Public bug reported:

None of the boot parameters worked, but with acpi_backlight=vendor the
backlight was dimmer (systemd-backlight didn't fail). The only way it
starts working is to do "setpci -v -H1 -s 00:01.00 BRIDGE_CONTROL=0"

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: linux-image-5.4.0-45-generic 5.4.0-45.49
ProcVersionSignature: Ubuntu 5.4.0-45.49-generic 5.4.55
Uname: Linux 5.4.0-45-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia wl
ApportVersion: 2.20.11-0ubuntu27.8
Architecture: amd64
AudioDevicesInUse:
 USER        PID ACCESS COMMAND
 /dev/snd/controlC1:  xma        1345 F.... pulseaudio
 /dev/snd/controlC0:  xma        1345 F.... pulseaudio
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Tue Sep  1 14:36:14 2020
InstallationDate: Installed on 2020-08-30 (1 days ago)
InstallationMedia: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
MachineType: Apple Inc. MacBookPro11,3
ProcFB: 0 EFI VGA
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.4.0-45-generic 
root=UUID=01482a85-82e8-44ce-a63d-a429cd6d5381 ro acpi_osi=
RelatedPackageVersions:
 linux-restricted-modules-5.4.0-45-generic N/A
 linux-backports-modules-5.4.0-45-generic  N/A
 linux-firmware                            1.187.3
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 06/10/2020
dmi.bios.vendor: Apple Inc.
dmi.bios.version: 162.0.0.0.0
dmi.board.asset.tag: Base Board Asset Tag#
dmi.board.name: Mac-2BD1B31983FE1663
dmi.board.vendor: Apple Inc.
dmi.board.version: MacBookPro11,3
dmi.chassis.type: 10
dmi.chassis.vendor: Apple Inc.
dmi.chassis.version: Mac-2BD1B31983FE1663
dmi.modalias: 
dmi:bvnAppleInc.:bvr162.0.0.0.0:bd06/10/2020:svnAppleInc.:pnMacBookPro11,3:pvr1.0:rvnAppleInc.:rnMac-2BD1B31983FE1663:rvrMacBookPro11,3:cvnAppleInc.:ct10:cvrMac-2BD1B31983FE1663:
dmi.product.family: Mac
dmi.product.name: MacBookPro11,3
dmi.product.sku: System SKU#
dmi.product.version: 1.0
dmi.sys.vendor: Apple Inc.

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


** Tags: amd64 apport-bug focal

** Attachment added: "backlight.tar"
   
https://bugs.launchpad.net/bugs/1893795/+attachment/5406558/+files/backlight.tar

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https://bugs.launchpad.net/bugs/1893795

Title:
  [Ubuntu 20.04.1 LTS] Backlight brightness can't be changed, even in
  /sys/

Status in linux package in Ubuntu:
  New

Bug description:
  None of the boot parameters worked, but with acpi_backlight=vendor the
  backlight was dimmer (systemd-backlight didn't fail). The only way it
  starts working is to do "setpci -v -H1 -s 00:01.00 BRIDGE_CONTROL=0"

  ProblemType: Bug
  DistroRelease: Ubuntu 20.04
  Package: linux-image-5.4.0-45-generic 5.4.0-45.49
  ProcVersionSignature: Ubuntu 5.4.0-45.49-generic 5.4.55
  Uname: Linux 5.4.0-45-generic x86_64
  NonfreeKernelModules: nvidia_modeset nvidia wl
  ApportVersion: 2.20.11-0ubuntu27.8
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC1:  xma        1345 F.... pulseaudio
   /dev/snd/controlC0:  xma        1345 F.... pulseaudio
  CasperMD5CheckResult: skip
  CurrentDesktop: ubuntu:GNOME
  Date: Tue Sep  1 14:36:14 2020
  InstallationDate: Installed on 2020-08-30 (1 days ago)
  InstallationMedia: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
  MachineType: Apple Inc. MacBookPro11,3
  ProcFB: 0 EFI VGA
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.4.0-45-generic 
root=UUID=01482a85-82e8-44ce-a63d-a429cd6d5381 ro acpi_osi=
  RelatedPackageVersions:
   linux-restricted-modules-5.4.0-45-generic N/A
   linux-backports-modules-5.4.0-45-generic  N/A
   linux-firmware                            1.187.3
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 06/10/2020
  dmi.bios.vendor: Apple Inc.
  dmi.bios.version: 162.0.0.0.0
  dmi.board.asset.tag: Base Board Asset Tag#
  dmi.board.name: Mac-2BD1B31983FE1663
  dmi.board.vendor: Apple Inc.
  dmi.board.version: MacBookPro11,3
  dmi.chassis.type: 10
  dmi.chassis.vendor: Apple Inc.
  dmi.chassis.version: Mac-2BD1B31983FE1663
  dmi.modalias: 
dmi:bvnAppleInc.:bvr162.0.0.0.0:bd06/10/2020:svnAppleInc.:pnMacBookPro11,3:pvr1.0:rvnAppleInc.:rnMac-2BD1B31983FE1663:rvrMacBookPro11,3:cvnAppleInc.:ct10:cvrMac-2BD1B31983FE1663:
  dmi.product.family: Mac
  dmi.product.name: MacBookPro11,3
  dmi.product.sku: System SKU#
  dmi.product.version: 1.0
  dmi.sys.vendor: Apple Inc.

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