Created attachment 134494 [details]
Xorg.0.log

Hi, 

I'm experiencing crashes on a thinkpad T440 when changing resolutions or CRTCs with an UltraDock (probably DisplayPort MST-related).

This is using X.org 1.19.3 with the modesetting driver on Debian kernel linux 4.12.0-2-amd64 (but also on earlier kernels).

When docking/undocking or otherwise connecting multiple displays, there are usually modesetting problems due to CRTC assignments. Fixing the CRTC assignments, however, usually crashes X, especially when the session is a bit older or when there was a suspend/resume somewhere before.

[   913.018] (II) modeset(0): Allocate new frame buffer 1920x1080 stride
[   913.019] (EE) 
[   913.019] (EE) Backtrace:
[   913.019] (EE) 0: /usr/lib/xorg/Xorg (xorg_backtrace+0x4a) [0x56217193bc7a]
[   913.019] (EE) 1: /usr/lib/xorg/Xorg (0x562171782000+0x1bda29) [0x56217193fa29]
[   913.019] (EE) 2: /lib/x86_64-linux-gnu/libpthread.so.0 (0x7f778c71c000+0x110c0) [0x7f778c72d0c0]
[   913.019] (EE) 3: /usr/lib/xorg/Xorg (0x562171782000+0xd7bd6) [0x562171859bd6]
[   913.019] (EE) 4: /usr/lib/xorg/Xorg (0x562171782000+0xd7d9b) [0x562171859d9b]
[   913.019] (EE) 5: /usr/lib/xorg/Xorg (0x562171782000+0xd4013) [0x562171856013]
[   913.019] (EE) 6: /usr/lib/xorg/Xorg (ProcRRSetScreenSize+0xe8) [0x5621718a7a48]
[   913.019] (EE) 7: /usr/lib/xorg/Xorg (0x562171782000+0x545e5) [0x5621717d65e5]
[   913.019] (EE) 8: /usr/lib/xorg/Xorg (0x562171782000+0x58568) [0x5621717da568]
[   913.019] (EE) 9: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xf1) [0x7f778c39d2b1]
[   913.019] (EE) 10: /usr/lib/xorg/Xorg (_start+0x2a) [0x5621717c421a]
[   913.019] (EE) 
[   913.019] (EE) Segmentation fault at address 0x22
[   913.019] (EE) 
Fatal server error:
[   913.019] (EE) Caught signal 11 (Segmentation fault). Server aborting

Usually reproducible with:
#!/bin/bash
xrandr -d :0 --output eDP-1 --off
xrandr -d :0 --output DP-1 --off
xrandr -d :0 --output DP-2-1 --off
xrandr -d :0 --output DP-2-2 --off
if xrandr -d :0|egrep -q '^DP-1 connected'; then
    xrandr -d :0 --output DP-2-1 --crtc 1 --mode 1920x1080
    xrandr -d :0 --output DP-1 --crtc 0 --mode 1920x1080 --right-of DP-2-1
    xrandr -d :0 --output DP-2-2 --crtc 2 --mode 1920x1080 --right-of DP-1
else
    xrandr -d :0 --output eDP-1 --crtc 0 --mode 1920x1080 --right-of DP-2-1
fi


Possibly related, the i915 kernel module is also stressing a bit about underruns:
[  809.933002] [drm:intel_set_cpu_fifo_underrun_reporting [i915]] *ERROR* uncleared fifo underrun on pipe B
[  809.933025] [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe B FIFO underrun
[  811.036711] [drm:intel_set_cpu_fifo_underrun_reporting [i915]] *ERROR* uncleared fifo underrun on pipe B
[  811.036732] [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe B FIFO underrun
[  913.602996] [drm:intel_set_cpu_fifo_underrun_reporting [i915]] *ERROR* uncleared fifo underrun on pipe C
[  913.603022] [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe C FIFO underrun