[Bug 221800] New: HP ProBook 445 G9: keyboard backlight hotkey no longer works after upgrade to Linux 7.1.4

https://bugzilla.kernel.org/show_bug.cgi?id=221800

            Bug ID: 221800
           Summary: HP ProBook 445 G9: keyboard backlight hotkey no longer
                    works after upgrade to Linux 7.1.4
           Product: Drivers
           Version: 2.5
          Hardware: AMD
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: Platform_x86
          Assignee: [email protected]
          Reporter: [email protected]
        Regression: No

Created attachment 310526
  --> https://bugzilla.kernel.org/attachment.cgi?id=310526&action=edit
Diagnostic logs from the working kernel (Linux 7.1.3-arch1-2), including dmesg,
kernel journal, input device information, hardware information, evtest output,
and system configuration for comparison w

Overview

After upgrading from Linux 7.1.3-arch1-2 to Linux 7.1.4-arch1-1, the keyboard
backlight hotkey on my HP ProBook 445 G9 stopped working.

This is a regression, as downgrading only the Linux kernel back to
7.1.3-arch1-2 immediately restores the functionality.

No BIOS changes or userspace changes were made between the working and broken
tests.

Steps to Reproduce
Boot Linux 7.1.4-arch1-1 on an HP ProBook 445 G9.
Log into the system.
Press the keyboard backlight hotkey.
Optionally monitor /dev/input/event4 using evtest while pressing the key.
Actual Results

The keyboard backlight does not toggle.

evtest reports only the following when the key is pressed:

Event: type 4 (EV_MSC), code 4 (MSC_SCAN), value 43

No corresponding EV_KEY event is generated.

Other function keys (brightness, volume, etc.) continue to function normally.

The keyboard backlight hardware itself is functional, as it turns on after a
cold boot.

Expected Results

Pressing the keyboard backlight hotkey should generate the appropriate key
event and toggle the keyboard backlight, as it does on Linux 7.1.3-arch1-2.

Regression Information

Working kernel

Linux 7.1.3-arch1-2

Broken kernel

Linux 7.1.4-arch1-1

The issue is completely reproducible.

Downgrading only the kernel from 7.1.4 to 7.1.3 restores the keyboard backlight
hotkey functionality immediately.

Hardware

Laptop:

HP ProBook 445 G9

BIOS Version:

U88 Ver. 01.22.01

BIOS Date:

2026-05-04

Distribution:

Arch Linux
Additional Information

The hp_wmi driver is loaded.

dmesg contains:

hp_wmi: query 0x4 returned error 0x5

The system does not expose a keyboard backlight LED interface under
/sys/class/leds.

/proc/bus/input/devices identifies the internal keyboard as AT Translated Set 2
keyboard and the HP WMI device as HP WMI hotkeys.

If additional logs or testing are required, I would be happy to provide them.

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