[Bug 2161797] Re: [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

** Tags added: kernel-daily-bug -- You received this bug notification because you are subscribed to linux in Ubuntu. Matching subscriptions: Bgg, Bmail, Nb 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 Status in linux package in Ubuntu: New Bug description: 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 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2161797/+subscriptions