Public bug reported:
Title:
[Regression 7.0.0-14 -> 7.0.0-28] Backlight brightness control broken on
Lenovo ThinkPad T480s (Intel UHD 620)
Package: linux (Ubuntu)
──────
#### Description:
Hardware Details:
• Laptop: Lenovo ThinkPad T480s (Device 2258)
• GPU: Intel Corporation Kaby Lake-R GT2 [UHD Graphics 620] (00:02.0, ID 5917)
• Working Kernel: 7.0.0-14-generic (Brightness keys and GNOME slider work
normally)
• Broken Kernel: 7.0.0-28-generic (Brightness control completely fails)
──────
#### Summary of Issue:
After updating to kernel 7.0.0-28-generic, screen brightness control
no longer works. The physical backlight level remains frozen.
• Brightness Fn keys (Fn+F5 / Fn+F6) and the GNOME Quick Settings slider do
not change hardware screen brightness.
• Writing to /sys/class/backlight/intel_backlight/brightness or
acpi_video0/brightness updates sysfs values in software, but physical panel
backlight hardware fails to react.
• Hardware is verified functional (brightness control works normally in
Windows and on kernel 7.0.0-14-generic).
──────
#### Kernel Regression Diagnostics & Log Excerpts:
1. DRM Minor Device Order Regression (simpledrm vs i915):
In kernel 7.0.0-28-generic, simpledrm registers on minor 0 (/dev/dri/card0)
during early boot, pushing the Intel i915 driver to minor 1 (/dev/dri/card1):
kernel: simple-framebuffer simple-framebuffer.0: [drm] Registered 1 planes
with drm panic
kernel: [drm] Initialized simpledrm 1.0.0 for simple-framebuffer.0 on minor 0
kernel: i915 0000:00:02.0: [drm] Initialized i915 1.6.0 for 0000:00:02.0 on
minor 1
2. ThinkPad ACPI Event Suppression:
kernel: thinkpad_acpi: This ThinkPad has standard ACPI backlight brightness
control, supported by the ACPI video driver
kernel: thinkpad_acpi: Disabling thinkpad-acpi brightness events by
default...
3. Sysfs & DRM Connector Paths:
• Active DRM path:
/devices/pci0000:00/0000:00:02.0/drm/card1/card1-eDP-1/intel_backlight
• /dev/dri/card0 is unpopulated/missing after boot, preventing
Mutter/gsd-power backlight binding.
──────
#### Steps to Reproduce:
1. Boot Lenovo ThinkPad T480s into Ubuntu using kernel 7.0.0-28-generic.
2. Attempt to adjust brightness via Fn hotkeys (Fn+F5/Fn+F6), GNOME Quick
Settings slider, or brightnessctl.
3. Physical screen brightness remains unchanged.
#### Expected Behavior:
Screen backlight hardware brightness increases/decreases when
adjusted.
#### Actual Behavior:
Physical screen brightness remains frozen at constant level.
ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: linux-image-7.0.0-14-generic (not installed)
ProcVersionSignature: Ubuntu 7.0.0-14.14-generic 7.0.0
Uname: Linux 7.0.0-14-generic x86_64
ApportVersion: 2.34.1-0ubuntu0.1
Architecture: amd64
AudioDevicesInUse:
USER PID ACCESS COMMAND
/dev/snd/controlC0: jason 8310 F.... wireplumber
/dev/snd/seq: jason 8298 F.... pipewire
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Thu Jul 30 18:38:51 2026
InstallationDate: Installed on 2026-07-30 (0 days ago)
InstallationMedia: Ubuntu 26.04 "Resolute Raccoon" - Release amd64 (20260423.1)
IwDevWlp61s0Link: Not connected.
MachineType: LENOVO 20L8S44F00
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=/vmlinuz-7.0.0-14-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: 09/05/2025
dmi.bios.release: 1.62
dmi.bios.vendor: LENOVO
dmi.bios.version: N22ET85W (1.62 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20L8S44F00
dmi.board.vendor: LENOVO
dmi.board.version: SDK0J40697 WIN
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: None
dmi.ec.firmware.release: 1.23
dmi.modalias:
dmi:bvnLENOVO:bvrN22ET85W(1.62):bd09/05/2025:br1.62:efr1.23:svnLENOVO:pn20L8S44F00:pvrThinkPadT480s:rvnLENOVO:rn20L8S44F00:rvrSDK0J40697WIN:cvnLENOVO:ct10:cvrNone:skuLENOVO_MT_20L8_BU_Think_FM_ThinkPadT480s:pfaThinkPadT480s:
dmi.product.family: ThinkPad T480s
dmi.product.name: 20L8S44F00
dmi.product.sku: LENOVO_MT_20L8_BU_Think_FM_ThinkPad T480s
dmi.product.version: ThinkPad T480s
dmi.sys.vendor: LENOVO
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug resolute wayland-session
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https://bugs.launchpad.net/bugs/2162541
Title:
Newest kernel breaks t480s backlight control
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