[Bug 2163019] Re: Screen brightness control completely broken on kernel 7.0.0-28 and 7.0.0-29; works perfectly on 7.0.0-22. Dell Inspiron 15 3511, Intel Tiger Lake Iris Xe.

*** This bug is a duplicate of bug 2161309 *** https://bugs.launchpad.net/bugs/2161309 ** This bug has been marked a duplicate of bug 2161309 Backlight regression -- You received this bug notification because you are subscribed to linux in Ubuntu. Matching subscriptions: Bgg, Bmail, Nb https://bugs.launchpad.net/bugs/2163019 Title: Screen brightness control completely broken on kernel 7.0.0-28 and 7.0.0-29; works perfectly on 7.0.0-22. Dell Inspiron 15 3511, Intel Tiger Lake Iris Xe. Status in linux package in Ubuntu: New Bug description: Summary: Backlight brightness control regressed between kernel 7.0.0-22 and 7.0.0-28 on a Dell Inspiron 15 3511 with Intel Tiger Lake-LP GT2 [Iris Xe Graphics]. The physical brightness keys are detected (on-screen slider moves), but the actual screen backlight does not change at all. Completely broken on 7.0.0-28 and 7.0.0-29. Works perfectly on 7.0.0-22. Hardware: Laptop: Dell Inspiron 15 3511 CPU/GPU: Intel Tiger Lake-LP GT2 [Iris Xe Graphics] (device ID 9a49) OS: Ubuntu 26.04 (resolute), dual boot with Windows Backlight interface: /sys/class/backlight/intel_backlight DMC firmware: i915/tgl_dmc_ver2_12.bin (v2.12) Regression window: ✅ Works: 7.0.0-22-generic ❌ Broken: 7.0.0-28-generic ❌ Broken: 7.0.0-29-generic The jump from 7.0.0-22 to 7.0.0-28 was introduced by an unattended upgrade on 2026-08-02. What was ruled out (extensive testing): Not a hardware fault — brightness works perfectly in Windows on the same machine (dual boot), and works in a live Ubuntu session from the original install ISO (kernel 7.0.0-14). Not a BIOS/firmware issue — booting the same hardware from a clean live USB with kernel 7.0.0-14 restored full brightness control, confirming the BIOS behaves correctly. The BIOS was also updated to the latest available version — no change. Not a firmware package regression — downgraded linux-firmware-intel-graphics from 0ubuntu2.1 back to 0ubuntu2 and confirmed the DMC firmware blob is identical between the working live session and the installed system (md5: de018e9fa626be8cf8d388c0bf767410). Brightness still broken on 7.0.0-28/29 with the old firmware package. Not a software/daemon interference — tested from a bare virtual terminal (Ctrl+Alt+F3, no desktop session, no extensions, no daemons running). Still broken. ACPI/SMI chain verified firing correctly — dumped and disassembled the DSDT. The _BCM brightness method routes through GENS → SMBF → ASMI(), which writes 0xE0 to I/O port 0xB2 (a genuine SMI trigger). Verified via the CPU's IA32_SMI_COUNT MSR (register 0x34, msr-tools) that the SMI counter increments by exactly 1 on every write to /sys/class/backlight/acpi_video0/brightness — the ACPI method and SMI are firing correctly. This was tested on the broken kernel (7.0.0-29); the issue is that the result is ignored. Kernel isolated as the sole cause — with the firmware package held at the old version, switching between 7.0.0-22 (works) and 7.0.0-28 (broken) is the only variable that changes the outcome. Kernel parameters tested (none fixed it on 7.0.0-28/29): acpi_backlight=native acpi_backlight=vendor acpi_backlight=video i915.enable_dpcd_backlight=1 i915.enable_dpcd_backlight=2 i915.enable_psr=0 i915.invert_brightness=1 Blacklisted xe module, ensured i915 has exclusive control Current workaround: Pinned to 7.0.0-22-generic via apt-mark hold. Brightness fully functional on that kernel. Reproduces on: Dell Inspiron 15 3511, Tiger Lake Iris Xe, Ubuntu 26.04, kernel ≥ 7.0.0-28. Happy to provide full dmesg output, ACPI DSDT dump, or any other diagnostics from either the working (7.0.0-22) or broken (7.0.0-29) kernel on request. This bug were spotted by a human and this report was generated with the assist of an AI after giving it all of the context. Thank you for making linux available for the community ❤️. ProblemType: Bug DistroRelease: Ubuntu 26.04 Package: linux-image-7.0.0-22-generic 7.0.0-22.22 ProcVersionSignature: Ubuntu 7.0.0-22.22-generic 7.0.0 Uname: Linux 7.0.0-22-generic x86_64 ApportVersion: 2.34.1-0ubuntu0.1 Architecture: amd64 AudioDevicesInUse:  USER PID ACCESS COMMAND  /dev/snd/controlC0: mohammed 4163 F.... wireplumber  /dev/snd/seq: mohammed 4147 F.... pipewire CasperMD5CheckResult: pass CurrentDesktop: ubuntu:GNOME Date: Fri Aug 7 11:37:32 2026 InstallationDate: Installed on 2026-05-19 (80 days ago) InstallationMedia: Ubuntu 26.04 "Resolute Raccoon" - Release amd64 (20260423.1) MachineType: Dell Inc. Inspiron 15 3511 ProcEnviron:  LANG=en_US.UTF-8  PATH=(custom, no user)  SHELL=/bin/bash  TERM=xterm-256color  XDG_RUNTIME_DIR=<set> ProcFB: 0 i915drmfb ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-7.0.0-22-generic root=UUID=af9f8c53-fb0e-4c6a-951d-7f651c38ed30 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: 05/27/2026 dmi.bios.release: 1.47 dmi.bios.vendor: Dell Inc. dmi.bios.version: 1.47.0 dmi.board.name: 0042CN dmi.board.vendor: Dell Inc. dmi.board.version: A00 dmi.chassis.type: 10 dmi.chassis.vendor: Dell Inc. dmi.modalias: dmi:bvnDellInc.:bvr1.47.0:bd05/27/2026:br1.47:svnDellInc.:pnInspiron153511:pvr:rvnDellInc.:rn0042CN:rvrA00:cvnDellInc.:ct10:cvr:sku0AB0:pfaInspiron: dmi.product.family: Inspiron dmi.product.name: Inspiron 15 3511 dmi.product.sku: 0AB0 dmi.sys.vendor: Dell Inc. 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