Commit 8ecd32b6 authored Apr 15, 2025 by Mario Limonciello

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
6ca01765 ("[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>

parent 50935044

Documentation/ABI/stable/sysfs-class-backlight

+6 −1

Original line number	Diff line number	Diff line
		@@ -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