachment 141496 [details]
Created attachment 141497 [details]
I'm pretty sure that this bug exists also in 18.2 (I've build git snapshot and had similar issues but attached logs are from 18.1).
*** This bug has been marked as a duplicate of bug 105381 ***