[Bug 221583] Lenovo 82KU: keyboard backlight works via Fn+Space but no kbd_backlight LED device exposed

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=221583

--- Comment #7 from Rong Zhang (i@rong.moe) ---
Created attachment 310266
  --> https://bugzilla.kernel.org/attachment.cgi?id=310266&action=edit
DSDT patch

Hi George,

Thanks for debugging. Based on the information, software control should be
possible by overriding the device's DSDT with appropriate ACPI methods properly
implemented, which is (unfortunately) not upstreamable.

But as the first step, please try writing to the address via /dev/mem and see
if the keyboard backlight brightness changes (replace 0xXX with the values you
found):

  sudo busybox devmem 0xFE00D42A 8 0xXX

Writing to EC registers could be dangerous, so please be cautious.

I copied and pasted some code from my device's DSDT to create a DSDT patch for
your device. If writing to the address works, you may use it to provide the
required ACPI method to ideapad-laptop. I'm not an expert in writing ASL and
didn't test it at all, so there is a chance that things could go wrong. Be
cautious.

See
https://www.kernel.org/doc/html/latest/admin-guide/acpi/initrd_table_override.html
and https://wiki.archlinux.org/title/DSDT for detailed instructions on
overriding DSDT.

I'd recommend overriding it using GRUB2. See also
https://www.gnu.org/software/grub/manual/grub/html_node/acpi.html
If it ain't work, try GRUB_EARLY_INITRD_LINUX_CUSTOM. See
https://www.gnu.org/software/grub/manual/grub/html_node/Simple-configuration.html

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