Comment 40 for bug 333386

Let me try to give more information. The big problem with backlight control is that there are many ways to do it. There is a generic implementation in the acpi subsystem and then there are several vendor specific drivers that do the same with all the different interfaces vendors came up with.
With Acer laptops this driver is acer-wmi (And a Thinkpad uses thinkpad-acpi, so any feedback from non-Acer users is nicely meant but, sorry, totally useless).
Back in Intrepid (when it was working for you) both drivers (the generic and the vendor specific) were allowed to touch the backlight. But this caused problems for some (values changed twice on each keypress). To fix that some patches from upstream were backported that would only allow the acpi or the vendor driver but not both. Beside of other problems, this also did change the acer-wmi driver the wrong way. Now the driver would only work if the acpi generic driver was also there. But this bug is fixed in Jaunty.

So for Acer, and sorry if this is mostly duplicated, at some point long Launchpad bugs get confusing:
1. With the old Intrepid kernel that works, which directories are populated there?
    (/proc/acpi/video, /sys/devices/platform/acer-wmi/backlight, both?)
2. Then going to Jaunty (as this has the acer-wmi bug fixed at least), which of the places above are there.

Can you do a "grep -r . <dir>" and "ls -la <dir>" for the existing directories above (put all the output into one file and add comments which is from which kernel. Then attach it here as an uncompressed file, that is the most convenient form to look at).
The first thing that has to work would be the change when echoing into one of those interfaces. If that doesn't work, then the brightness applet cannot work.