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[Bug 2142146] [NEW] DisplayPort MST over USB-C regression in 6.17.0-14 — external monitors cycle on/off (works on 6.14.0-37)

Public bug reported:

Three external monitors connected via DisplayPort MST through a USB-C
docking station (Anker, ID 291a:8392) fail to work correctly on kernel
6.17.0-14-generic. Only the internal display and one external monitor
are usable. The other two displays repeatedly turn off and on in a loop,
as if the system is continuously attempting and failing to negotiate the
DP MST link.

The same hardware configuration works correctly on kernel
6.14.0-37-generic with no issues.

Symptoms on 6.17.0-14:

Of three external DP MST outputs (DP-1-1-1, DP-1-1-2, DP-1-1-3), only
one or two activate; the third is connected but has no active mode

Connected monitors show severely degraded EDID — e.g. a 3440x1440
display only reports 640x480, 800x600, and 1024x768 as standard modes

All displays (including working ones) cycle off and on repeatedly

NVIDIA log shows all DFP ports cycling between connected and
disconnected

Two USB Billboard devices appear (VIA Labs, ID 2109:8817), suggesting
USB-C DisplayPort Alt Mode is not fully negotiating

Hardware:

Laptop: Lenovo Legion Pro 7 16IAX10H

GPU: NVIDIA GeForce RTX 5070 Ti Laptop GPU

NVIDIA driver: 580.126.09 (open kernel modules)

Dock: Anker USB-C Docking Device (USB ID 291a:8392) with VIA Labs USB3.1
hub (ID 2109:0822)

Monitors: 3 external displays connected via DP MST through the dock
(3440x1440, 1920x1080, 1600x1200)

Versions:

Ubuntu 24.04.3 LTS (HWE kernel)

Working: 6.14.0-37-generic

Broken: 6.17.0-14-generic

Steps to reproduce:

Connect three external monitors to an Anker USB-C dock via DP MST

Boot with kernel 6.17.0-14-generic

Observe that only one or two external displays activate, and all
displays cycle on/off repeatedly

Reboot into 6.14.0-37-generic — all three displays work immediately

** Affects: linux (Ubuntu)
Importance: Undecided
Status: New

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Title:
DisplayPort MST over USB-C regression in 6.17.0-14 — external monitors
cycle on/off (works on 6.14.0-37)

Status in linux package in Ubuntu:
New

Bug description:
Three external monitors connected via DisplayPort MST through a USB-C
docking station (Anker, ID 291a:8392) fail to work correctly on kernel
6.17.0-14-generic. Only the internal display and one external monitor
are usable. The other two displays repeatedly turn off and on in a
loop, as if the system is continuously attempting and failing to
negotiate the DP MST link.

The same hardware configuration works correctly on kernel
6.14.0-37-generic with no issues.

Symptoms on 6.17.0-14:

Of three external DP MST outputs (DP-1-1-1, DP-1-1-2, DP-1-1-3), only
one or two activate; the third is connected but has no active mode

Connected monitors show severely degraded EDID — e.g. a 3440x1440
display only reports 640x480, 800x600, and 1024x768 as standard modes

All displays (including working ones) cycle off and on repeatedly

NVIDIA log shows all DFP ports cycling between connected and
disconnected

Two USB Billboard devices appear (VIA Labs, ID 2109:8817), suggesting
USB-C DisplayPort Alt Mode is not fully negotiating

Hardware:

Laptop: Lenovo Legion Pro 7 16IAX10H

GPU: NVIDIA GeForce RTX 5070 Ti Laptop GPU

NVIDIA driver: 580.126.09 (open kernel modules)

Dock: Anker USB-C Docking Device (USB ID 291a:8392) with VIA Labs
USB3.1 hub (ID 2109:0822)

Monitors: 3 external displays connected via DP MST through the dock
(3440x1440, 1920x1080, 1600x1200)

Versions:

Ubuntu 24.04.3 LTS (HWE kernel)

Working: 6.14.0-37-generic

Broken: 6.17.0-14-generic

Steps to reproduce:

Connect three external monitors to an Anker USB-C dock via DP MST

Boot with kernel 6.17.0-14-generic

Observe that only one or two external displays activate, and all
displays cycle on/off repeatedly

Reboot into 6.14.0-37-generic — all three displays work immediately

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