Comment # 24 on bug 1015020 from Max Staudt

This is a real problem on your hardware, however I'm afraid we can't progress
without hardware that exhibits the same issue.

I'll close this bug as there is nothing more that we can do. Sorry.

As a workaround, you can set your backlight manually using sysfs, as you've
already recognized.

Another thing you can do, in case your X11 desktop environment has a power
manager that is listening to these keys, is to simulate the keypresses with
other keys. You can bind shortcuts that call a tool such as this:

  https://gist.github.com/260841.git

Compile the tool and call it with

  ./fake_key KEYSYM

where KEYSYM can be either XF86MonBrightnessUp or XF86MonBrightnessDown . On my
XFCE system, this triggers xfce4-power-manager's brightness control.

In case you stumble upon the change in the kernel that broke your system, or
any other information that tells us how to fix your problem, you're very
welcome to tell us in this bug.