Public bug reported: SUMMARY
Panel backlight control stopped working after a kernel update. Brightness hotkeys and the desktop slider respond visually, and writes to /sys/class/backlight/intel_backlight/brightness are accepted and read back correctly, but the physical panel brightness never actually changes. ENVIRONMENT - Laptop: Lenovo ThinkPad, Meteor Lake platform, hybrid Intel/NVIDIA - iGPU: Intel Meteor Lake-P [Intel Arc Graphics], PCI ID 8086:7d55, driver in use: i915 - dGPU: NVIDIA AD107GLM [RTX 500 Ada Generation Laptop GPU] - OS: Ubuntu 26.04 (resolute) - Backlight device: /sys/class/backlight/intel_backlight, type = raw - Display server: Wayland / GNOME KNOWN-GOOD / KNOWN-BAD KERNELS 7.0.0-27-generic: working 7.0.0-28-generic: broken 7.0.0-29-generic: broken Regression introduced between 7.0.0-27 and 7.0.0-28, still present in 7.0.0-29. linux-firmware-intel-graphics was also upgraded in the same apt transaction as the 27->28 kernel bump, so the root cause could be in i915 itself or in the bundled firmware blob. STEPS TO REPRODUCE 1. Boot 7.0.0-28-generic or later. 2. cat /sys/class/backlight/intel_backlight/type -> raw 3. sudo tee /sys/class/backlight/intel_backlight/brightness <<< 32767 cat .../brightness -> correctly shows 32767, but screen does not change 4. sudo tee /sys/class/backlight/intel_backlight/brightness <<< 1000 cat .../brightness -> correctly shows 1000, screen still does not change EXPECTED Panel brightness should track the sysfs value, as it does on 7.0.0-27-generic. ACTUAL Value is written and read back correctly; physical backlight is unaffected. ISOLATION ALREADY PERFORMED - Reproduces identically in prime-select intel, on-demand, and nvidia modes -> not a PRIME/GPU-switching issue. - lspci -k confirms i915 (not xe) is bound in all cases. - Direct sysfs writes behave the same as hotkeys/GUI slider -> not a desktop/logind/permissions issue. - i915.enable_dpcd_backlight=1 and =2 kernel parameters tested, no change. - Reproduces identically across nvidia-driver-595-open, 610-open, and 610 (proprietary) -> not NVIDIA-driver-specific. - Confirmed by kernel downgrade: reinstalling and booting 7.0.0-27-generic (Intel-only, no NVIDIA module for that kernel) restores working brightness control. Rebooting into 28 or 29 reproduces the failure immediately. DMESG (broken kernel) thinkpad_acpi: This ThinkPad has standard ACPI backlight brightness control, supported by the ACPI video driver ACPI: video: Video Device [GFX0] (multi-head: yes rom: no post: no) input: Video Bus as /devices/pci0000:00/acpi.video_bus.0/input/input14 No i915-specific backlight/panel/edp/pwm messages appear in dmesg on the broken kernels. RELATED (BUT LIKELY DISTINCT) REPORT https://github.com/NVIDIA/open-gpu-kernel-modules/issues/1087 describes an NVIDIA-driver-specific backlight regression on the 595 series. This report reproduces independent of NVIDIA involvement (Intel-only mode is also broken), so it is likely a separate i915/kernel issue rather than the same root cause. ProblemType: Bug DistroRelease: Ubuntu 26.04 Package: linux-image-7.0.0-29-generic 7.0.0-29.29 ProcVersionSignature: Ubuntu 7.0.0-29.29-generic 7.0.12 Uname: Linux 7.0.0-29-generic x86_64 ApportVersion: 2.34.1-0ubuntu0.1 Architecture: amd64 AudioDevicesInUse: USER PID ACCESS COMMAND /dev/snd/controlC0: gian 6457 F.... pipewire gian 6477 F.... wireplumber /dev/snd/seq: gian 6457 F.... pipewire CasperMD5CheckResult: pass CurrentDesktop: ubuntu:GNOME Date: Wed Aug 5 16:07:04 2026 InstallationDate: Installed on 2026-04-24 (103 days ago) InstallationMedia: Ubuntu 26.04 "Resolute Raccoon" - Release amd64 (20260423.1) MachineType: LENOVO 21G20009UK ProcFB: 0 i915drmfb ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-7.0.0-29-generic root=/dev/mapper/ubuntu--vg-ubuntu--lv ro quiet splash crashkernel=2G-4G:320M,4G-32G:512M,32G-64G:1024M,64G-128G:2048M,128G-:4096M SourcePackage: linux UpgradeStatus: No upgrade log present (probably fresh install) dmi.bios.date: 04/24/2026 dmi.bios.release: 1.27 dmi.bios.vendor: LENOVO dmi.bios.version: R2DET42W (1.27 ) dmi.board.asset.tag: Not Available dmi.board.name: 21G20009UK dmi.board.vendor: LENOVO dmi.board.version: SDK0T76530 WIN dmi.chassis.asset.tag: No Asset Information dmi.chassis.type: 10 dmi.chassis.vendor: LENOVO dmi.chassis.version: None dmi.ec.firmware.release: 1.18 dmi.modalias: dmi:bvnLENOVO:bvrR2DET42W(1.27):bd04/24/2026:br1.27:efr1.18:svnLENOVO:pn21G20009UK:pvrThinkPadP14sGen5:rvnLENOVO:rn21G20009UK:rvrSDK0T76530WIN:cvnLENOVO:ct10:cvrNone:skuLENOVO_MT_21G2_BU_Think_FM_ThinkPadP14sGen5:pfaThinkPadP14sGen5: dmi.product.family: ThinkPad P14s Gen 5 dmi.product.name: 21G20009UK dmi.product.sku: LENOVO_MT_21G2_BU_Think_FM_ThinkPad P14s Gen 5 dmi.product.version: ThinkPad P14s Gen 5 dmi.sys.vendor: LENOVO ** Affects: linux (Ubuntu) Importance: Undecided Status: New ** Tags: amd64 apport-bug resolute wayland-session ** Attachment added: "lspci-verbose.txt" https://bugs.launchpad.net/bugs/2162908/+attachment/5989730/+files/lspci-verbose.txt -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2162908 Title: Panel backlight control non-functional on Meteor Lake Arc Graphics after kernel 7.0.0-28 (regression from 7.0.0-27) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2162908/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
