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Old 08-15-2011, 03:08 AM   #1
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Question No backlight controls in /sys/class (ATI Mobility Radeon HD 5650, 3.0.1 kernel)

Firstly, I'm running Gentoo amd64 on an HP Pavilion dv6, with the ATI drivers (11.7, but I've had the same problems since 11.5), and a 3.0.1 vanilla kernel (again, problems since 2.6.37). I only got the laptop a few months ago, so I haven't tried any newer drivers.

I have mucked around with the kernel config, and put in all the obviously backlight-related options, but I can't work out how to get backlight control working. At the moment, nothing shows up in /sys/class/backlight/; it's just an empty directory. The backlight buttons don't do anything, and when it boots, the laptop just sits at full brightness, which makes it very difficult to use when not plugged in.
I know the backlight works únder Linux, because it has responded properly in OpenSUSE, Fedora 15, and Ubuntu.

So, my question, what do I need to do to enable backlight support? Apart from this issue, and the fingerprint-reader and some multimedia buttons, the hardware runs Gentoo flawlessly, and it's a shame to have such a seemingly simple problem persist.
Old 08-20-2011, 12:38 PM   #2
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Hello hrasdt, welcome to LQ,

there are special modules for laptops available in the kernel. You can find them in the kernelsources, when doing "make xconfig" in "Device Drivers"->"x86 Platform Specific Drivers" you'll find at least two modules which are related to HP, hp_wmi and hp_accel. Often these vendor-specific modules are necessary when one wants to use the function-keys. You should try to load them via modprobe.

If it doesn't work, I'd recommend to boot one of the Live-DVDs where you know it works (Suse or Ubuntu) and check which modules they load, I'd recommend when the live-DVD is booted to execute
lsmod | sort > lsmod_suse.txt
and store the textfile on the system. Then after rebooting Gentoo you can compare it with the output of the same command.

Hope this helps

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Old 08-30-2011, 04:15 AM   #3
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It turns out, I actually had all of the ACPI options enabled in Power Management->ACPI except for the backlight controls.
To anyone led by Google here, don't forget to check the ACPI bits as well as the graphics card-specific stuff.
Thanks markush for the tip, I used a kernel seed to work out which bits to enable. Thread will be closed!


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