Thanks for testing Ubuntu 26.04 and confirming the issue is still
reproducible.
At this point we have a consistent pattern across:
Ubuntu 24.04 (6.17 HWE)
Ubuntu 24.04 updated HWE (6.17.0-29)
Ubuntu 26.04 LTS
The issue is:
- limited to USB-C DisplayPort Alt Mode / MST configuration
- present only during early display initialisation (GDM/login screen)
- not present after the session starts
- not reproducible over HDMI on the same hardware
This strongly suggests a timing or initial modeset issue in the amdgpu
display pipeline during early boot rather than a userspace (GDM/Mutter)
regression. Probably, as we both think, to do with that odd refresh
rate/bandwidth over the USB-C.
I have updated the summary to reflect that this appears to be an amdgpu
DC/MST initialisation issue affecting login-time display setup, and I've
marking the bug as Triaged with High importance for kernel/display team
attention.
I believe there's enough information here now though for a developer to
take a look at this, so I'll leave this as Triaged now with all the
information you've gathered, it's been a great help.
Thank you.
** Summary changed:
- [amdgpu] Green lines on GDM login screen over USB-C DP MST — DSC bandwidth
validation failure
+ [amdgpu] Green lines on GDM login screen over USB-C DP MST — DSC bandwidth
validation failure(?)
** Tags added: resolute
** Also affects: mesa (Ubuntu Resolute)
Importance: Undecided
Status: New
** Also affects: linux-hwe-6.17 (Ubuntu Resolute)
Importance: Undecided
Status: New
** Changed in: mesa (Ubuntu Resolute)
Status: New => Invalid
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https://bugs.launchpad.net/bugs/2150386
Title:
[amdgpu] Green lines on GDM login screen over USB-C DP MST — DSC
bandwidth validation failure(?)
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