Re: [RFC] platform/x86: asus-wmi: no kbd backlight on Zenbook S 16 UM5606WA

[Catalin Cereanu]

Following up on my own RFC: after sending it I came across prior art for
the first of the three questions I raised, which makes that part much
less open-ended than I posed it.

The FA401 series has the same shape of problem with a different device -
WMI device 0x00100057 is not reported by DSTS even though the EC does
support it - and that was handled with a DMI quirk ORing a quirk_entry
flag into the availability test rather than trusting the probe:

  https://lore.kernel.org/platform-driver-x86/20260902174718.16228-1-idotohors@xxxxxxxxx/

If the same approach is acceptable for the keyboard backlight here, the
equivalent would match DMI_SYS_VENDOR "ASUSTeK COMPUTER INC." and
DMI_BOARD_NAME "UM5606WA", and do roughly:

    static struct quirk_entry quirk_asus_um5606wa = {
            .kbd_backlight_available = true,
    };

with, in asus_wmi_led_init():

    asus->kbd_led_avail = !kbd_led_read(asus, &led_val, NULL) ||
                          asus->driver->quirks->kbd_backlight_available;

That would restore sysfs brightness control, since the DEVS setter at
0x00050021 works regardless of what DSTS reports.

It would not restore the Fn+F4 key, which stays gated on KBLC in
firmware, so questions 2 and 3 in the original mail still stand as
written.

Happy to send this as a proper patch if the shape looks right.

Thanks,
Catalin Cereanu