I'm not so certian it is a kernel error now after some digging deeper.
I've tried all of the following kernels:

3.11.10-03111015-generic
3.12.20-031220-generic
3.14.2-031402-generic
3.14.23-031423-generic
3.16.0-10-generic
3.16.0-14-generic
3.16.0-18-generic
3.16.0-23-generic
3.16.0-24-generic
3.17.2-031702-generic
3.18.0-031800rc4-generic


I tried downloading and booting from the currently available Utopic iso, and as 
well as the latest daily image of Vivid.

On the Utopic iso boot, running kernel 3.16.0-23-generic (according to
uname -a), the brightness controls function absolutely fine.

On the Vivid iso boot, the brightness controls no longer work.


Booting my machine from the same kernel version as what was working on the iso, 
3.16.0-23, the controls also still don't work.

Additionally, I am noticing the following dmesg errors:


On brightness down:
[   64.883616] ACPI Error: Method parse/execution failed [\_SB_.PCI0.GFX0.DWBL] 
(Node ffff88011a433500), AE_AML_UNINITIALIZED_ELEMENT (20140424/psparse-536)
[   64.883626] ACPI Error: Method parse/execution failed 
[\_SB_.PCI0.LPCB.EC0_._Q0E] (Node ffff88011a442c58), 
AE_AML_UNINITIALIZED_ELEMENT (20140424/psparse-536)

On brightness up:
[   64.888952] ACPI Error: Method parse/execution failed [\_SB_.PCI0.GFX0.UPBL] 
(Node ffff88011a4334d8), AE_AML_UNINITIALIZED_ELEMENT (20140424/psparse-536)
[   64.888959] ACPI Error: Method parse/execution failed 
[\_SB_.PCI0.LPCB.EC0_._Q0F] (Node ffff88011a442c80), 
AE_AML_UNINITIALIZED_ELEMENT (20140424/psparse-536)


Could there have been a userland change that is causing some mismatch or 
somesuch?

** Changed in: linux (Ubuntu)
       Status: Incomplete => Confirmed

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https://bugs.launchpad.net/bugs/1392101

Title:
  Brightness controls not working on intel igfx

Status in “linux” package in Ubuntu:
  Confirmed

Bug description:
  Brightness controls not working for me in Vivid Vervet as of 1-2 weeks ago.
  Was confirmed working on Utopic Unicorn.

  Hotkeys for brightness up & down non-functioning,
  Hotkey for display on/off still works.

  Gnome control center (brightness & lock) does not show brightness
  slider.

  What does work is
  "echo $value > /sys/class/backlight/intel_backlight/brightness"
  as root for any $value of 0 - 2047

  Running on an Asus S400C Laptop.

  kernel: 
  x86_64 3.16.0-24-generic

  lspci:
  00:02.0 VGA compatible controller: Intel Corporation 3rd Gen Core processor 
Graphics Controller (rev 09)

  lsb_release:
  Description:  Ubuntu Vivid Vervet (development branch)
  Release:      15.04

  ProblemType: Bug
  DistroRelease: Ubuntu 15.04
  Package: linux-image-3.16.0-24-generic 3.16.0-24.32
  ProcVersionSignature: Ubuntu 3.16.0-24.32-generic 3.16.4
  Uname: Linux 3.16.0-24-generic x86_64
  ApportVersion: 2.14.7-0ubuntu10
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC0:  martin     1970 F.... pulseaudio
  CurrentDesktop: Unity
  Date: Wed Nov 12 16:06:27 2014
  HibernationDevice: RESUME=UUID=a7b59988-1d75-4f19-8fd8-3c9a08c38e8d
  InstallationDate: Installed on 2014-08-21 (83 days ago)
  InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Alpha amd64 (20140820)
  MachineType: ASUSTeK COMPUTER INC. S400CA
  ProcFB: 0 inteldrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.16.0-24-generic.efi.signed 
root=UUID=e2aa386f-a710-4fb4-ba6e-5d233c6b7b25 ro quiet splash vt.handoff=7
  RelatedPackageVersions:
   linux-restricted-modules-3.16.0-24-generic N/A
   linux-backports-modules-3.16.0-24-generic  N/A
   linux-firmware                             1.138
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 01/27/2014
  dmi.bios.vendor: American Megatrends Inc.
  dmi.bios.version: S400CA.210
  dmi.board.asset.tag: ATN12345678901234567
  dmi.board.name: S400CA
  dmi.board.vendor: ASUSTeK COMPUTER INC.
  dmi.board.version: 1.0
  dmi.chassis.asset.tag: No Asset Tag
  dmi.chassis.type: 10
  dmi.chassis.vendor: ASUSTeK COMPUTER INC.
  dmi.chassis.version: 1.0
  dmi.modalias: 
dmi:bvnAmericanMegatrendsInc.:bvrS400CA.210:bd01/27/2014:svnASUSTeKCOMPUTERINC.:pnS400CA:pvr1.0:rvnASUSTeKCOMPUTERINC.:rnS400CA:rvr1.0:cvnASUSTeKCOMPUTERINC.:ct10:cvr1.0:
  dmi.product.name: S400CA
  dmi.product.version: 1.0
  dmi.sys.vendor: ASUSTeK COMPUTER INC.

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