I'm also having the same issue. My external screen is blank when using docking on the 5.11.0-34-generic kernel.
** Attachment added: "2021-08-14_i915_drm_link_training_unsuccessful_error.log" https://bugs.launchpad.net/ubuntu/+source/linux-signed-hwe-5.11/+bug/1939297/+attachment/5525093/+files/2021-08-14_i915_drm_link_training_unsuccessful_error.log -- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux-signed-hwe-5.11 in Ubuntu. https://bugs.launchpad.net/bugs/1939297 Title: External monitors are black (connected through USB-C hub) Status in linux-signed-hwe-5.11 package in Ubuntu: Confirmed Bug description: 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) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux-signed-hwe-5.11/+bug/1939297/+subscriptions -- Mailing list: https://launchpad.net/~kernel-packages Post to : kernel-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~kernel-packages More help : https://help.launchpad.net/ListHelp
