[Bug 2051074] [NEW] Mirror mode doesn't work when panel only supports one refresh rate with reduced blanking

[Launchpad Bug Tracker]

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[Impact]
On systems equipped with reduced blanking panel, mirror mode can result to no image on external monitor.
The reason is due to current fallback modes don't have pixelclock for reduced blanking, so inadequate refresh rate is used to mode set, and result with atomic commit failure.

[Fix]
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3449
Add reduced blanking (RB) to fallback modes, so mutter can use RB mode to meet pixelclock requirement in order to pick the right refresh rate.

[Test]
Connect an external monitor to affected system. Use the monitor hotkey to switch to mirror mode or use GNOME control center to choose mirror mode. With the patch included, the issue is no longer observed.

[Where problems could occur]
The modelines of reduced blanking have reversed Hsync and Vsync flags, so if the driver doesn't handle those flags correctly, the atomic modeset/commit may fail.

I haven't observe any issue on Intel/AMD/Nvidia GPU so far.

** Affects: hwe-next
     Importance: Undecided
         Status: New

** Affects: mutter (Ubuntu)
     Importance: Undecided
         Status: Fix Committed

** Affects: mutter (Ubuntu Jammy)
     Importance: Undecided
         Status: New

** Affects: mutter (Ubuntu Mantic)
     Importance: Undecided
         Status: New

** Affects: mutter (Ubuntu Noble)
     Importance: Undecided
         Status: Fix Committed


** Tags: fixed-in-mutter-46.beta fixed-upstream multimonitor noble oem-priority originate-from-2044228 patch somerville
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Mirror mode doesn't work when panel only supports one refresh rate with reduced blanking
https://bugs.launchpad.net/bugs/2051074
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