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[David Chen <david.chen@xxxxxxxxxxxxx>]

I am having the similar issue on Ubuntu 13.10.  Under
/sys/class/backlight, I have 2 interfaces: intel_backlight and
dell_backlight.  By echo brightness values into 2 different interfaces,
the intel one is the actual brightness control interface.  Tried to
remove dell-laptop kernel module, and adding acpi_backlight=vendor
kernel option, brightness hotkey starts working.

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https://bugs.launchpad.net/bugs/1185927

Title:
  8086:0046 [Dell XPS 15 L501x] Unable to change backlight by default

Status in “linux” package in Ubuntu:
  Incomplete

Bug description:
  While I disabled my nvidia card in BIOS, I am unable to change the
  backlight by default.

  WORKAROUND: Use both kernel parameters:
  acpi_backlight=vendor dell_laptop.backlight=0

  Having just 'acpi_backlight=vendor' means that if I quickly and
  repeatedly press the key to change brightness, it eventually lowers,
  but sometimes the down key makes it higher and visa versa, making it
  difficult to change. Adding 'dell_laptop.backlight=0' makes the
  controls work perfectly normally. But when I boot, I get the message:
  dell_laptop: '0' invalid parameter for 'backlight', at which my
  computer then continues to boot and works normally, with the
  brightness control working.

  ProblemType: Bug
  DistroRelease: Ubuntu 13.04
  Package: linux-image-3.8.0-22-generic 3.8.0-22.33
  ProcVersionSignature: Ubuntu 3.8.0-22.33-generic 3.8.11
  Uname: Linux 3.8.0-22-generic x86_64
  ApportVersion: 2.9.2-0ubuntu8
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC1:  benjamin   4172 F.... pulseaudio
   /dev/snd/controlC0:  benjamin   4172 F.... pulseaudio
  Date: Thu May 30 19:07:26 2013
  HibernationDevice: RESUME=UUID=74004122-cb29-4972-92c0-8bd036223958
  InstallationDate: Installed on 2013-04-26 (34 days ago)
  InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
  MachineType: Dell Inc. XPS L501X
  MarkForUpload: True
  ProcFB: 0 inteldrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.8.0-22-generic root=UUID=df9e5312-fd68-4a0f-99fb-8b6667021bea ro quiet splash acpi_backlight=vendor dell_laptop.backlight=0
  RelatedPackageVersions:
   linux-restricted-modules-3.8.0-22-generic N/A
   linux-backports-modules-3.8.0-22-generic  N/A
   linux-firmware                            1.106
  RfKill:
   0: phy0: Wireless LAN
    Soft blocked: no
    Hard blocked: no
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 10/15/2010
  dmi.bios.vendor: Dell Inc.
  dmi.bios.version: A03
  dmi.board.name: 00CKNG
  dmi.board.vendor: Dell Inc.
  dmi.board.version: A03
  dmi.chassis.type: 8
  dmi.chassis.vendor: Dell Inc.
  dmi.chassis.version: A03
  dmi.modalias: dmi:bvnDellInc.:bvrA03:bd10/15/2010:svnDellInc.:pnXPSL501X:pvrA03:rvnDellInc.:rn00CKNG:rvrA03:cvnDellInc.:ct8:cvrA03:
  dmi.product.name: XPS L501X
  dmi.product.version: A03
  dmi.sys.vendor: Dell Inc.

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