Comment 17 for bug 1552040

My workaround is to enable GuC firmware loading and boot without the dock attached.

Kernel 4.15.0.30 Ubuntu 18.04.1 Xorg/Gnome Lenovo 4th gen X1 20FBxxx Skylake i7 Intel built-in video

Looks like this is some kind of race condition related to DisplayPort Multi-Stream mode (DP-MST). This is why multi-monitor docking stations are affected. A race is happening, with system attempting modesetting upon disconnect of the mst dock, which leaves the connector enabled. Error messages upon disconnect:

[drm:intel_dp_start_link_train [i915]] *ERROR* failed to enable link training
[drm:intel_mst_pre_enable_dp [i915]] *ERROR* failed to allocate vcpi

Next connection leads to black screen. Multiple error messages upon connect:

[drm:intel_wait_ddi_buf_idle [i915]] *ERROR* Timeout waiting for DDI BUF C idle bit

Related cases I've found:

https://patchwork.kernel.org/patch/8603521/
https://bugzilla.redhat.com/show_bug.cgi?id=1470960
https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/1552040
https://bugs.freedesktop.org/show_bug.cgi?id=73705

Workaround:

sudo vi /etc/modprobe.d/i915.conf

options i915 enable_guc_loading=1 enable_guc_submission=1

sudo update-initramfs -u
sudo reboot

always reboot while disconnected from DP MST dock