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[Bin Li <2110585@xxxxxxxxxxxxxxxxxx>]

** Tags added: jira-sutton-2411 oem-priority

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https://bugs.launchpad.net/bugs/2110585

Title:
  Stop using 'actual_brightness' in systemd

Status in OEM Priority Project:
  New
Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Noble:
  Confirmed
Status in systemd source package in Plucky:
  Confirmed
Status in systemd source package in Questing:
  Fix Released

Bug description:
  [Impact]

  At shutdown time systemd will record the most recently used brightness
  value and then attempt to restore it the next boot.

  In order to do this; it currently will read the 'actual_brightness'
  value and use that to save.

  This normally works fine; but GPU drivers don't always show the same
  value in 'brightness' and 'actual_brightness' meaning that the wrong
  value may be restored the next boot.

  This problem is very prominently found on AMD APUs starting with
  kernel 6.15 due to a new feature called "custom brightness curves".
  Custom brightness curves are brightness values programmed by an OEM in
  the firmware to make the panel behave optimally.  The
  'actual_brightness' of some 'brightness' values will be lower.

  The same issue can occur on systems using "panel power savings" which
  behaves similarly.

  For example if the brightness a user requested was 150, the
  actual_brightness might be 130. So the next boot the brightness will
  be "set" to 130, but the actual brightness might be 115. If the user
  reboots again it will be set to 115 for the next boot but the actual
  brightness might be 100. That is this gets worse and worse each reboot
  cycle until the system eventually boots up at minimum brightness.

  [Test Plan]

  Verify brightness save/restore works properly on Intel UMA, AMD UMA,
  and either I+N or A+N systems.

  [Where problems could occur]

  If a driver relies upon actual_brightness rather than brightness
  (which is against kernel API [1] then the brightness might no longer
  be saved and restored properly.

  [1] https://origin.kernel.org/doc/html/next/admin-guide/abi-
  stable.html#abi-sys-class-backlight-backlight-actual-brightness

  [Other Info]

  This has been fixed in upstream systemd in this commit:
  https://github.com/systemd/systemd/commit/9a224c307b36610e3675390eb2620e74d0f4efb0
  It's been brought into stable systemd releases carried by other distros as well.
  No regressions have been reported upstream that I'm aware of.

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