Ok, so in
powertop (version 1.97), in both the "overview" and "device stats" sections, I see two devices, each called "display backlight". One shows the correct percentage of use, e.g. if I lower screen brightness then the use goes down, but the other is always at 100%! The 100% one is always consuming around 2.3 watts, so of course I am interested in killing this thing off, whatever it is.
So my question is: why is powertop doing this? Is there some technical way in which I do have two displays? I have a MacBook Air, and I've never even once attached it to an external display.
Update: Even stranger is that when I start up my system, the "display backlight" at 100% is only using 800-900 mW; however, over time, it will work its way up to 2.3 W.