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I'm using Thinkpad P14s and a docking station (usb-c dock) with two external monitors. Everything was working fine for several months but after some Ubuntu system upgrades this setup broke. Now both external monitors and the laptop's own monitor goes blank as soon as I plug the docking station in. The external monitors don't seem to receive any signal. The laptop's own monitor comes back to life when I unplug the docking station.

Earlier I was using a setup where the internal monitor is disabled and the external monitors are an extended display when plugged in. So I'm thinking if the configuration still disables the internal monitor as soon as I plugged in and for some reason the external monitors just don't seem to be functioning.

I saw an old post with similar description and then it seemed to have been an issue in the kernel (Thinkpad not working with docking station). I wonder if anybody else has got similar issues? Also is there any way to view and edit the display configurations / profiles with command line?

I'm using the out-of-the-box Ubuntu Desktop 20.04.

EDIT (more details):

Kernel:

$ uname -r
5.11.0-27-generic

Dmesg full of "Link Training Unsuccessful" message:

$ dmesg | grep "Link Training"
[271436.154505] i915 0000:00:02.0: [drm] *ERROR* Link Training Unsuccessful
quinz
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    What version of the kernel are you running? I think this relates to a bug which also affected me, see here.

    To find out kernel version, run the command uname -r in the terminal.

    Also, check the logs for the message Link Training Unsuccessful (run dmesg | grep "Link Training")

     – Bib-lost Aug 28 '21 at 09:26
  • Thanks @Bib-lost, I added the details at the end of the post. It seems that dmesg has plenty of those error lines you mentioned. – quinz Aug 30 '21 at 08:31
  • The bug report mentions a fix to be merged in 5.11 but that doesn't seem to be the case - at least for the build that I've got from Ubuntu upgrades. – quinz Aug 30 '21 at 09:11

1 Answers1

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This is a known bug in the kernel with DP over USB, which affected me recently, see here. A patch is available but does not seem to have reached the mainline Ubuntu repositories yet.

For me, the easiest and probably most stable immediate solution was to instead use the older kernel 5.4, which Ubuntu 20.04 supports out of the box and should be supported until the end of life of 20.04, and for which the bug was not there. See here for explanation on how to boot your system into an older kernel. Unless you have specific use for the newest features of the 5.11 kernel, I would try this first.

Bib-lost
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