Public bug reported:
# System
- Lenovo ThinkPad T14s Gen 3 (`21CRS08P00`)
- AMD Rembrandt / Radeon 680M (`1002:1681`)
- BIOS `R22ET81W (1.51)`, dated 2026-04-10
- Lenovo ThinkPad Universal Thunderbolt 4 Smart Dock
- Ubuntu 26.04 LTS
- Kernel `7.0.0-28-generic`, package version `7.0.0-28.28`
- `linux-firmware` version `20260319.git217ca6e4.1ubuntu`
# Expected result
The dock should enumerate reliably on either supported laptop USB-C
port. Its USB devices and both external displays should remain
connected, without AMDGPU warnings or GNOME atomic-commit failures.
# Actual result
Observed outcomes include:
1. USB setup fails with `device not accepting address ..., error -71`; no
Thunderbolt device or DP MST topology is created.
2. A docked boot shows `failed to allocate DP resource for port 7`, `DMUB
notification skipped due to no handler: type=HPD`, and `DM_MST: Differing MST
start`, then eventually recovers.
3. After successful enumeration, the entire dock disconnects and re-enumerates
at runtime. Both external displays blank, GNOME reports failed atomic commits,
and AMDGPU reports DPIA allocation-table and DCN compbuf warnings.
# Reproduction / observations
1. Boot Ubuntu with the dock's rear/upstream port and both external displays
already connected, or boot undocked and connect the dock after login.
2. Observe either successful enumeration or an early USB `error -71` failure.
The result is port/path-sensitive:
- Rear/upstream dock port to one laptop USB-C port: failed after the initial
USB attempt with `error -71`; no Thunderbolt or display event followed.
- Rear/upstream dock port to the other laptop USB-C port: full dock
enumeration and both displays worked, after one recoverable AMDGPU `Sending
link address failed with -5` message.
3. During a separate successful `7.0.0-28-generic` boot, the dock later
disconnected and re-enumerated. Both displays went off and came back.
# Relevant kernel messages
Runtime disconnect on `7.0.0-28-generic`:
```text
DM_MST: stopping TM
USB devices below usb 8-1 and usb 9-1 disconnect
thunderbolt 0-2: device disconnected
gnome-shell: Page flip failed: drmModeAtomicCommit: Invalid argument
WARNING in update_dpia_stream_allocation_table [amdgpu]
REG_WAIT timeout in dcn31_program_compbuf_size
WARNING in dcn31_program_compbuf_size [amdgpu]
Lenovo ThinkPad Universal Thunderbolt 4 Smart Dock detected again
DM_MST: starting TM; DP14, 4-lane link detected
```
Docked-boot initialization on `7.0.0-28-generic`:
```text
thunderbolt 0000:34:00.6: 0: failed to allocate DP resource for port 7
amdgpu: [drm] DMUB notification skipped due to no handler: type=HPD
amdgpu: [drm] DM_MST: Differing MST start on aconnector
```
Failed hot-plug through one laptop USB-C port:
```text
usb 8-1: Device not responding to setup address.
usb 8-1: device not accepting address 2, error -71
```
# Historical information
Retained journals from kernels `6.17.0-22` and `6.17.0-23` already
contain USB enumeration failures, Thunderbolt disconnects, and AMDGPU
AUX/MST/DPCD failures. The `dcn31_program_compbuf_size` warning is newer
locally: it first appears with `7.0.0-15-generic` and recurs through
`7.0.0-27-generic`.
# Notes
The sequence suggests an interaction between USB4/Thunderbolt DP tunnel
setup and AMDGPU DPIA/MST initialization, but that is only an inference
from log ordering, not a root-cause claim. Please triage with both the
Ubuntu kernel and relevant upstream AMDGPU/Thunderbolt components in
mind.
A one-time oops in `trigger_hpd_mst_set [amdgpu]` occurred only after
manually writing an AMDGPU debugfs MST trigger. It was not part of
normal operation, so I will report it separately if needed rather than
using it as a workaround or conflating it with this issue.
# Attachments
1. The Apport report generated with `ubuntu-bug linux`.
2. Full kernel journals for: the runtime-disconnect boot, the docked boot
showing DP-resource/MST-ordering messages, the failed hot-plug path, and the
successful alternate-port path.
3. `sudo boltctl list --verbose` (if Bolt is installed) and `lspci -nnk`.
4. Do not use the AMDGPU debugfs MST HPD trigger; it caused a separate
diagnostic-only oops in this environment.
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Attachment added: "kernel amdgpu thunderbolt usb4 displayport mst
external-monito"
https://bugs.launchpad.net/bugs/2161425/+attachment/5984981/+files/ubuntu-linux-attachments-20260721.zip
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https://bugs.launchpad.net/bugs/2161425
Title:
Lenovo Thunderbolt 4 dock intermittently fails or disconnects, causing
USB device loss and both Radeon 680M-driven external MST displays to
blank
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