[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 #34 from Rong Zhang (i@rong.moe) ---
Hmm, let's try some radical approaches. There is still a chance that some
registers near 0x8A have something to do with the keyboard backlight
brightness.

1. Set physical keyboard brightness to Low
2. Backup 0x8A: sudo busybox devmem 0xFE00D48A 8
3. sudo busybox devmem 0xFE00D42A 8 0x05
4. sudo busybox devmem 0xFE00D48A 8 0x00
5. sudo busybox devmem 0xFE00D42A 8
6. sudo busybox devmem 0xFE00D42A 8 0x05
7. sudo busybox devmem 0xFE00D48A 8 0xFF
8. sudo busybox devmem 0xFE00D42A 8
9. Restore the backup of 0x8A: sudo busybox devmem 0xFE00D48A 8 <backup value>

REPEAT the same procedure, but replace 0x8A with 0x8B.
REPEAT with 0x8C.
REPEAT with 0x8D.

It could be dangerous as it massages a lot of bits. Be sure you have SSH access
before trying this, as it could turn off the backlight of the screen.

If massaging any of the four registers switches the brightness level to High,
stop experimenting and restore the backup of the register value.

If still no luck, you'd probably have to accept that it may not be possible to
set the brightness to High using software. I don't have any other idea, sigh.
Optimistically speaking, you can still switch between Off and Low using
software and get OSD work for all three states. If you can take that, I think I
can make a DSDT patch to achieve it.

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