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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.

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HDMI port ..... –  mikewhatever Feb 14 '12 at 9:43
    
No, that's not it. I have all the ports turned off using powertop, but I still have that mysterious, extra "display backlight". –  Chan-Ho Suh Feb 14 '12 at 10:26
    
I don't see any reason here why you would think it's a bug and not a misconfiguration, as it turned out to be. For the benefit of others who will run into the same issue, the solution is to install the proprietary Nvidia drivers. Problem solved. –  Chan-Ho Suh Mar 9 '12 at 4:17
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up vote 2 down vote accepted

I have a MacBook Pro running Leopard and Ubuntu 11.10 in dual boot. System monitor also gives me the same 2 cpu working, and I had one of them always at 100%. I changed the NVIDIA propietary driver to the "current-updates" version and it helped, now both cpu run around 30%. Also, battery life seems to have been extended and I have less heat problems (still, the laptop heats up way more when running Ubuntu than when running MacOS)

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I should have paid closer attention to your answer. The Nvidia proprietary driver solved my issue, which I thought was unrelated to yours. –  Chan-Ho Suh Mar 9 '12 at 4:19
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