From mboxrd@z Thu Jan  1 00:00:00 1970
From: Aaron Lu <aaron.lu@intel.com>
Subject: Re: Multiple video backlight devices on Dell laptop
Date: Tue, 14 Jan 2014 10:49:40 +0800
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References: <CAJrk0Bv5w25vipp8vTmoZEegb-7Va+FLvek=QnTvSfHDFjvroA@mail.gmail.com>
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To: Bradley Baetz <bbaetz@gmail.com>, linux-acpi@vger.kernel.org

On 01/11/2014 11:22 AM, Bradley Baetz wrote:
> Hi,
> 
> I have a Dell Vostro 3560 laptop, with hybrid graphics (intel + radeon).
> 
> This exposes 3 backlight devices - acpi_video0, acpi_video1, and
> intel_backlight. Only the intel_backlight/brightness file actually
> changes the backlight. Is this a kernel bug, or should userspace
> 'know' which backlight file to use based on which graphics device is
> active? XFCE picks the first one it finds (acpi_video0), which doesn't
> work.

Solving it from userspace:
You can specify which backlight interface to use by editing /etc/xorg.conf:
 Section "Device"
    Identifier  "Intel Graphics"
    Driver      "intel"
    Option      "AccelMethod"     "sna"
    Option      "Backlight"       "intel_backlight" # use your backlight that works here
    Driver      "intel"
    BusID       "PCI:0:2:0"
 EndSection

See https://wiki.archlinux.org/index.php/Backlight

solving it from kernel space:
Since the firmware claims support of win8, you can forget the
acpi_backlight=vendor cmdline option and just use video.use_native_backlight=1.

Hope it helps.
-Aaron