Public bug reported:
Regression: screen brightness cannot be changed on Linux (F5/F6 keys,
GUI slider, or direct sysfs writes all silently no-op) since a kernel
update, though it worked fine before.
Hardware: Lenovo ThinkPad T470p (20J6001AGE), Intel HD Graphics 630
(Kaby Lake GT2, i915), BIOS R0FET61W (1.41, 2023-10-10) — confirmed
already at latest available firmware via fwupd.
Confirmed regression window: kernel package upgraded from linux-image-generic
7.0.0-27.27 to 7.0.0-30.30 on 2026-08-18 via a routine update.
- Booting 7.0.0-27-generic: brightness keys and slider work correctly.
- Booting 7.0.0-30-generic: brightness control is completely non-functional.
Symptoms under 7.0.0-30-generic:
- /sys/class/backlight/intel_backlight/brightness accepts writes and
actual_brightness reflects the new value, but the physical panel brightness
never changes.
- Tested all three acpi_backlight= kernel parameter modes (native, vendor,
video): native and video register a backlight device with the same no-effect
behavior; vendor registers no backlight device at all.
- Forcing thinkpad_acpi.brightness_enable=1 (with acpi_backlight=vendor) still
produces no backlight device.
- Tested acpi_osi="!Windows 2012" workaround: no effect.
- dmesg/journalctl show no ACPI errors around GFX0/video/backlight
initialization — everything reports as registering successfully.
- Windows (dual-boot, same hardware, same panel) controls brightness correctly,
ruling out a hardware/EC/panel fault.
This strongly suggests a regression in kernel backlight/ACPI-video quirk
handling between 7.0.0-27 and 7.0.0-30 specific to this Lenovo T470p
BIOS/EC combination, since the generic ACPI _BCM call path appears to
execute without error but the EC does not apply it.
Workaround in use: booting 7.0.0-27-generic (pinned via apt-mark manual
to prevent removal).
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/controlC0: jskenderi 8521 F.... wireplumber
/dev/snd/seq: jskenderi 8497 F.... pipewire
CasperMD5CheckResult: unknown
CurrentDesktop: ubuntu:GNOME
Date: Wed Sep 2 13:27:39 2026
InstallationDate: Installed on 2019-09-08 (2551 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
MachineType: LENOVO 20J6001AGE
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-7.0.0-27-generic
root=UUID=9d7b01c1-6390-42c5-9578-7f97feab5d6c ro quiet splash
acpi_backlight=native
crashkernel=2G-4G:320M,4G-32G:512M,32G-64G:1024M,64G-128G:2048M,128G-:4096M
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: 10/10/2023
dmi.bios.release: 1.41
dmi.bios.vendor: LENOVO
dmi.bios.version: R0FET61W (1.41 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20J6001AGE
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: 1.5
dmi.modalias:
dmi:bvnLENOVO:bvrR0FET61W(1.41):bd10/10/2023:br1.41:efr1.5:svnLENOVO:pn20J6001AGE:pvrThinkPadT470p:rvnLENOVO:rn20J6001AGE:rvrSDK0J40697WIN:cvnLENOVO:ct10:cvrNone:skuLENOVO_MT_20J6_BU_Think_FM_ThinkPadT470p:pfaThinkPadT470p:
dmi.product.family: ThinkPad T470p
dmi.product.name: 20J6001AGE
dmi.product.sku: LENOVO_MT_20J6_BU_Think_FM_ThinkPad T470p
dmi.product.version: ThinkPad T470p
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/2166176
Title:
Backlight/brightness control regression on Lenovo ThinkPad T470p (Kaby
Lake i915): works on 7.0.0-27-generic, broken on 7.0.0-30-generic
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