On 2025-04-15 15:20, Mario Limonciello wrote:
> From: Mario Limonciello <mario.limoncie...@amd.com>
>
> 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
> 6ca017658b1f9 ("[PATCH] backlight: Backlight Class Improvements").
>
> Cc: Lee Jones <l...@kernel.org>
> Cc: Lennart Poettering <lenn...@poettering.net>
> Cc: richard.pur...@linuxfoundation.org
> Link: https://github.com/systemd/systemd/pull/36881
> Signed-off-by: Mario Limonciello <mario.limoncie...@amd.com>
> ---
> v2:
> * Add more explanation
Reviewed-by: Harry Wentland <harry.wentl...@amd.com>
Harry
> ---
> Documentation/ABI/stable/sysfs-class-backlight | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/ABI/stable/sysfs-class-backlight
> b/Documentation/ABI/stable/sysfs-class-backlight
> index 6102d6bebdf9a..40b8c46b95b28 100644
> --- a/Documentation/ABI/stable/sysfs-class-backlight
> +++ b/Documentation/ABI/stable/sysfs-class-backlight
> @@ -26,7 +26,12 @@ Date: March 2006
> KernelVersion: 2.6.17
> Contact: Richard Purdie <rpur...@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
