[Bug 218569] New: Early KMS Resolution Issue on MST dock connected 4K Monitor

[bugzilla-daemon at kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=218569

            Bug ID: 218569
           Summary: Early KMS Resolution Issue on MST dock connected 4K
                    Monitor
           Product: Drivers
           Version: 2.5
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri at kernel-bugs.osdl.org
          Reporter: erenoglu at gmail.com
        Regression: No

My laptop, a Thinkpad P14s Gen2 AMD has two usb-c and an hdmi port. I use one
USB-C port to connect to Thinkpad Dock Type 40B0 which has an internal MST hub.
The Dock has 2xDP connectors and 1xHDMI. The dock is connected to a 4K TV (LG
C2), using a DP to HDMI cable.

The system works fine when Wayland or Xorg starts and displays 3840x2160 at 60hz.

I noticed a lot of monitor sync loss when changing modes. At UEFI boot it's at
1280x1024, then at kernel boot it switches to 1920x1080 and monitor loses
signal during this transition, then SDDM or GDM starts it switches to
3840x2160, and when I login to KDE, it loses signal again for a few seconds
then shows the desktop again at 3840x2160 at 60.
So this is a nuisance for losing monitor sync but not effecting normal
operation.

I then tried to use the HDMI port on the laptop. I unplugged the cable from the
dock and plugged a separate HDMI cable directly to the port on the Laptop. Then
I noticed it was fully smooth. After UEFI, mode immediately switched to
3840x2160 at 60 with early kms and then SDDM launched without losing any signal.
Then logging into desktop also is smooth and does not lose any signal.

So I concluded something is wrong when one uses a Monitor connected to a Dock
with MST Hub function, connected to a USB-C port on the laptop
(no-Thunderbolt). Problems are:
1) When using the MST docking station, early kms does not switch to native 4K
resolution (3840x2160 at 60hz). I tried feeding an EDID at kernel command line, it
did not work.
2) Monitor loses signal sync when switching modes when using Dock. Does not
happen when using HDMI port on laptop.

This does not happen if I use a simpler usb-c hub without MST support. So a
simple usb-c hub with one HDMI port out works fine.

Problematic setup:
Laptop --usb-c-- dock_with_mst -- dp2hdmi cable -- hdmi_port_on_4K_Monitor
Working setup:
Laptop -- hdmi -- hdmi_port_on_4K_Monitor

Kernel Version: Linux: 6.7.8-arch1-1

bug reported to Arch as well but I'm directed to report upstream:
https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/issues/32

Steps to reproduce:
1) Use any recent kernel and use any docking station with an MST hub.
2) Connect a monitor to the docking station.
3) Connect Dock to the laptop using USB-C port (no thunderbolt support, ie amd
laptop)
4) Use early kms by adding amdgpu to mkinitcpio.conf modules section
5) Watch monitor resolution using Monitor functions while booting

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