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Hi,
changing the screen brightness in the Ubuntu settings does not change the 
brightness of the screen.
Changing the brightness for example with the fn keys changes the value in 
/sys/class/backlight/dell_backlight/brightness but not the screen brightness. 
There is also a /sys/class/backlight/intel_backlight but changing the 
brightness file there manually also has no effect. 

The acpi_backlight=vendor option did not work out.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: linux-image-5.4.0-21-generic 5.4.0-21.25
ProcVersionSignature: Ubuntu 5.4.0-21.25-generic 5.4.27
Uname: Linux 5.4.0-21-generic x86_64
ApportVersion: 2.20.11-0ubuntu26
Architecture: amd64
AudioDevicesInUse:
 USER        PID ACCESS COMMAND
 /dev/snd/controlC0:  benedict   1744 F.... pulseaudio
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Sat Apr 11 20:38:51 2020
InstallationDate: Installed on 2020-04-08 (2 days ago)
InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Beta amd64 (20200402)
MachineType: Dell Inc. XPS 15 7590
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.4.0-21-generic 
root=UUID=8bbcafe3-01fd-4b8e-8924-1c0d0efa9270 ro quiet splash 
acpi_backlight=vendor vt.handoff=7
RelatedPackageVersions:
 linux-restricted-modules-5.4.0-21-generic N/A
 linux-backports-modules-5.4.0-21-generic  N/A
 linux-firmware                            1.187
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 11/11/2019
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 1.4.0
dmi.board.name: 0T8KGX
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 10
dmi.chassis.vendor: Dell Inc.
dmi.modalias: 
dmi:bvnDellInc.:bvr1.4.0:bd11/11/2019:svnDellInc.:pnXPS157590:pvr:rvnDellInc.:rn0T8KGX:rvrA00:cvnDellInc.:ct10:cvr:
dmi.product.family: XPS
dmi.product.name: XPS 15 7590
dmi.product.sku: 0905
dmi.sys.vendor: Dell Inc.

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


** Tags: amd64 apport-bug focal
-- 
screen backlight can't be changed 
https://bugs.launchpad.net/bugs/1872229
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