i915 external monitor not working in usb-c/thunderbolt dock on X1 gen6

Submitted by tmai@math.upb.de

Assigned to Stanislav Lisovskiy

Link to original bug (#111250)

Description

Created attachment 144909
dmesg while reproducing the error twice

Hi,

I cannot get external monitors to work on my X1 gen6 on Linux (Ubuntu) when they are connected over a dock. I tried the official dock from lenovo as well as an third-party usb-c adapter. The systems immediately freezes if I hotplug an external monitor or it freezes when I try to configure the external monitor (which is not enabled in the settings up to this point). It unfreezes when the cable is disconnected. In some cases booting with the external monitor ended up at a log-in screen with reduced screen resolution (below full HD).

Kernel: 5.3.0-994-generic
(from here: https://kernel.ubuntu.com/~kernel-ppa/mainline/drm-tip/2019-07-27/)
However, it affects various other kernel versions as well.

OS: Kubuntu 19.04

Machine: Lenovo X1 (Gen6); 20KGS03800

Display connector: HDMI and DP

dmesg is attached (external monitor was connected twice after boot up)

Additional info:

External monitors were working on Kubuntu 18.04 earlier this year. The other day, I tried the livesystem 18.04.2 which is already affected by the bug.

Please, let me know if I can provide any further information from my end or point me to any other place where I should report this bug instead.

Thanks to anyone who looks into this.

Attachment 144909, "dmesg while reproducing the error twice":
dmesg