[Bug 222015] New: ASUS TUF Gaming A18 (FA808UH): keyboard backlight requires HID feature report 0x46 0x01 to initialize on cold boot

[Bug 222015] New: ASUS TUF Gaming A18 (FA808UH): keyboard backlight requires HID feature report 0x46 0x01 to initialize on cold boot

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=222015

            Bug ID: 222015
           Summary: ASUS TUF Gaming A18 (FA808UH): keyboard backlight
                    requires HID feature report 0x46 0x01 to initialize on
                    cold boot
           Product: Drivers
           Version: 2.5
          Hardware: AMD
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: Platform_x86
          Assignee: drivers_platform_x86@kernel-bugs.osdl.org
          Reporter: imatvey.root@gmail.com
        Regression: No

Created attachment 310851
  --> https://bugzilla.kernel.org/attachment.cgi?id=310851&action=edit
dmesg

Problem
-------
Keyboard backlight (asus::kbd_backlight LED class device) does not work on a
cold boot under Linux. Writing a brightness value to
/sys/class/leds/asus::kbd_backlight/brightness succeeds and the value is
reflected in the sysfs file, but the physical backlight stays off.

This is reproducible on a vanilla Arch kernel (7.2.4-arch1-2) and was also
observed on linux-zen, on a fully updated EndeavourOS installation.

Hardware
--------
System:   ASUS TUF Gaming A18 FA808UH_FA808UH
Board:    FA808UH
BIOS:     FA808UH.313 (02/04/2026)
CPU/GPU:  AMD Ryzen + NVIDIA (ASUS TUF A18, 2025/2026 model)

Root cause / evidence
---------------------
1. Booting Windows (bare metal, not a VM) turns the keyboard backlight on
   immediately. This proves the hardware and firmware are fine.

2. After a *warm reboot* from Windows into Linux, the backlight remains on
   and asusctl / brightnessctl start working normally. This shows that the
   embedded controller (EC) state is initialized by Windows and persists
   across a warm reboot.

3. On a *cold boot* directly into Linux, the EC is never initialized, so
   the LED class device accepts writes but they have no effect.

4. Sending a single HID feature report to the keyboard controller fixes the
   problem immediately and permanently for the rest of the boot session:

       hidapitester --vidpid 0B05/19B6 --open --send-feature 70,1 --close

   The tool writes a 64-byte feature report whose first two bytes are
   0x46 0x01. After this, writing brightness to sysfs works as expected.

Workaround
----------
A systemd oneshot service that runs hidapitester at boot restores normal
behavior. This is not a real fix, only a userspace workaround.

Request
-------
Please add a quirk to asus-wmi (or hid-asus) to send the equivalent
feature report (0x46 0x01) to USB device 0B05:19B6 during driver
initialization on the ASUS TUF Gaming A18 FA808UH / FA808UP, so the
keyboard backlight works out of the box on cold boot.

Attachments
-----------
I can provide the following if needed:
  - full dmesg from a cold boot
  - lsusb -v output for the keyboard device

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