Comment 0 for bug 1485246

Control of screen backlight is broken with a default clean install of Ubuntu 15.04 on my Samsung Series 7 Laptop (NP770Z5E). Setting kernal parameter acpi_backlight=vendor fixes the issue after two reboots. Debugging information attached as per - https://wiki.ubuntu.com/Kernel/Debugging/Backlight

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: linux-image-3.19.0-25-generic 3.19.0-25.26
ProcVersionSignature: Ubuntu 3.19.0-25.26-generic 3.19.8-ckt2
Uname: Linux 3.19.0-25-generic x86_64
ApportVersion: 2.17.2-0ubuntu1.2
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: mark 1699 F.... pulseaudio
Date: Sat Aug 15 21:36:20 2015
HibernationDevice: RESUME=UUID=c4899b44-3e19-4e62-921d-cf0a1cef97ec
InstallationDate: Installed on 2015-08-12 (3 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
MachineType: SAMSUNG ELECTRONICS CO., LTD. 770Z5E/780Z5E
ProcFB:
 0 inteldrmfb
 1 radeondrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.19.0-25-generic root=UUID=5a02817b-1f9b-4abc-93a3-b10d5d9278ed ro quiet splash acpi_backlight=vendor vt.handoff=7
PulseList:
 Error: command ['pacmd', 'list'] failed with exit code 1: Home directory not accessible: Permission denied
 No PulseAudio daemon running, or not running as session daemon.
RelatedPackageVersions:
 linux-restricted-modules-3.19.0-25-generic N/A
 linux-backports-modules-3.19.0-25-generic N/A
 linux-firmware 1.143.3
SourcePackage: linux
UdevLog: Error: [Errno 2] No such file or directory: '/var/log/udev'
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 09/05/2013
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: P05ABU.142.130905.SK
dmi.board.asset.tag: No Asset Tag
dmi.board.name: NP770Z5E-S02UK
dmi.board.vendor: SAMSUNG ELECTRONICS CO., LTD.
dmi.board.version: SEC_SW_REVISION_1234567890ABCD
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 9
dmi.chassis.vendor: SAMSUNG ELECTRONICS CO., LTD.
dmi.chassis.version: N/A
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrP05ABU.142.130905.SK:bd09/05/2013:svnSAMSUNGELECTRONICSCO.,LTD.:pn770Z5E/780Z5E:pvrP05ABU:rvnSAMSUNGELECTRONICSCO.,LTD.:rnNP770Z5E-S02UK:rvrSEC_SW_REVISION_1234567890ABCD:cvnSAMSUNGELECTRONICSCO.,LTD.:ct9:cvrN/A:
dmi.product.name: 770Z5E/780Z5E
dmi.product.version: P05ABU
dmi.sys.vendor: SAMSUNG ELECTRONICS CO., LTD.