[PATCH] docs: backlight: Clarify `actual_brightness`

[Mario Limonciello]

On 4/15/2025 10:53 AM, Lee Jones wrote:
> On Tue, 08 Apr 2025, Mario Limonciello wrote:
> 
>> From: Mario Limonciello <mario.limonciello at 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 at poettering.net>
>> Cc: richard.purdie at linuxfoundation.org
>> Link: https://github.com/systemd/systemd/pull/36881
>> Signed-off-by: Mario Limonciello <mario.limonciello at 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 at 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?

Is this question for your background or are you saying these should be 
enumerated here in the documentation?

Richard Purdie mentioned:

"The challenge back then was that things like framebuffer blanking would 
turn off the backlight but the single brightness attribute was 
implemented inconsistently. Some drivers would report no backlight when 
blanking was active, some would report the value stored in brightness. 
This had multiple issues"

For AMD the two cases that can cause this on amdgpu are Custom 
brightness curves and Adaptive Backlight Management (ABM).


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