Linux kernel 4.5 looks like it’s turning out to be a good one for those of us using the newer ASUS Zenbooks (I have a Broadwell UX305). As of Release Candidate 3, it includes a change in the ambient light sensor driver and a fix for the keyboard’s airplane function button. This means that automatic screen brightness will work! The F2 key’s airplane mode button should also be working as well. For automatic brightness, You’ll have to be running GNOME 3.18 though (or something that recognizes iio-sensor-proxy), and have the know-how to get newer mainline kernels if your distribution doesn’t carry the latest and greatest. Linux 4.5 won’t be released likely until sometime in March.

I gave rc3 a spin a few weeks ago and the ambient light sensor is, in fact, working. I had some problems using it with the overhead fluorescent lighting in my house, but I think that’s a byproduct of my lighting’s limited and directed illumination pattern.

Update: The acpi-als driver fix got backported to kernel 4.4.3, so if you run Arch, you should already have automatic brightness on your Zenbook.