debcrafters-packages team mailing list archive

[Mario Limonciello <2110585@xxxxxxxxxxxxxxxxxx>]

I've adjusted the text to better explain the situation with panel power
savings as that can be reproduced now with current kernel.

** Description changed:

  [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.
+ This problem is very prominently found on AMD APUs due to two features:
+ * Panel power savsings
+ * Custom Brightness curves
  
- The same issue can occur on systems using "panel power savings" which
- behaves similarly.
+ Panel power savings has been introduced in kernel 6.8 and is activated
+ by power profiles daemon dynamically.
+ 
+ Custom brightness curves are brightness values programmed by an OEM in
+ the firmware to make the panel behave optimally.  Support for this is
+ introduced in kernel 6.15.
+ 
+ The 'actual_brightness' of some 'brightness' values will be lower in
+ both cases.
  
  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. Use a laptop with an eDP panel, AMD laptop.  Verify the presence of
+ panel power savings feature by looking at
+ /sys/class/drm/card0-eDP/amdgpu/panel_power_savings.
  
- 1. Look at value of brightness for device in /sys/class/backlight/. Note it down.
+ 2. Manually set power savings to maximum:
+ 
+ # echo 4 | sudo tee /sys/class/drm/card0-eDP/amdgpu/panel_power_savings
+ 
+ 3. 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.
+ 4. Reboot look at value again, ensure the brightness value is the same
+ as 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.

-- 
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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 due to two features:
  * Panel power savsings
  * Custom Brightness curves

  Panel power savings has been introduced in kernel 6.8 and is activated
  by power profiles daemon dynamically.

  Custom brightness curves are brightness values programmed by an OEM in
  the firmware to make the panel behave optimally.  Support for this is
  introduced in kernel 6.15.

  The 'actual_brightness' of some 'brightness' values will be lower in
  both cases.

  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.

  1. Use a laptop with an eDP panel, AMD laptop.  Verify the presence of
  panel power savings feature by looking at
  /sys/class/drm/card0-eDP/amdgpu/panel_power_savings.

  2. Manually set power savings to maximum:

  # echo 4 | sudo tee
  /sys/class/drm/card0-eDP/amdgpu/panel_power_savings

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

  4. Reboot look at value again, ensure the brightness value is the same
  as 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.

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