Hi Lukas,
Thanks for retesting with the latest kernel. I've completed a source
investigation into this bug and have some findings to share.
The green lines on your GDM login screen appear to be caused by an
incorrect DSC (Display Stream Compression) bandwidth calculation in the
AMDGPU driver's USB-C/DP MST handling. The current 6.17 kernel uses the
wrong value when initialising monitors at login, which leads to DSC
validation failure. This is present in all linux-hwe-6.17 versions for
noble — no fix yet in that tree.
However, the fix does appear to exist in the linux-7.0.0 kernel tree
(Ubuntu 26.04 LTS). Could you test with a Ubuntu 26.04 LTS live USB or
CD? Just boot it, connect your monitors, and check whether the GDM login
screen displays cleanly — no need to log in.
If this confirms the fix, upgrading to Ubuntu 26.04 LTS would be the
quickest path forward.
Either way, this bug is now being marked as Triaged with High importance
as it is ready for a developer to review. Thanks again for the detailed
report — it made narrowing down the root cause much easier.
** Changed in: mesa (Ubuntu)
Status: Incomplete => Invalid
** Changed in: linux-hwe-6.17 (Ubuntu)
Status: Incomplete => Triaged
** Changed in: linux-hwe-6.17 (Ubuntu)
Importance: Undecided => High
** Also affects: mesa (Ubuntu Noble)
Importance: Undecided
Status: New
** Also affects: linux-hwe-6.17 (Ubuntu Noble)
Importance: Undecided
Status: New
** Summary changed:
- [amdgpu] Green lines flickering on login screen
+ [amdgpu] Green lines on GDM login screen over USB-C DP MST — DSC bandwidth
validation failure
** Description changed:
+ UPDATE: Source investigation identifies likely root cause — incorrect
+ DSC (Display Stream Compression) bandwidth calculation in the AMDGPU
+ driver's MST/USB-C code path. The current linux-hwe-6.17 kernel uses the
+ wrong value for DSC pic_width, causing DC_FAIL_DSC_VALIDATE during GDM
+ login screen initialisation. Fix confirmed present in linux-7.0.0
+ (Ubuntu 26.04 LTS).
+
+
+ === Original Description ===
+
I have a desktop pc with AMD Ryzen 7 9800X3D cpu, Micro-Star
International Co., Ltd. MS-7E59 motherboard and no graphic card other
than the integrated chip in cpu. I have ubuntu 24.04 installed on the
machine and when I turn on the computer and login screen comes up, there
are green horizontal lines across the screen to the point where the text
on the screen is not readable. When I manage to login and OS fully
starts up, these lines do not show anymore and system behaves fine
otherwise. Only place where these green horizontal lines show up are on
login screen.
I have one HDMI port on my motherboard and when I connect monitor
through the HDMI, the issue does not occur. My setup uses two screens,
currently both connected through 10Gbps USB-C to USB-C cables.
xrandr.txt file shows the refresh rate is 29.94Hz which may be the
issue
ProblemType: Bug
DistroRelease: Ubuntu 24.04
Package: gdm3 46.2-1ubuntu1~24.04.7
ProcVersionSignature: Ubuntu 6.17.0-22.22~24.04.1-generic 6.17.13
Uname: Linux 6.17.0-22-generic x86_64
ApportVersion: 2.28.1-0ubuntu3.8
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Sun Apr 26 19:15:56 2026
InstallationDate: Installed on 2026-01-26 (90 days ago)
InstallationMedia: Ubuntu 24.04.3 LTS "Noble Numbat" - Release amd64
(20250805.1)
SourcePackage: gdm3
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.gdm3.Init.Default: 2026-04-26T18:45:55.681416
mtime.conffile..etc.gdm3.custom.conf: 2026-04-26T19:15:53.425337
** Changed in: mesa (Ubuntu Noble)
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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