debcrafters-packages team mailing list archive

[Mario Limonciello <2110585@xxxxxxxxxxxxxxxxxx>]

>  How could we confirm AMD laptop supports custom brightness curve
(look in kernel log)? From the 'backlight' keyword, I only got these
logs below.

Custom brightness curve starts with 6.15 (it hasn't been backported).
But here is how you tell:

❯ sudo dmesg | grep custom
[    5.436063] amdgpu 0000:c1:00.0: amdgpu: [drm] Using custom brightness curve

However the reason that this needs to be backported now is because
adaptive backlight modulation (panel power savings) has the same effect
that user requested brightness doesn't equal actual brightness.  You can
discover that this feature is enabled by this sysfs file.

/sys/class/drm/card0-eDP-1/amdgpu/panel_power_savings

-- 
You received this bug notification because you are a member of
Debcrafters packages, which is subscribed to systemd in Ubuntu.
https://bugs.launchpad.net/bugs/2110585

Title:
  [SRU] Stop using 'actual_brightness' in systemd

Status in OEM Priority Project:
  Triaged
Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Noble:
  Incomplete
Status in systemd source package in Plucky:
  Incomplete
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.

  0. Use a laptop with an eDP panel, AMD laptop that supports custom
  brightness curve (look in kernel log)

  1. Look at value of brightness for device in /sys/class/backlight/. Note it down.
  $ grep . /sys/class/backlight/*/* 2>/dev/null

  2. Unplug the power supply, look at the value of brightness again. The
  actual_brightness will be decreased on AMD laptops that supportes
  custom brightnes curve.

  3. Reboot without power supply, look at value again, ensure the
  actual_brightnesss and brightness are the same with the ones before
  reboot.

  [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.

To manage notifications about this bug go to:
https://bugs.launchpad.net/oem-priority/+bug/2110585/+subscriptions