[arch-general] Brightness keys not working on laptop

[David Rosenstrauch]

I can't seem to get the screen brightness up and down keys working on my 
laptop.  (A new HP EliteBook x360 1030 G4.)

Controlling the screen brightness programmatically does work.  I.e., I'm 
able to set up mappings in my DE (XFCE) for specific key combos to call 
out to the "light" application and increase/decrease brightness that 
way.  But the built-in brightness function keys (F3 and F4, without 
pressing fn) don't work at all.  xev doesn't register any keypress when 
I hit those keys, and evtest always reports an unknown/wrong scan and 
key code:

Event: time 1583360957.168127, -------------- SYN_REPORT ------------
Event: time 1583360959.602902, type 4 (EV_MSC), code 4 (MSC_SCAN), value 81
Event: time 1583360959.602902, type 1 (EV_KEY), code 465 (KEY_FN_ESC), 
value 1

I've been trying to debug this for several weeks now, but without 
success.  I'm also pretty sure that I saw these keys working about 2 
weeks ago.  But although I've tried rolling back to versions of some key 
packages (e.g., kernel, libinput, and systemd) from that date range, 
I've not been able to get them working again.  I've searched for bugs 
and forum posts on the issue, but haven't found anything that fixes it. 
I've also tried mucking with some of the suggestions at 
https://wiki.archlinux.org/index.php/Map_scancodes_to_keycodes but no 
joy there either.  I'd file a bug on this, but I have no idea which 
package is causing the issue.

I'm fairly certain these hotkeys are controlled by the kernel hp_wmi 
module, and I do see some error message lines in dmesg about that:

[    5.683232] hp_wmi: query 0x4 returned error 0x5
[    5.697112] hp_wmi: query 0x4 returned error 0x5
[    5.704773] hp_wmi: query 0xd returned error 0x5
[    5.730490] hp_wmi: query 0x1b returned error 0x5

But I'm not certain that those error messages are actually related to my 
issue, and googling on them didn't turn up anything of much relevance.

Anyone have any suggestions/pointers on what might be broken here and/or 
how to fix?

Thanks,

DR