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

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** Tags added: patch

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https://bugs.launchpad.net/bugs/2051074

Title:
  Mirror mode doesn't work when panel only supports one refresh rate
  with reduced blanking

Status in HWE Next:
  New
Status in mutter package in Ubuntu:
  Fix Committed
Status in mutter source package in Jammy:
  New
Status in mutter source package in Mantic:
  New
Status in mutter source package in Noble:
  Fix Committed

Bug description:
  [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.

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