From mboxrd@z Thu Jan  1 00:00:00 1970
From: Aaron Lu <aaron.lu@intel.com>
Subject: Re:
Date: Mon, 23 Dec 2013 09:44:48 +0800
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References: <CANsOggaW_Fve_AAt3=Zae62bcZeOeHiQpPrmg3B3YTV0H0gY_A@mail.gmail.com>
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To: Alex Barattini <alex.barattini@gmail.com>, linux-acpi@vger.kernel.org

On 12/22/2013 12:48 AM, Alex Barattini wrote:
> [1.] One line summary of the problem:
> 
> 8086:0166 [Dell Inspiron 15R 5521] Impossible to adjust the screen backlight
> 
> [2.] Full description of the problem/report:
> 
> The change in the level of illumination of the screen, through the
> corresponding option in the control panel, has no effect.

Does this patch help?
http://www.spinics.net/lists/linux-acpi/msg47755.html