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 ** Bug watch added: Red Hat Bugzilla #1470960 https://bugzilla.redhat.com/show_bug.cgi?id=1470960 ** Bug watch added: freedesktop.org Bugzilla #73705 https://bugs.freedesktop.org/show_bug.cgi?id=73705 -- You received this bug notification because you are a member of Ubuntu-X, which is subscribed to xserver-xorg-video-intel in Ubuntu. https://bugs.launchpad.net/bugs/1552040 Title: [Lenovo ThinkPad T440s] Laptop sometimes doesn't detect external monitor To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/1552040/+subscriptions _______________________________________________ Mailing list: https://launchpad.net/~ubuntu-x-swat Post to : ubuntu-x-swat@lists.launchpad.net Unsubscribe : https://launchpad.net/~ubuntu-x-swat More help : https://help.launchpad.net/ListHelp
