Public bug reported: Summary
A second monitor connected via a Lenovo ThinkPad USB-C Dock (40AY) is
detected by the kernel, has a CRTC and framebuffer allocated, shows DPMS
On and link-status Good, but displays no picture. The primary monitor
(Samsung Odyssey G93SC 5120x1440) works correctly through the same dock.
Both monitors are MST endpoints through the dock's USB4 DP tunnel.
The identical dock and monitor configuration works correctly on Fedora
43 (kernel 6.19.8).
Hardware
- Machine: Beelink SER9 (AMD Ryzen AI 9 HX 370)
- GPU: AMD Radeon 890M (Strix Point, DCN 4.01) - amdgpu driver
- Dock: Lenovo ThinkPad Universal USB-C Dock (40AY), connected via USB4
- Primary monitor: Samsung Odyssey G93SC (49" 5120x1440) - dock DP output -
works
- Secondary monitor: GWD ARZOPA (14" 1920x1080) - dock HDMI output (DP-HDMI
PCON) — blank
Reproduction
1. Connect Beelink SER9 to Lenovo 40AY dock via USB4
2. Connect Samsung to dock's DisplayPort output
3. Connect ARZOPA to dock's HDMI output
4. Boot Ubuntu 25.10 - Samsung displays, ARZOPA is detected but blank
5. Unplug USB4 from SER9, plug into Beelink SER8 running Fedora 43 — both
monitors work
What works / what doesn't
┌──────────────────────────────────────────────────┬────────┐
│ Configuration │ Result │
├──────────────────────────────────────────────────┼────────┤
│ ARZOPA via dock HDMI → Ubuntu 25.10 │ Blank │
├──────────────────────────────────────────────────┼────────┤
│ ARZOPA via dock DP (with adapter) → Ubuntu 25.10 │ Blank │
├──────────────────────────────────────────────────┼────────┤
│ ARZOPA via native HDMI (no dock) → Ubuntu 25.10 │ Works │
├──────────────────────────────────────────────────┼────────┤
│ ARZOPA via dock HDMI → Fedora 43 │ Works │
├──────────────────────────────────────────────────┼────────┤
│ Same issue on Beelink SER8 (8745HS) with Ubuntu │ Blank │
└──────────────────────────────────────────────────┴────────┘
DRM state while blank (everything looks correct)
DP-9: connected, enabled, DPMS On
CRTC 416 → Encoder 494 (DPMST) → Connector 451 (DP-9)
Framebuffer 506 allocated, 1920x1080
link-status: Good
Both DP-9 and DP-10 are MST virtual endpoints (DPMST), not physical
connectors.
Kernel / boot messages
[drm] DP-HDMI FRL PCON supported
[drm] pre_validate_dsc:1667 MST_DSC dsc precompute is not needed
[drm] pre_validate_dsc:1742 MST_DSC crtc[0] needs mode_change
[drm] MST_DSC Send DSC enable to synaptics
What has been tested and ruled out
- Kernels: 6.17.0-14, 6.17.0-19, 6.19.10 mainline — all fail
- Mesa: 25.2.8 (stock), 26.0.3 (kisak PPA) — both fail
- Mutter: 49.0-2ubuntu5 (pre-tiled-monitor-patch), 49.0-2ubuntu5.3 — both
fail
- Weston compositor (from TTY, bypassing GNOME entirely) — also fails → issue
is below compositor
- DMCUB firmware: updated to match Fedora's linux-firmware 20260309 — still
fails
- EVDI/DisplayLink: blacklisted and confirmed unloaded — still fails
- monitors.xml: deleted for fresh detection — still fails
- amdgpu.dcdebugmask=0x4 (disable DSC) — still fails
- ICC profiles removed — still fails
- GNOME Shell extensions disabled — still fails
- Mutter display toggle (disable/re-enable via DBus) — still fails
Comparison with working Fedora 43
┌────────────────────────┬────────────────────────────────┬────────────────────────────────┐
│ │ Ubuntu 25.10 (broken) │ Fedora 43
(works) │
├────────────────────────┼────────────────────────────────┼────────────────────────────────┤
│ Kernel │ 6.19.10-061910-generic │
6.19.8-200.fc43.x86_64 │
├────────────────────────┼────────────────────────────────┼────────────────────────────────┤
│ Kernel config (AMD) │ Identical │ Identical
│
├────────────────────────┼────────────────────────────────┼────────────────────────────────┤
│ amdgpu params │ dcfeaturemask=2, dcdebugmask=0 │ dcfeaturemask=2,
dcdebugmask=0 │
├────────────────────────┼────────────────────────────────┼────────────────────────────────┤
│ Mesa │ 26.0.3 │ 25.3.6
│
├────────────────────────┼────────────────────────────────┼────────────────────────────────┤
│ linux-firmware │ 20250901 (Sep 2025) │ 20260309 (Mar
2026) │
├────────────────────────┼────────────────────────────────┼────────────────────────────────┤
│ DMUB firmware │ 0x09002C01 │ 0x08005700
(different GPU) │
├────────────────────────┼────────────────────────────────┼────────────────────────────────┤
│ MST + DSC to Synaptics │ Yes │ Yes
│
└────────────────────────┴────────────────────────────────┴────────────────────────────────┘
System info
Ubuntu 25.10 (Questing Quokka)
Kernel: 6.19.10-061910-generic
GPU: AMD Strix [Radeon 880M / 890M] (rev c1)
DMUB: 0x09002C01
linux-firmware: 20250901.git993ff19b-0ubuntu1.9
Mutter: 49.0-2ubuntu5.3
Mesa: 26.0.3~kisak1~q
** Affects: ubuntu
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/2146857
Title:
Second monitor blank via USB-C dock MST detected, CRTC active, no
picture (works on Fedora 43)
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