Bug 521568

Summary:	KWin Wayland / NVIDIA: "atomic commit failed: Permission denied" during login transition from SDDM Wayland to User Session
Product:	[Plasma] kwin	Reporter:	wentf9
Component:	xwayland	Assignee:	KWin default assignee <kwin-bugs-null>
Status:	REPORTED ---
Severity:	normal	CC:	kdedev
Priority:	NOR
Version First Reported In:	6.6.5
Target Milestone:	---
Platform:	CachyOS
OS:	Linux
See Also:	https://bugs.kde.org/show_bug.cgi?id=521594 https://bugs.kde.org/show_bug.cgi?id=524540
Latest Commit:		Version Fixed/Implemented In:
Sentry Crash Report:
Attachments:	output by kscreen-doctor -o output by drm_info output by kscreen-doctor -o when the bug happens in tty

Description wentf9 2026-06-17 11:51:18 UTC

### System Information
* **OS:** CachyOS (Arch Linux based)
* **KDE Plasma Version:** 6.x (Wayland)
* **KDE Frameworks Version:** 6.x
* **Qt Version:** 6.x
* **Graphics Hardware:** NVIDIA GeForce RTX 5070 Ti (GB203 architecture)
* **Driver Version:** 610.43.02 (Proprietary)
* **Display Manager:** SDDM (configured to run on Wayland with `kwin_wayland`)

### Description
When logging in from SDDM (which is configured to run on Wayland using `kwin_wayland`) to a user Plasma Wayland session, the screen goes black and the monitor loses signal.

Investigation of `journalctl` shows that during the session transition, the SDDM greeter compositor exits, but the incoming user `kwin_wayland` session fails to commit frames to the DRM device due to a permission error:
```text
sddm-helper-start-wayland[960]: "kwin_wayland_drm: atomic commit failed: 权限不够\n"
sddm-helper-start-wayland[960]: Stopping... "kwin_wayland"
```

The system remains running in the background (no signal to display).

**Workaround:**
Pressing `Ctrl + Alt + F3` to switch to TTY3, and then `Ctrl + Alt + F2` to switch back to the GUI session on TTY2 immediately restores the signal and displays the desktop. This suggests switching VTs forces a reset or release of the DRM Master lock, allowing the user's `kwin_wayland` to successfully gain control.

Changing SDDM's display server back to `x11` completely bypasses this issue, suggesting a race condition/deadlock in DRM Master transfer between the SDDM Wayland greeter and the user's Wayland compositor on NVIDIA.

### Steps to Reproduce
1. Configure SDDM to run on Wayland (using `kwin_wayland` as the compositor).
2. Boot the system and reach the login screen.
3. Enter the password and press Enter.
4. Observe the screen going black/losing signal.
5. Switch to TTY3 and back to TTY2 to restore the screen.

Comment 1 TraceyC 2026-08-21 18:19:37 UTC

Thanks for the bug report. To investigate, we need  the output of this command:
kscreen-doctor -o

Also, please get the output of drm_info when the bug happens. You can run it from ssh if you can't get to a TTY

Comment 2 wentf9 2026-08-23 07:57:40 UTC

Created attachment 195393 [details]
output by kscreen-doctor -o

Comment 3 wentf9 2026-08-23 07:59:34 UTC

Created attachment 195394 [details]
output by drm_info

Comment 4 wentf9 2026-08-23 08:02:18 UTC

Created attachment 195395 [details]
output by kscreen-doctor -o when the bug happens in tty

Comment 5 TraceyC 2026-08-24 19:38:58 UTC

Thanks for the details. The kwin developers will take it from here.