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It seems to be a recurrent problem and I try everything I can to solve this by myself but I do not have any solution left so here it goes.
I have installed archlinux and gnome 3, everything is up to date but I can't set the screen brightness (I am using the xf86-video-ati drivers with a Radeon mobility HD4300). I can't set it via the gnome control center, via xbacklight or setpci. When I try the fn keys, I get this message:
"No backlights were found on your system",
By googling, I tried to add the acpi_backlight=vendor and acpi_osi=Linux or acpi_osi="" to the kernel line but still the same message. I installed the linux-kamal kernel but still not working. I have no entry in /sys/class/backlight and no video folder in /proc/acpi so I can't set the brightness with echo <some value>.
Maybe someone could help me since I don't know what to do now and the brightness is locked to 100%, really hurting my eyes.
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http://www.linlap.com/wiki/hp+probook+4510s
http://www.linlap.com/wiki/hp+elitebook+8460p
Try to disable acpi boot kernel with option “acpi=off” .
When it works, there must be bug in BIOS or ACPI on at model,
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