As requested, here's a bug report for the problem I'm currently trying to solve
with upstream devs.

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).

Turns out that the process that's stuck is "ksplashqml" and when I switch to
text console and kill it, I see that everything has actually 
started up, just the progressbar screen was obscuring it.

Bisecting the kernel between 4.3 and 4.4 leads 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:
5b5561b3660db734652fbd02b4b6cbe00434d96b "drm/radeon: Fixup hw vblank 
counter/ts for new drm_update_vblank_count() (v2)"
fa4270d8e0257b4b76f11baa2866f4313d29aaf5 "drm: Don't zero vblank 
timestamps from the irq handler"
235fabe09b46469adad2c9e4cb0563758155187c "drm: Add DRM_DEBUG_VBL()"
4dfd64862ff852df7b1198d667dda778715ee88f "drm: Use vblank timestamps to 
guesstimate how many vblanks were missed"
8e36f9d33c134d5c6448ad65b423a9fd94e045cf "drm/amdgpu: Fixup hw vblank 
counter/ts for new drm_update_vblank_count() (v3)"

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

The radeon card is "CAICOS" with 1GB memory. Further details appear in upstream
thread http://comments.gmane.org/gmane.linux.kernel/2127512