Lenovo T470, display and kbd brightness control issues

Hello, I need help diagnosing an issue… when I unplug my laptop from AC, KDE’s display brightness won’t work on the display anymore and when it does, its just on/off level, its not the normal full backlight.
I’ve had this issue from long time and have no idea on what to try. I do see display and keyboard backlight controls in KDE, but the first one now controls my Logitech g15 display brightness and the one for the keyboard, does nothing.

Any help is appreciated.

What have you tried, it is this:


Or is it something else, if so please add a screen capture.

This screen brightness control is also not working for me, but the screen dims after minutes of inactivity.

What is working is changing the brightness via the command line:

$ cat /sys/class/backlight/intel_backlight/brightness
$ cat /sys/class/backlight/intel_backlight/maxbrightness
$ su
# echo 400 > /sys/class/backlight/intel_backlight/brightness

I think the reason why the KDE control does not work is that to write /sys/class/backlight/intel_backlight/brightness root access is needed and changing the brightness in the Energy Saving window does not ask me for the root password.

Reading a bit further on the Internet I found 3 Ways to Fix the Laptop Brightness Problem In Ubuntu.

Based on that I added “acpi_backlight=vendor” to my kernel cmdline using Yast → Boot loader → Kernel Parameters

After that the Screen brightness control in the window shared above works. What not works are the hardware keys, but is no real problem to me.