[Intel-gfx] [REGRESSION] drm_atomic_helper_wait_for_flip_done *ERROR* [CRTC:69:pipe C] flip_done timed out

[dmummenschanz at web.de]

Hi,

I have a Lenovo T470 notebook with a KBL (GT2) CPU connected to a docking station with two IPS monitors connected.
According to xrandr the primary laptop screen is connected to eDP-1 and the monitors are connected to DP-2-1 and DP-2-2.

This setup worked fine until kernel 4.19-rc1. After booting into the framebuffer console the screen connected to DP-2-2 won't come on. 

When I try to startx my system locks up!

I've cranked the debug level up to drm.debug=0xe. I'm not an expert but this looks suspicious:
...
[    4.465645] [drm:intel_mst_enable_dp [i915]] active links 2
[    4.465655] [drm:intel_dp_check_mst_status [i915]] got esi 02 00 00
[    4.466964] [drm:intel_mst_enable_dp [i915]] *ERROR* Timed out waiting for ACT sent
[    4.478043] [drm:drm_dp_check_act_status [drm_kms_helper]] failed to get ACT bit 1 after 30 retries
....
and later:
[   14.669161] [drm:drm_atomic_helper_wait_for_flip_done [drm_kms_helper]] *ERROR* [CRTC:69:pipe C] flip_done timed out
[   14.669269] [drm:verify_connector_state.isra.86 [i915]] [CONNECTOR:100:DP-3]
[   14.669328] [drm:intel_atomic_commit_tail [i915]] [CRTC:55:pipe B]
....
and finally after firing up X:
[   17.741227] [drm:edp_panel_vdd_off_sync [i915]] Turning eDP port A VDD off
[   17.741347] [drm:edp_panel_vdd_off_sync [i915]] PP_STATUS: 0x80000008 PP_CONTROL: 0x00000007
[   24.909159] [drm:drm_atomic_helper_wait_for_dependencies [drm_kms_helper]] *ERROR* [CRTC:69:pipe C] flip_done timed out

Full dmesg is provided here: https://pastebin.com/nDwvQPy6

Xorg.log doesn't reveal anything interesting. The last line in the log before freezing:
[    73.632] (II) xfree86: Adding drm device (/dev/dri/card0)

Full log: https://pastebin.com/9RaAYhjP

This is what xrandr shows with kernel 4.18 (with 4.19 I can't get that far):

https://pastebin.com/cYSf9CyS

I've tried various settings like switching monitors or switching the dockingstation nothing helps.

Actually I've encountered this issue months earlier with drm-tip roughly about the time it 
got merged with 4.18-rc1. I've opened a but last month but it didn't draw any attention:

https://bugs.freedesktop.org/show_bug.cgi?id=107365

Now infortunalely this regression has made it into mainline :(.
I'll be happy to provide more information or do more tests but for now I'm out of ideas.

Regards