Single GPU Base Mosaic not showing up - possible Xorg/driver conflict on Ubuntu 24.04

Graphics / Linux Linux
1

Subject: Base Mosaic option missing in nvidia-settings - single RTX A400 with two displays, GPU initialization failure in logs

Body:

I am trying to configure Base Mosaic on a single NVIDIA RTX A400 GPU with two displays connected (DP-0 and DP-6), but I cannot see the “Configuration” or “Base Mosaic” option in nvidia-settings (neither in Basic nor Advanced mode).

Environment:

  • OS: Ubuntu 24.04 (also tried Xorg session, not Wayland)

  • Driver: 595.58.03 (also tried 550 series)

  • GPU: NVIDIA RTX A400 (single card, professional series)

  • Displays: AOC 2060W3 (1920x1080) and HP S24e (1920x1080), both connected via DP

What I have tried:

  • Switched to “Ubuntu on Xorg” at login

  • Added nvidia-drm.modeset=0 iommu=off to GRUB_CMDLINE_LINUX_DEFAULT, ran update-grub and rebooted

  • Generated xorg.conf with sudo nvidia-xconfig --base-mosaic --metamodes="DP-0: 1920x1080 +0+0, DP-6: 1920x1080 +1920+0"

  • Manually added Option "BaseMosaic" "on" and Option "MetaModes" in /etc/X11/xorg.conf

  • Tried nvidia-xconfig --enable-all-gpus --multigpu=on --separate-x-screens=false

  • Cleaned /usr/share/X11/xorg.conf.d/ directory

Problem:
Despite all attempts, the “Configuration” dropdown (where Base Mosaic should be selectable) never appears in nvidia-settings → X Server Display Configuration. The two displays only work as separate extended screens (X Screen 0), not merged.

From nvidia-bug-report.log:
The log shows what appears to be a GPU initialization failure (see attached full log). Relevant snippet:

text

[     4.442] (WW) NVIDIA(0): Failed to initialize Base Mosaic!  Reason: Only one GPU
[     4.442] (WW) NVIDIA(0):     detected.  Only one GPU will be used for this X screen.

Question:
Is this GPU initialization failure the reason Base Mosaic is not showing up? Or is this a known compatibility issue between Ubuntu 24.04’s Xorg and NVIDIA Mosaic? Should I downgrade to Ubuntu 22.04, or are there other steps I can try on 24.04?

Thank you for any insights.