Re: [PATCH] docs: backlight: Clarify `actual_brightness`

[Lee Jones <lee@kernel.org>]

On Tue, 08 Apr 2025, Mario Limonciello wrote:

> From: Mario Limonciello <mario.limonciello@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: Lennart Poettering <lennart@poettering.net>
> Cc: richard.purdie@linuxfoundation.org
> Link: https://github.com/systemd/systemd/pull/36881
> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> ---
>  Documentation/ABI/stable/sysfs-class-backlight | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/ABI/stable/sysfs-class-backlight b/Documentation/ABI/stable/sysfs-class-backlight
> index 6102d6bebdf9a..909cacde85041 100644
> --- a/Documentation/ABI/stable/sysfs-class-backlight
> +++ b/Documentation/ABI/stable/sysfs-class-backlight
> @@ -26,7 +26,9 @@ 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. This
> +		takes into account other factors besides the value programmed
> +		for 'brightness' and may not match the value in 'brightness'.

Which factors?  Under what circumstances may the values not match?

>  Users:		HAL
>  
>  What:		/sys/class/backlight/<backlight>/max_brightness
> -- 
> 2.43.0
> 

-- 
Lee Jones [李琼斯]