Public bug reported:
Ubuntu 24.04.4, mutter 46.2-1ubuntu0.24.04.16, gnome-shell
46.0-0ubuntu6~24.04.14,
kernel 7.0.0-30-generic, Intel Arrow Lake-P graphics (i915).
I have an LG UltraFine 5K connected over two DP streams (DP-2 and DP-3). On the
Xorg session it comes up as one 5120x2880 display like it should. On the Wayland
session it comes up as two monitors: DP-2 at 2560x2880 and DP-3 at 3840x2160,
which is the panel's single-stream fallback mode. With my second monitor plugged
in, Settings > Displays shows three displays. The GDM greeter does the same
thing. Nothing about it is logged.
The kernel side is fine. Both connectors carry a proper TILE property:
DP-3 (obj 540): 1:1:2:1:0:0:2560:2880 (main tile)
DP-2 (obj 531): 1:1:2:1:1:0:2560:2880
and mutter has read it, GetResources on org.gnome.Mutter.DisplayConfig shows
'tile' on both outputs. Forcing a re-probe of DP-2 (echo off/detect to its sysfs
status) makes mutter rebuild monitors and the result is the same. There is no
"Failed to create tiled monitor output" warning, the tiled path isn't reached.
Cause is in
debian/patches/ubuntu/x11-Add-support-for-fractional-scaling-using-Randr.patch.
It introduces META_MONITOR_MANAGER_CAPABILITY_TILING and changes
rebuild_monitors()
in src/backends/meta-monitor-manager.c from upstream's
if (output_info->tile_info.group_id)
to
if (has_tiling && output_info->tile_info.group_id)
Only the xrandr backend (randr >= 1.5) and the test dummy backend set that
capability. meta_monitor_manager_native_get_capabilities() never does, so under
Wayland the tiled branch is skipped and each tile becomes a MetaMonitorNormal.
Upstream has no gate here at all. Same code in 46.2-1ubuntu0.24.04.12, so this
is not related to the LP #2131575 change.
Fix is one line in src/backends/native/meta-monitor-manager-native.c, set the
capability there too. I rebuilt 46.2-1ubuntu0.24.04.16 with it and the UltraFine
is one 5120x2880 monitor again under Wayland, per-monitor scaling works. Debdiff
attached.
Should hit any DisplayPort tiled monitor on the Wayland session (Dell UP2715K,
UP3218K, Apple Studio Display in tiled mode, ...).
** Affects: mutter (Ubuntu)
Importance: Undecided
Status: New
** Tags: noble patch wayland
** Patch added: "mutter_46.2-1ubuntu0.24.04.17.debdiff"
https://bugs.launchpad.net/bugs/2166435/+attachment/5997302/+files/mutter_46.2-1ubuntu0.24.04.17.debdiff
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https://bugs.launchpad.net/bugs/2166435
Title:
Tiled monitor (LG UltraFine 5K) shows up as two displays on Wayland,
fine on Xorg
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