Public bug reported:
Ubuntu release:
Kubuntu 26.04 LTS
Kernel:
Linux 7.0.0-28-generic
The issue also existed on earlier 7.0 kernels used since upgrading to Kubuntu
26.04.
Hardware:
Lenovo ThinkPad T14
AMD Ryzen 5 PRO 4650U
AMD Renoir Radeon Vega integrated graphics
PCI ID: 1002:1636
amdgpu driver
Dock:
UtechSmart UCN3474 USB-C docking station
USB hubs reported as:
2109:2817 VIA Labs USB 2.0 Hub
2109:0817 VIA Labs USB 3.0 Hub
Problem:
The system freezes after entering the LUKS disk-encryption passphrase and
before the SDDM login screen whenever two HDMI monitors are connected
simultaneously to the docking station.
The freeze occurs during boot before the graphical login screen appears.
The system becomes unresponsive and one monitor must be disconnected
before boot can continue.
Expected result:
The system should reach the SDDM login screen and start normally with both
external monitors connected through the dock.
Actual result:
The system freezes after unlocking the encrypted disk and before the login
screen.
Important observations:
1. The same dock and two-monitor configuration worked correctly under Kubuntu
24.04 LTS.
2. The problem started after upgrading/installing Kubuntu 26.04 LTS.
3. One HDMI monitor connected directly to the laptop and one HDMI monitor
connected to the dock works correctly.
4. Either HDMI output on the dock works when used individually.
5. Both monitors work through the dock if the second monitor is connected after
the Plasma session has already started.
6. The problem only occurs when both HDMI displays are connected to the dock
during boot.
7. Testing the kernel parameter amdgpu.dc=0 did not solve the problem.
8. Blacklisting the evdi module did not solve the problem.
9. The dock is using DisplayPort MST rather than DisplayLink for these outputs.
Relevant dmesg lines:
amdgpu 0000:07:00.0: initializing kernel modesetting
amdgpu 0000:07:00.0: Display Core initialized on DCN 2.1
[drm] DM_MST: starting TM
[drm] DM_MST: DP12, 2-lane link detected
[drm] Downstream port present 1, type 2
[drm] Downstream port present 1, type 2
GPU:
07:00.0 VGA compatible controller:
AMD/ATI Renoir Radeon Vega Series
Kernel driver in use: amdgpu
BIOS:
R1BET87W (1.56)
Release date: 06/04/2026
Regression:
Yes. This configuration worked in Kubuntu 24.04 LTS and fails in Kubuntu 26.04
LTS.
Workaround:
Connect one monitor directly to the laptop HDMI port and the second monitor to
the docking station, or disconnect one monitor during boot and reconnect it
after login.
ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: linux-image-7.0.0-28-generic 7.0.0-28.28
ProcVersionSignature: Ubuntu 7.0.0-28.28-generic 7.0.12
Uname: Linux 7.0.0-28-generic x86_64
ApportVersion: 2.34.0-0ubuntu2
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: KDE
Date: Thu Jul 23 12:34:06 2026
MachineType: LENOVO 20UD0009US
ProcFB: 0 amdgpudrmfb
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-7.0.0-28-generic
root=/dev/mapper/dabs2601vg-root ro quiet splash
PulseList: Error: command ['pacmd', 'list'] failed with exit code 1: No
PulseAudio daemon running, or not running as session daemon.
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 06/04/2026
dmi.bios.release: 1.56
dmi.bios.vendor: LENOVO
dmi.bios.version: R1BET87W(1.56 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20UD0009US
dmi.board.vendor: LENOVO
dmi.board.version: SDK0J40697 WIN
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: None
dmi.ec.firmware.release: 1.56
dmi.modalias:
dmi:bvnLENOVO:bvrR1BET87W(1.56):bd06/04/2026:br1.56:efr1.56:svnLENOVO:pn20UD0009US:pvrThinkPadT14Gen1:rvnLENOVO:rn20UD0009US:rvrSDK0J40697WIN:cvnLENOVO:ct10:cvrNone:skuLENOVO_MT_20UD_BU_Think_FM_ThinkPadT14Gen1:pfaThinkPadT14Gen1:
dmi.product.family: ThinkPad T14 Gen 1
dmi.product.name: 20UD0009US
dmi.product.sku: LENOVO_MT_20UD_BU_Think_FM_ThinkPad T14 Gen 1
dmi.product.version: ThinkPad T14 Gen 1
dmi.sys.vendor: LENOVO
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug resolute wayland-session
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https://bugs.launchpad.net/bugs/2161661
Title:
System freezes before login when two HDMI monitors are connected
through USB-C MST dock on AMD Renoir
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