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[Richard Laager <rlaager@xxxxxxxxxx>]

I can confirm this on Vivid with the proprietary nvidia driver (version
346) installed. With switchable graphics disabled, I have an acpi_video0
backlight that does nothing. That is, the contents of
/sys/class/backlight/acpi_video0/backlight changes, but the screen's
brightness does not. If I enable switchable graphics, then I have only a
/sys/class/backlight/intel_backlight, and it works.

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https://bugs.launchpad.net/bugs/1411514

Title:
  [Dell Precision M4800] Brightness can't be controlled on 14.04.1

Status in HWE Next Project:
  Triaged
Status in linux package in Ubuntu:
  Confirmed

Bug description:
  CID: 201305-13527 Dell Precision M4800

  The screen brightness

  The brightness control does not work at all on 14.04.1

  Steps:
  1. Install 14.04.1 + update
  2. Test the brightness control with hotkeys
  3. Test the brightness control by using the "Brightness and Lock" applet.

  Expected results:
  * Brightness control should work in step 2 and 3

  Actual result:
  * Brightness level would change with the hotkey, but the backlight level won't, nothing happened when you scroll the scrollbar in the "Brightness and Lock" applet.

  Debugging information:
  ubuntu@201305-13527:~$ sudo showkey -s
  kb mode was ?UNKNOWN?
  [ if you are trying this under X, it might not work
  since the X server is also reading /dev/console ]

  press any key (program terminates 10s after last keypress)...
  0x9c
  0xe0 0x54 0xe0 0xd4
  0xe0 0x4c 0xe0 0xcc
  0x1d
  ^Ccaught signal 2, cleaning up...
  ubuntu@201305-13527:~$

  ubuntu@201305-13527:~$ sudo showkey -k
  kb mode was ?UNKNOWN?
  [ if you are trying this under X, it might not work
  since the X server is also reading /dev/console ]

  press any key (program terminates 10s after last keypress)...
  keycode 28 release
  keycode 225 press
  keycode 225 release
  keycode 224 press
  keycode 224 release
  keycode 29 press
  ^Ccaught signal 2, cleaning up...
  ubuntu@201305-13527:~$

  ubuntu@201305-13527:~$ ls /sys/class/backlight/
  acpi_video0

  ProblemType: Bug
  DistroRelease: Ubuntu 14.04
  Package: linux-image-3.13.0-44-generic 3.13.0-44.73
  ProcVersionSignature: Ubuntu 3.13.0-44.73-generic 3.13.11-ckt12
  Uname: Linux 3.13.0-44-generic x86_64
  ApportVersion: 2.14.1-0ubuntu3.6
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC1:  ubuntu     1635 F.... pulseaudio
                        ubuntu     2242 F.... pulseaudio
   /dev/snd/controlC0:  ubuntu     1635 F.... pulseaudio
                        ubuntu     2242 F.... pulseaudio
  CRDA:
   country TW:
   	(2402 - 2472 @ 40), (3, 27)
   	(5270 - 5330 @ 40), (3, 17), DFS
   	(5735 - 5815 @ 40), (3, 30)
  CurrentDesktop: Unity
  Date: Fri Jan 16 00:56:25 2015
  HibernationDevice: RESUME=UUID=4e3c0183-4d3f-495e-8d6a-57e2b23e6412
  InstallationDate: Installed on 2015-01-16 (0 days ago)
  InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
  MachineType: Dell Inc. Precision M4800
  ProcFB: 0 nouveaufb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.13.0-44-generic root=UUID=3edd6366-27b1-41af-88d4-ac1abe216d8f ro quiet splash vt.handoff=7
  RelatedPackageVersions:
   linux-restricted-modules-3.13.0-44-generic N/A
   linux-backports-modules-3.13.0-44-generic  N/A
   linux-firmware                             1.127.11
  RfKill:
   0: phy0: Wireless LAN
   	Soft blocked: no
   	Hard blocked: no
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 12/03/2014
  dmi.bios.vendor: Dell Inc.
  dmi.bios.version: A11
  dmi.board.name: 0FVDR2
  dmi.board.vendor: Dell Inc.
  dmi.board.version: X02
  dmi.chassis.type: 9
  dmi.chassis.vendor: Dell Inc.
  dmi.modalias: dmi:bvnDellInc.:bvrA11:bd12/03/2014:svnDellInc.:pnPrecisionM4800:pvr01:rvnDellInc.:rn0FVDR2:rvrX02:cvnDellInc.:ct9:cvr:
  dmi.product.name: Precision M4800
  dmi.product.version: 01
  dmi.sys.vendor: Dell Inc.

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