External monitors are black (connected through USB-C hub)

[Tomas Jenicek]

After upgrading the kernel from 5.8 to 5.11 (5.11.0-25-generic), external monitors (both HDMI and DP) connected through USB-C hub (ThinkPad Thunderbolt 3 Dock Gen 2) do not function properly. I can see them in the Displays settings utility, but they are not receiving any signal. The laptop is ThinkPad T490s and uses Intel UHD Graphics 620 (Whiskey Lake) as a graphic card with the default driver (no Additional Drivers). I see the following errors in dmesg repeated several times per second

[ 5.383739] i915 0000:00:02.0: [drm] *ERROR* Link Training Unsuccessful
[ 5.907908] i915 0000:00:02.0: [drm] *ERROR* CPU pipe B FIFO underrun
[ 854.832801] i915 0000:00:02.0: [drm] *ERROR* CPU pipe C FIFO underrun

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: linux-image-5.11.0-25-generic 5.11.0-25.27~20.04.1
ProcVersionSignature: Ubuntu 5.11.0-25.27~20.04.1-generic 5.11.22
Uname: Linux 5.11.0-25-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.18
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Mon Aug 9 14:01:47 2021
InstallationDate: Installed on 2020-07-26 (378 days ago)
InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
SourcePackage: linux-signed-hwe-5.11
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Jay Tuckey (jay-tuckey)]

I'm hitting the same issue. Here's a log from the command:
~[08:50]> journalctl --boot=-1 > 2021-08-10_i915_drm_link_training_unsuccessful_error.log

Booting back into the 5.8 kernel allows my dock display to work again.

It looks like this might be the same issue: https://bugs.archlinux.org/task/70135

[Mathias Bjerke (mathbje)]

I'm also having the same issue. My external screen is blank when using docking on the 5.11.0-34-generic kernel.

[Tomas Jenicek (tojenicek)]

Seems fixed in 5.11.0-37-generic

[Jay Tuckey (jay-tuckey)]

I've just booted up on the 5.11.0-37-generic and it seems to be fixed for me also:
~[08:44]> uname -a
Linux jay-t15-u 5.11.0-37-generic #41~20.04.2-Ubuntu SMP Fri Sep 24 09:06:38 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

Other people with this issue - worth updating to the latest kernel and testing?

[dr_eja (drewnak)]

I did the update to "5.11.0-37-generic #41~20.04.2-Ubuntu SMP" and it resolved the issue for me, too.