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docs: backlight: Clarify `actual_brightness` 

Currently userspace software systemd treats `brightness` and
`actual_brightness` identically due to a bug found in an out of tree
driver.

This however causes problems for in-tree drivers that use brightness
to report user requested `brightness` and `actual_brightness` to report
what the hardware actually has programmed.

Clarify the documentation to match the behavior described in commit
6ca017658b ("[PATCH] backlight: Backlight Class Improvements").

Cc: Lee Jones <lee@kernel.org>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: richard.purdie@linuxfoundation.org
Link: https://github.com/systemd/systemd/pull/36881
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Link: https://lore.kernel.org/r/20250415192101.2033518-1-superm1@kernel.org
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
This commit is contained in:
Mario Limonciello 2025-04-15 14:20:59 -05:00
parent 50935044e5
commit 8ecd32b668
1 changed files with 6 additions and 1 deletions
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7
Documentation/ABI/stable/sysfs-class-backlight
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@ -26,7 +26,12 @@ Date: March 2006
KernelVersion: 2.6.17
Contact: Richard Purdie <rpurdie@rpsys.net>
Description:
Show the actual brightness by querying the hardware.
Show the actual brightness by querying the hardware. Due
to implementation differences in hardware this may not
match the value in 'brightness'. For example some hardware
may treat blanking differently or have custom power saving
features. Userspace should generally use the values in
'brightness' to make decisions.
Users: HAL
What: /sys/class/backlight/<backlight>/max_brightness