Arch Linux

ryex

Member

Registered: 2016-09-09

Posts: 1

Fn Brightness keys triggering hardware switch?

and Hello! first  off, yes I did register just to ask this question. (very clever spam prevention by the way, I like).

I have at my fingertips an Acer Aspire One 722 (A0722) netbook from around 2012. on it keyboard there are two fn brightness keys on the left and right arrows.

The problem I have is that no matter which one I press it toggles the brightness between max and about 20%.

Now at first I though this was a simple ACPI misconfiguration and indeed /sys/class/backlight/acpi_video0 missbehaves.

cat /sys/class/backlight/acpi_video0/max_brightness returns 9

and trying to set the brightness manually by writing to /sys/class/backlight/acpi_video0/brightness does not work as expected.
instead of a linear progression writing an even number sets the backlight to max and writing an odd ones sets it to about 20%.

at this point I noticed that /sys/class/backlight/radeon_bl0 existed so I tried using it and it DID behave correctly with a max_brightness of 255 and even turning the backlight completely off if 0 was written to /sys/class/backlight/radeon_bl0/brightness

the obvious solution at this point was to force ACPI to use the native driver for the backlight. so I booted with the `acpi_backlight=native` kernel peram
to my surprise this did not work as expected. /sys/class/backlight/acpi_video0 was gone as it was supposed to be but the fn keys functioned exactly as they had before.

I work in openbox on this laptop so and afterwards the effects of editing the radeoon device triggered.

the result is this

20% >> fn+right(brightness-up) > >100%flash -> 30% >> fn+right(brightness-up) >> 20%flash -> 40% >  fn+right(brightness-up) >  100%flash -> 50%  >> ... >> 100% >>  fn+right(brightness-up) >>powermanager thinks we're at 100% so changes nothing but screen brightness starts toggling.

I did some more experimentation the fn keys do their toggeling things even in a tty, they even work when gnome loads. the fn keys do NOT work in the bios setup.

I get the feeling the keys are attached to a hardware switch of some kind kinda like the do with wireless cards, but this behavior is not observed when running windows ( it had windows when I got it before I put Arch on it)

any thoughts on how to get around this?

Last edited by ryex (2016-09-09 22:28:39)

basica

Member

From: Australia

Registered: 2012-10-31

Posts: 217

Re: Fn Brightness keys triggering hardware switch?

I had an issue a couple years ago with my lappy where the brightness keys would turn off my laptop. Quite annoying to say the least. After a lot of research I found the issue was basically to do with how the kernel was detecting my set up, and essentially it was confusing it for something else. Frustratingly it would work in Ubuntu, but not Arch. A few months later the Arch kernel worked A-OK, in the mean time I had to create my own xmodmap.

I think in your case, it's likely to be a kernel problem as well; I would try using an LTS kernel and a mainline one to see if you get any better results. Off the top of my head I know there are boot parameters you can set to the kernel, which may also effect the behavior and I know this can fix some issues.