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

Kernel 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

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

Related cases I've found:


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

** Bug watch added: Red Hat Bugzilla #1470960

** Bug watch added: Bugzilla #73705

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  [Lenovo ThinkPad T440s] Laptop sometimes doesn't detect external

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