Public bug reported:
Brightness control works correctly with Linux kernel `7.0.0-27-generic` but
stops working after booting into `7.0.0-28-generic`.
The brightness slider and sysfs values change normally, but the physical
display brightness does not change.
This appears to be an Intel i915 graphics driver regression introduced
in kernel `7.0.0-28`.
## Hardware
Laptop:
* Lenovo ThinkPad T450s
Graphics:
* Intel integrated graphics
* Intel Broadwell/ULT GPU
* PCI device ID: 1616
Display:
* Internal eDP panel
## Operating System
Ubuntu 26.04 LTS (Resolute)
Desktop session:
* GNOME
* Wayland
## Affected kernels
### Working kernel
```
Linux 7.0.0-27-generic
```
Brightness works normally.
Kernel command line:
```
BOOT_IMAGE=/boot/vmlinuz-7.0.0-27-generic
root=UUID=06789976-622b-4eb6-9e48-b0a08facff37 ro quiet splash
i915.enable_psr=0
crashkernel=2G-4G:320M,4G-32G:512M,32G-64G:1024M,64G-128G:2048M,128G-:4096M
```
### Broken kernel
```
Linux 7.0.0-28-generic
```
Brightness control does not affect the physical display.
Kernel command line:
```
BOOT_IMAGE=/boot/vmlinuz-7.0.0-28-generic
root=UUID=06789976-622b-4eb6-9e48-b0a08facff37 ro quiet splash
i915.enable_psr=0
crashkernel=2G-4G:320M,4G-32G:512M,32G-64G:1024M,64G-128G:2048M,128G-:4096M
```
## Reproduction steps
1. Boot into kernel:
```
7.0.0-28-generic
```
2. Change brightness using GNOME brightness controls or:
```
brightnessctl set 50%
```
3. The brightness value changes, but the display brightness remains
unchanged.
4. Boot into:
```
7.0.0-27-generic
```
5. Repeat the same test.
Result:
* Brightness works correctly on 7.0.0-27.
## Backlight interface
The system exposes:
```
/sys/class/backlight/intel_backlight
```
Output:
```
lrwxrwxrwx intel_backlight ->
../../devices/pci0000:00/0000:00:02.0/drm/card1/card1-eDP-1/intel_backlight
```
Maximum brightness:
```
1023
```
On kernel 7.0.0-28:
```
brightness:
627
actual_brightness:
627
```
Changing values works:
```
echo 100 | sudo tee /sys/class/backlight/intel_backlight/brightness
100
echo 800 | sudo tee /sys/class/backlight/intel_backlight/brightness
800
```
However, the physical screen brightness does not change.
## i915 driver information
Working kernel:
```
modinfo i915
srcversion: 042D149A3C0CF120B518B19
vermagic: 7.0.0-27-generic SMP preempt mod_unload modversions
```
Broken kernel:
```
modinfo i915
srcversion: 7F187B928024528F00EC733
vermagic: 7.0.0-28-generic SMP preempt mod_unload modversions
```
## i915 logs from broken kernel
```
i915 0000:00:02.0: [drm] Found broadwell/ult (device ID 1616) integrated
display version 8.00 stepping N/A
i915 0000:00:02.0: [drm] Initialized i915 1.6.0 for 0000:00:02.0 on
minor 1
i915drmfb (fb0) is primary device
```
No obvious i915 errors appear.
## Additional testing
The following kernel parameter was already enabled:
```
i915.enable_psr=0
```
The issue remains.
The following was tested:
```
echo 100 | sudo tee /sys/class/backlight/intel_backlight/brightness
echo 800 | sudo tee /sys/class/backlight/intel_backlight/brightness
```
The sysfs values change, but hardware brightness does not.
## Expected behavior
Changing brightness should physically change the display brightness.
## Actual behavior
Brightness values update in sysfs and GNOME UI, but the display
brightness remains unchanged.
## Regression range
Last working:
```
7.0.0-27-generic
```
First broken:
```
7.0.0-28-generic
```
## Possible cause
This looks like a regression in the Intel i915 backlight handling
introduced between kernel versions 7.0.0-27 and 7.0.0-28.
ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: linux-image-7.0.0-27-generic 7.0.0-27.27
ProcVersionSignature: Ubuntu 7.0.0-27.27-generic 7.0.6
Uname: Linux 7.0.0-27-generic x86_64
ApportVersion: 2.34.1-0ubuntu0.1
Architecture: amd64
AudioDevicesInUse:
USER PID ACCESS COMMAND
/dev/snd/controlC1: jotech 4200 F.... wireplumber
/dev/snd/controlC0: jotech 4200 F.... wireplumber
/dev/snd/seq: jotech 4191 F.... pipewire
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Sat Aug 1 15:46:19 2026
InstallationDate: Installed on 2026-05-12 (81 days ago)
InstallationMedia: Ubuntu 26.04 "Resolute Raccoon" - Release amd64 (20260423.1)
MachineType: LENOVO 20BWS4BP00
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-7.0.0-27-generic
root=UUID=06789976-622b-4eb6-9e48-b0a08facff37 ro quiet splash
i915.enable_psr=0
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: 08/14/2019
dmi.bios.release: 1.37
dmi.bios.vendor: LENOVO
dmi.bios.version: JBET73WW (1.37 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20BWS4BP00
dmi.board.vendor: LENOVO
dmi.board.version: SDK0J40705 WIN
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: None
dmi.ec.firmware.release: 1.4
dmi.modalias:
dmi:bvnLENOVO:bvrJBET73WW(1.37):bd08/14/2019:br1.37:efr1.4:svnLENOVO:pn20BWS4BP00:pvrThinkPadT450s:rvnLENOVO:rn20BWS4BP00:rvrSDK0J40705WIN:cvnLENOVO:ct10:cvrNone:skuLENOVO_MT_20BW_BU_Think_FM_ThinkPadT450s:pfaThinkPadT450s:
dmi.product.family: ThinkPad T450s
dmi.product.name: 20BWS4BP00
dmi.product.sku: LENOVO_MT_20BW_BU_Think_FM_ThinkPad T450s
dmi.product.version: ThinkPad T450s
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/2162631
Title:
Intel backlight regression on Lenovo ThinkPad T450s after upgrading
from Linux 7.0.0-27 to 7.0.0-28
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