Public bug reported:
After upgrading from Ubuntu 25.10 to 26.04, display brightness control stopped
having any physical effect on this laptop's internal eDP panel.
The entire software chain works correctly. Writes to
/sys/class/backlight/intel_backlight/brightness succeed, actual_brightness reads
back the value that was written, and GNOME/Mutter's DisplayConfig Backlight
property tracks it. The panel simply does not change. The brightness slider and
the Fn brightness keys therefore appear to do nothing.
The distinguishing symptom is that i915 computes a different backlight
PWM period:
WORKING kernels intel_backlight max_brightness = 7500
BROKEN kernel intel_backlight max_brightness = 1023
7500 is the BIOS-programmed PWM period for this panel. 1023 (2^10 - 1) looks
like a
synthetic fallback, which suggests that on 7.0.0-28 i915 is no longer reading
the
BIOS/VBT-programmed value and instead computes a period the panel cannot follow.
I tested every 7.0.0-x kernel available in resolute/main, plus the 25.10 kernel.
All were booted on the same machine with the identical stock kernel command line
("quiet splash", no acpi_backlight= or i915.* parameters). Only the kernel
differs.
6.17.0-8-generic max_brightness = 7500 GOOD (Ubuntu 25.10 GA kernel)
7.0.0-14-generic max_brightness = 7500 GOOD
7.0.0-22-generic max_brightness = 7500 GOOD
7.0.0-27-generic max_brightness = 7500 GOOD
7.0.0-28-generic max_brightness = 1023 BAD
The regression therefore landed in the 7.0.0-27.27 -> 7.0.0-28.28 upload.
7.0.0-28 is the current 26.04 kernel, so all 26.04 users on affected panels are
hit.
I am currently running 7.0.0-27-generic as a workaround, which has fully
working
ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: linux-image-7.0.0-28-generic 7.0.0-28.28
ProcVersionSignature: Ubuntu 7.0.0-28.28-generic 7.0.12
Uname: Linux 7.0.0-28-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.34.1-0ubuntu0.1
Architecture: amd64
AudioDevicesInUse:
USER PID ACCESS COMMAND
/dev/snd/controlC0: ubuntu 4288 F.... wireplumber
/dev/snd/seq: ubuntu 4267 F.... pipewire
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Sat Jul 25 18:32:06 2026
InstallationDate: Installed on 2025-10-27 (271 days ago)
InstallationMedia: Ubuntu 25.10 "Questing Quokka" - Release amd64 (20251007)
Lsusb:
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 05c8:03ab Cheng Uei Precision Industry Co., Ltd
(Foxlink) HP Wide Vision HD Camera
Bus 001 Device 003: ID 8087:0aa7 Intel Corp. Wireless-AC 3168 Bluetooth
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
MachineType: HP HP Pavilion Laptop 15-ck0xx
ProcEnviron:
LANG=en_US.UTF-8
PATH=(custom, no user)
SHELL=/usr/bin/zsh
TERM=xterm-256color
XDG_RUNTIME_DIR=<set>
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-7.0.0-28-generic
root=/dev/mapper/username--vg-username--lv ro quiet splash
crashkernel=2G-4G:320M,4G-32G:512M,32G-64G:1024M,64G-128G:2048M,128G-:4096M
SourcePackage: linux
UpgradeStatus: Upgraded to resolute on 2026-07-25 (0 days ago)
dmi.bios.date: 08/09/2021
dmi.bios.release: 15.15
dmi.bios.vendor: Insyde
dmi.bios.version: F.15
dmi.board.asset.tag: Type2 - Board Asset Tag
dmi.board.name: 841C
dmi.board.vendor: HP
dmi.board.version: 64.14
dmi.chassis.type: 10
dmi.chassis.vendor: HP
dmi.chassis.version: Chassis Version
dmi.ec.firmware.release: 64.14
dmi.modalias:
dmi:bvnInsyde:bvrF.15:bd08/09/2021:br15.15:efr64.14:svnHP:pnHPPavilionLaptop15-ck0xx:pvrType1ProductConfigId:rvnHP:rn841C:rvr64.14:cvnHP:ct10:cvrChassisVersion:sku3QQ89EA#ABZ:pfa103C_5335KVHPPavilion:
dmi.product.family: 103C_5335KV HP Pavilion
dmi.product.name: HP Pavilion Laptop 15-ck0xx
dmi.product.sku: 3QQ89EA#ABZ
dmi.product.version: Type1ProductConfigId
dmi.sys.vendor: HP
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Tags: regression-update
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Bugs, which is subscribed to Ubuntu.
https://bugs.launchpad.net/bugs/2161797
Title:
[regression 7.0.0-27 -> 7.0.0-28] i915 derives wrong backlight PWM
period on Kaby Lake-R (max_brightness 1023 instead of 7500), panel
ignores all brightness changes
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