linux-4.4 bisected: kwin5 stuck on kde5 loading screen with radeon

[Vlastimil Babka <vbabka@suse.cz>]

Hi,

since kernel 4.4 I'm unable to login to kde5 desktop (on openSUSE 
Tumbleweed). There's a screen with progressbar showing the startup, 
which normally fades away after reaching 100%. But with kernel 4.4, the 
progress gets stuck somewhere between 1/2 and 3/4 (not always the same).
Top shows that kwin is using few % of CPU's but mostly sleeps in poll().
When I kill it from another console, I see that everything has actually 
started up, just the progressbar screen was obscuring it. The windows 
obviously don't have decorations etc. Starting kwin manually again shows 
me again the progressbar screen at the same position.

I have suspected that kwin is waiting for some event, but nevertheless 
tried bisecting the kernel between 4.3 and 4.4, which lead to:

# first bad commit: [4dfd64862ff852df7b1198d667dda778715ee88f] drm: Use 
vblank timestamps to guesstimate how many vblanks were missed

I can confirm that 4.4 works if I revert the following commits:
63154ff230fc9255cc507af6277cd181943c50a1 "drm/amdgpu: Fixup hw vblank 
counter/ts for new drm_update_vblank_count() (v3)"

d1145ad1e41b6c33758a856163198cb53bb96a50 "drm/radeon: Fixup hw vblank 
counter/ts for new drm_update_vblank_count() (v2)"

31ace027c9f1f8e0a2b09bbf961e4db7b1f6cf19 "drm: Don't zero vblank 
timestamps from the irq handler"

ac0567a4b132fa66e3edf3f913938af9daf7f916 "drm: Add DRM_DEBUG_VBL()"

4dfd64862ff852df7b1198d667dda778715ee88f "drm: Use vblank timestamps to 
guesstimate how many vblanks were missed"

All clean reverts, just needs some fixup on top to use abs() instead of 
abs64() due to 79211c8ed19c055ca105502c8733800d442a0ae6.

Unfortunately I don't know if this is a kernel problem or kwin problem. 
I tried to CC maintainers of both, advices what to try or what info to 
provide welcome. The card is "CAICOS" with 1GB memory.

Thanks,
Vlastimil