Public bug reported:

I've taken a bold guess in the title of this bug.

While investigating why on 14.04 LTS, I have working backlight on the
latest 3.13 kernel, but not on the 3.16, I tried acpi_backlight=vendor.

With default boot parameters, I just get
/sys/class/backlight/intel_backlight/, and the backlight works correctly
on 3.13, but does not change the brightness on 3.16 (the OSD does show
that the hotkeys are trying to change the brightness).

With acpi_backlight=vendor, I get both intel and /lenovo/.

The key point for this bug report is what happens with the lenovo one:

The hotkeys work and make the backlight change, but not consistently.
Repeatedly using the hotkey to increase the brightness causes the level
to increase, but then occasionally reset to zero (including resetting as
shown in the OSD - i.e. the OSD is always showing the level that the
backlight actually is).

Occasionally the OSD shows the "enable touchpad" icon when pressing the
increase brightness hotkey repeatedly.  The same action also causes the
wifi to enable/disable randomly.

If I press the hotkey with pauses in between, things behave normally.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: linux-image-3.13.0-67-generic 3.13.0-67.110
ProcVersionSignature: Ubuntu 3.13.0-67.110-generic 3.13.11-ckt27
Uname: Linux 3.13.0-67-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.14.1-0ubuntu3.18
Architecture: amd64
AudioDevicesInUse:
 USER        PID ACCESS COMMAND
 /dev/snd/controlC0:  neale      3106 F.... pulseaudio
CurrentDesktop: Unity
Date: Sun Nov 15 11:16:55 2015
HibernationDevice: RESUME=UUID=1fd50851-1727-4432-a99c-d0be8aafc983
InstallationDate: Installed on 2013-10-23 (752 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Release amd64 (20131016.1)
MachineType: LENOVO IdeaPad Z580
ProcFB: 0 inteldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.13.0-67-generic 
root=UUID=d66417de-a540-485b-b469-aabd51a8844b ro quiet splash 
acpi_backlight=vendor
RelatedPackageVersions:
 linux-restricted-modules-3.13.0-67-generic N/A
 linux-backports-modules-3.13.0-67-generic  N/A
 linux-firmware                             1.127.18
SourcePackage: linux
StagingDrivers: rts5139
UpgradeStatus: Upgraded to trusty on 2014-03-17 (607 days ago)
dmi.bios.date: 12/25/2012
dmi.bios.vendor: Phoenix Technologies Ltd.
dmi.bios.version: 5FCN95WW
dmi.board.asset.tag: No Asset Tag
dmi.board.name: Lenovo
dmi.board.vendor: LENOVO
dmi.board.version: 31900003WIN8 STD MLT
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: Lenovo Z580
dmi.modalias: 
dmi:bvnPhoenixTechnologiesLtd.:bvr5FCN95WW:bd12/25/2012:svnLENOVO:pnIdeaPadZ580:pvrLenovoZ580:rvnLENOVO:rnLenovo:rvr31900003WIN8STDMLT:cvnLENOVO:ct10:cvrLenovoZ580:
dmi.product.name: IdeaPad Z580
dmi.product.version: Lenovo Z580
dmi.sys.vendor: LENOVO

** Affects: linux (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug staging trusty

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https://bugs.launchpad.net/bugs/1516370

Title:
  Vendor backlight on Lenovo Z580 having random hotkeys issues

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