Bug 455024

Summary: Brightness slider lets me disable the backlight
Product: [Plasma] plasmashell Reporter: alexandermasokin
Component: Power and Battery widgetAssignee: ratijas <me>
Status: RESOLVED UPSTREAM    
Severity: normal CC: kde, me, nate
Priority: NOR Keywords: usability
Version First Reported In: 5.24.5   
Target Milestone: 1.0   
Platform: Arch Linux   
OS: Linux   
See Also: https://bugs.kde.org/show_bug.cgi?id=447475
Latest Commit: Version Fixed/Implemented In:
Sentry Crash Report:

Description alexandermasokin 2022-06-08 12:36:10 UTC 
SUMMARY
When I click the battery and brightness icon, and adjust the brightness, it goes all the way down to 0% and I can't see anything!

STEPS TO REPRODUCE
1. Click battery and brightness icon in the taskbar
2. Drag brightness slider all the way down to the lowest setting (0%)

OBSERVED RESULT
Backlight is now off, and it is now impossible to see anything unless you have light theme enabled and shine a torch directly at the screen.

EXPECTED RESULT
Backlight should remain on at least 1% brightness.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.24.5
KDE Frameworks Version: 5.94.0
Qt Version: 5.15.4

ADDITIONAL INFORMATION
Kernel: 5.18.1-zen1-1-zen x86_64
CPU: Intel Core i7-10750H
GPU: Intel Corporation CometLake-H GT2 [UHD Graphics] (rev 05)
Display server: X11
Backlight: /sys/class/backlight/intel_backlight
Comment 1 Nate Graham 2022-06-08 18:45:12 UTC 
You're talking about the System Tray applet, right? In there, the slider does go down to 0%, but internally it's actually setting the backlight value to 1, not 0. This would suggest that your display hardware turns off the backlight at a value above 0, which is unfortunate. Sadly this isn't standardized at all.
Comment 2 alexandermasokin 2022-06-09 13:55:28 UTC 
(In reply to Nate Graham from comment #1)
> You're talking about the System Tray applet, right? In there, the slider
> does go down to 0%, but internally it's actually setting the backlight value
> to 1, not 0. This would suggest that your display hardware turns off the
> backlight at a value above 0, which is unfortunate. Sadly this isn't
> standardized at all.
Yes, the system tray applet for battery and brightness. Why not just limit the slider to 1-100%? 1% is barely visible at all in daylight but perfect for using my computer in complete darkness.
Comment 3 ratijas 2022-06-10 10:22:45 UTC 
> Yes, the system tray applet for battery and brightness. Why not just limit the slider to 1-100%?

>> In there, the slider does go down to 0%, but internally it's actually setting the backlight value to 1%, not 0%.

It already does exactly that. Seems like your hardware treats that 1% as a signal to turn the screen completely off.

Please, test your setup more and tell us what's the lowest limit when the screen is not turned off yet. You may write raw values to sysfs backlight 'files', use xbacklight or whatever the tool you are using.
Comment 4 Nate Graham 2022-06-10 16:06:26 UTC 
Indeed, I would be interested in knowing that as well. Turning off the backlight at nonzero brightness values is very odd behavior from the hardware.
Comment 5 alexandermasokin 2022-06-12 19:27:10 UTC 
/sys/class/backlight/intel_backlight/brightness: 232 sets the backlight to the lowest level without switching it off. 231 switches it off completely. max_brightness is 120000.
Comment 6 ratijas 2022-06-12 22:35:46 UTC 
Both values are slightly below the 2% threshold: 0,019(3) vs. 0,01925.  I guess we could bump the minimum brightness to 2% then.
Comment 7 Nate Graham 2023-04-10 01:05:03 UTC 
This has since been fixed by a change in the Intel GPU driver.