With my Lenovo L530, the brightness doesn't work well. When I press Fn + F8 or F9 to modify the screen brightness, xfce4-power-manager (or another component) modifies /sys/class/backlight/acpi_video0/brightness, but the brightness changes only for certain values multiple of 5.
# cat max_brightness
# cat brightness
# echo 89 > brightness
It doesn't work. If the brightness is at 100 and I press Fn + F8, the value is 89, and the screen brightness doesn't change.
# echo 80
Now it works. Most multiples of 5 work, but not all. For example 95 doesn't work.
I use Debian testing (up-to-date), with Linux version 3.11.10, and Xfce 4.10.
I think it is a regression in the kernel.
With Linus' kernel tree v3.13-rcX, please test adding video.use_native_backlight=1 to kernel cmdline and report back, thanks.
I've rebooted with an old kernel, the 3.2.46, and the screen brightness is updated correctly, on each press on Fn + F8 or F9. The difference: max_brightness == 15.
Now I'll reboot on the 3.13-rc4.
On Linux 3.13-rc4 without video.use_native_backlight=1, the bug occurs.
With video.use_native_backlight=1, it works.
# cat /sys/class/backlight/intel_backlight/max_brightness
We can see in this directory that the brightness value is updated when pressing Fn + F8 or F9.
(the /sys/class/backlight/acpi_video0/ directory doesn't exist)
*** This bug has been marked as a duplicate of bug 51231 ***
Note that video.use_native_backlight=1 is required, it doesn't work out-of-the-box. Is it possible to fix that?
Yes, there is a patch to change the default value of use_native_backlight to 1:
Once that is applied, you do not need anything extra.
For now, you can also use a xorg.conf file to tell X driver that the intel_backlight interface should be used instead of acpi_video0:
$ cat /etc/Xorg/Xorg.conf
Option "Backlight" "intel_backlight"
Okay, thank you for the information.
Please attach the output of acpidump and dmidecode:
# acpidump > acpidump.txt
# dmidecode > dmi.txt
Created attachment 126431 [details]
Output of acpidump
Created attachment 126441 [details]
Output of dmidecode