Public bug reported:
1) Release of Ubuntu
Ubuntu 26.04 LTS (resolute)
2) Package version
Affected: linux-image-7.0.0-28-generic 7.0.0-28.28
Working: linux-image-7.0.0-14-generic
3) What I expected to happen
Writing a lower value to /sys/class/backlight/intel_backlight/brightness
reduces the panel backlight.
4) What happened instead
The write is accepted and actual_brightness reports the new value, but the
panel brightness does not change and stays at maximum.
Description
On a Lenovo ThinkPad T480 (i7-8650U, Kaby Lake-R, UHD Graphics 620 +
GeForce MX150), display backlight brightness cannot be changed under
kernel 7.0.0-28-generic. The sysfs interface accepts writes and reports
them as applied, but the panel brightness does not change and remains at
maximum.
Booting 7.0.0-14-generic on the same installation, with no other
changes, restores working brightness control immediately.
Steps to reproduce
Boot 7.0.0-28-generic.
Write a low value to the backlight interface: echo 300 | sudo tee
/sys/class/backlight/intel_backlight/brightness
Observe that the panel brightness is unchanged.
Actual result
Both brightness and actual_brightness report the newly written value, so
the kernel believes the change was applied:
$ B=/sys/class/backlight/intel_backlight
$ echo 300 | sudo tee $B/brightness
300
$ cat $B/brightness $B/actual_brightness
300
300
The same failure occurs through the desktop brightness slider and the
Fn+F5/F6 keys. The panel stays at full brightness in all cases.
Regression
The failure follows the kernel version only. Both kernels are installed
from the archive on the same system, and switching between them is the
only variable.
7.0.0-14-generic brightness control works
7.0.0-28-generic brightness control does not work
Investigation already performed
The following were tested and ruled out, so the report can be narrowed
to i915:
NVIDIA driver ruled out. The proprietary nvidia-driver-580 is installed.
Booting 7.0.0-28-generic with
modprobe.blacklist=nvidia,nvidia_drm,nvidia_modeset does not restore brightness
control, so the NVIDIA driver is not involved.
Backlight interface selection ruled out. Only intel_backlight is registered by
default. Booting with acpi_backlight=video replaces it with acpi_video0, which
also accepts writes without affecting the panel. acpi_backlight=native and
acpi_backlight=vendor were also tested with no effect.
Hardware ruled out. Brightness control works normally under Windows on the same
machine, so the panel, the eDP cable and the backlight PWM circuit are
functional.
Permissions ruled out. Writes were performed as root, and actual_brightness
reflects the written value.
Relevant dmesg
thinkpad_acpi: This ThinkPad has standard ACPI backlight brightness control,
supported by the ACPI video driver
i915 0000:00:02.0: [drm] Found kabylake (device ID 5917) integrated display
version 9.00 stepping C0
i915 0000:00:02.0: [drm] Finished loading DMC firmware
i915/kbl_dmc_ver1_04.bin (v1.4)
[drm] Initialized i915 1.6.0 for 0000:00:02.0 on minor 1
No backlight or PWM errors are logged.
System information
Model Lenovo ThinkPad T480
CPU Intel Core i7-8650U (Kaby Lake-R)
GPU Intel UHD Graphics 620 (device ID 5917) + NVIDIA MX150
Desktop KDE Plasma on Wayland
Mesa 26.0.3-1ubuntu1
Bad kernel 7.0.0-28-generic
Good kernel 7.0.0-14-generic
Secure Boot disabled
Workaround
Boot 7.0.0-14-generic.
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: vex 2469 F.... pipewire
vex 2477 F.... wireplumber
/dev/snd/pcmC0D0p: vex 2469 F...m pipewire
/dev/snd/seq: vex 2469 F.... pipewire
CasperMD5CheckResult: unknown
CurrentDesktop: KDE
Date: Sun Aug 2 04:11:28 2026
InstallationDate: Installed on 2026-08-01 (0 days ago)
InstallationMedia: Kubuntu 26.04 "Resolute Raccoon" - Release amd64 (20260423)
MachineType: LENOVO 20L6SE7N00
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-7.0.0-28-generic
root=UUID=8414ab7b-7ac1-487e-8096-a6c27c795e8e ro thinkpad_acpi.fan_control=1
quiet splash
PulseList: Error: command ['pacmd', 'list'] failed with exit code 1: No
PulseAudio daemon running, or not running as session daemon.
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 03/17/2025
dmi.bios.release: 1.54
dmi.bios.vendor: LENOVO
dmi.bios.version: N24ET79W (1.54 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20L6SE7N00
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: 1.22
dmi.modalias:
dmi:bvnLENOVO:bvrN24ET79W(1.54):bd03/17/2025:br1.54:efr1.22:svnLENOVO:pn20L6SE7N00:pvrThinkPadT480:rvnLENOVO:rn20L6SE7N00:rvrSDK0J40697WIN:cvnLENOVO:ct10:cvrNone:skuLENOVO_MT_20L6_BU_Think_FM_ThinkPadT480:pfaThinkPadT480:
dmi.product.family: ThinkPad T480
dmi.product.name: 20L6SE7N00
dmi.product.sku: LENOVO_MT_20L6_BU_Think_FM_ThinkPad T480
dmi.product.version: ThinkPad T480
dmi.sys.vendor: LENOVO
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug resolute wayland-session
--
You received this bug notification because you are a member of Ubuntu
Bugs, which is subscribed to Ubuntu.
https://bugs.launchpad.net/bugs/2162650
Title:
i915 backlight control silently ineffective on ThinkPad T480 with
7.0.0-28-generic (works on 7.0.0-14)
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2162650/+subscriptions
--
ubuntu-bugs mailing list
[email protected]
https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
