Arch Linux

delta38

Member

Registered: 2021-05-14

Posts: 1

Kernel >=5.11 + amdgpu + USB-C Dock: Problems Discovering Displays

I'm running Gnome 3 with Wayland on my ThinkPad T495 (AMD Ryzen 3500U) and have trouble connecting to my external monitors ever since upgrading to Kernel 5.11. The monitors are connected via DisplayPort to an HP Elite USB-C Dock G4.

The behaviour looks as follows: Upon booting, only one of the monitors is switched on for my LUKS password prompt. GDM seems to have a hard time enabling graphics, as the screen stays black for a few seconds, but it finally succeeds on the main monitor. After logging in, the second monitor still stays in standby, although Gnome seems to somehow recognize it (it's available in system settings + I can move mouse/windows to it). Dmesg shows the following error:

[   71.843372] [drm:drm_dp_get_mst_branch_device [drm_kms_helper]] *ERROR* failed to lookup MSTB with lct 2, rad 00

My setup worked fine with 5.10.16. It first broke when trying out 5.11 and still persists with the latest 5.12.3. Downgrading allows me to use both monitors again, but ideally I'd like to upgrade my Kernel some time in the near future

Has anyone ever experienced similar issues or could point me in the direction of a solution?

ansemjo

Member

Registered: 2022-04-22

Posts: 4

Re: Kernel >=5.11 + amdgpu + USB-C Dock: Problems Discovering Displays

I've had – or rather: I'm having the same issues as well. Unfortunately, I don't have a real solution yet.
I'm running a ThinkPad P14s Gen 2 on a ThinkPad Thunderbolt 3 Dock Gen 2 and I have two monitors connected in a daisy-chain via DisplayPort. It happens regularly with almost any other USB-C "Dock" though, including standalone USB-C Monitors.

Even though USB-C is supposed to be symmetrical, I can sometimes solve my issue by simply turning the cable's end around 180° or plug it into the other USB-C port on the laptop. Yesterday, I solved it by plugging the DisplayPort cable into the other DP port on the Dock?!

A somewhat-reliable way to fix the issue for me has always been a fresh reboot with the dock attached. I do remember a time around the kernel versions you mentioned, where even after a reboot only one or none of the displays would turn on.

So, yeah. I don't have a solution but you're not alone.