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When I boot my Linux with acpi_osi=Linux, I can control backlight brightness from almost_zero to moderately_bright. (Actually, the maximum revolves around the brightness level that is set before Linux starts)

When I boot my Linux with acpi_osi=Linux acpi_backlight=vendor, I can control brigtness from middle to maximum_bright

How to be able to control it from almost_zero to maximum_bright?

                 very_dim:   dim:   normal:  bright:  very_bright:
acpi_backlight:                          *************************
only acpi_osi:   ***************************
I want:          *************************************************
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2 Answers 2

up vote 1 down vote accepted

Seems like after loading the "i915" module the brightness is controlled by it.

The solution is using /sys/class/backlight/intel_backlight/brightness to control the brightess. It supports values from 0 to 250, allowing the full range on brightnesses.

(Why I overlooked it when I was searching for that control the first time?.. May be it was older kernel?)

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I've struggled with this problem quite a bit but from the kernel perspective. Basically, it's caused by the Linux kernel and the BIOS not communicating properly via ACPI. In other words, a the netbook has a buggy ACPI implementation, and the kernel cannot communicate to it properly. This has been a chronic problem since netbooks were introduced, but is less as much today. There are a few things you can try to solve this problem yourself:

  1. Try upgrading your Linux distribution or kernel (you didn't say what you're running) as it's possible a later kernel may have already solved this problem.

  2. Check if there are any BIOS updates from ASUS which fix the ACPI implementation. Unfortunately such an update will likely require you to use Windows to install it. That's a separate problem ;)

  3. Try modifying the backlight during the POST-stage of bootup. I've seen it where the hotkeys offer the full range of brightness levels only during very early bootup stages (i.e., before GRUB has kicked in.) Chances are, such a change will be maintained after Linux boots, but will not be modifiable beyond the initial range you reported.

Anyway, failing all of this, follow standard kernel bug-reporting procedures.

Hope this helps!

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I afraid that if reported as a bug then most likely developers consider dim backlight as unwanted weird mode and lock it to "normal -> very bright", as it behaves before Linux boot. I want non-standard mode which probably currently relies on "bug" and requires rebooting to set and unset. –  Vi. Aug 4 '12 at 0:26

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