Public bug reported:
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)
** Affects: linux-signed-hwe-5.11 (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug focal
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https://bugs.launchpad.net/bugs/1939297
Title:
External monitors are black (connected through USB-C hub)
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