Comment 2 for bug 524214

More testing has yielded this find:

It seems as though the laptop brightness works like my old Dell 700m. Plug in the power cable, the brightness immediately increases (to the preset value), unplug it, it immediately dims, with no transition between brightness states.

With the XPS 1340, whenever brightness settings are changed, the brightness values change immediately (i.e. from 7 to 15) with no transition, but Ubuntu (or acpi) resets it back down and incrementally increases it from 7 to 15. Ditto for brightness decreases.

So it seems like acpi can't decide whether or not it should incrementally change the brightness, or just set it once and forget it.

Hopefully, I can provide logs, if someone can tell me where to look!