Arch Linux

ralph_13

Member

Registered: 2015-02-06

Posts: 73

[SOLVED] Cannot decrease backlight

Some background info first:

I got this laptop (asus X401U) used with windows 8 installed. Backlight controls wouldn't work already - notification appears on screen as if it was going up/down but no actual effect. When the charger was unplugged, though, the backlight would drop to about, say, 20%; and there was absolutely no way to increase it again, aside from rebooting on line power. Power manager options would have absolutely no effect whatsoever on this.

$ ls /sys/class/backlight
asus-nb-wmi  radeon_bl0

Main difference I noticed for these two is that asus-nb-wmi's max brightness is 100, while the latter is 255; consistent with the max value on my previous asus laptop. The graphics card is radeon hd 7290 with radeon drivers. Changing brightness values on either device has no effect.

Now, the weirdest part is that systemd generates 3 backlight save/load services: one for asus-nb-wmi, one for radeon_bl0 and another one for acpi_video0; the latter of which fails as expected because the device doesn't exist. But if it isn't there, why is systemd trying to start the service in the first place? Makes no sense.

I also tried using setpci with no success. Maybe partly because I didn't understand how the last part of the command used as an example in the wiki, after the gpu's address (F4.B=0) is supposed to work.

Anyway, I'm guessing this is probably a hardware related issue, but if windows could (sort of) decrease the brightness, then there must be some way, right? I also thought that maybe the backlight devices could be in conflict but I can't seem to find anyone that's ever experienced something like that.

I'm really lost on this one.

Last edited by ralph_13 (2017-08-09 02:27:24)

Head_on_a_Stick

Member

From: London

Registered: 2014-02-20

Posts: 7,732

Website

Re: [SOLVED] Cannot decrease backlight

Try:

# modprobe -v asus-nb-wmi

and perhaps the kernel parameters listed in the ArchWiki:

https://wiki.archlinux.org/index.php/Ba … ne_options

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Are you infected with Wetiko?

ralph_13

Member

Registered: 2015-02-06

Posts: 73

Re: [SOLVED] Cannot decrease backlight

I had already tried loading the asus-nb-wmi module, forgot to mention that, sorry. Not sure if relevant, but when I change the brightness value for asus-nb-wmi, the actual_brightness value stays the same. On the other hand, for the radeon_bl0 device, it changes with brightness (although there is still no actual effect).

$ lsmod | grep asus_nb_wmi
asus_nb_wmi            28672  0
asus_wmi               24576  1 asus_nb_wmi
i8042                  24576  1 asus_nb_wmi

Will try the kernel parameters and report back in a while.

edit: A bit confused here, am I supposed to use acpi_backlight for the modules, even though I don't have a device called acpi_video in /sys/class/backlight?

Last edited by ralph_13 (2017-08-08 21:49:10)

ralph_13

Member

Registered: 2015-02-06

Posts: 73

Re: [SOLVED] Cannot decrease backlight

Ok, after trying all the kernel parameters, acpi_backlight=video did the trick (why is it always in the last place you look?). Now /sys/class/backlight lists acpi_video0 and it works as expected. Thanks for the help!