Arch Linux

Pacane

Member

Registered: 2016-10-22

Posts: 1

Adjust brightness on Macbook Pro

Hey guys, I have installed Arch with i3 on my MacBook Pro, and I still can't figure out how to adjust the brightness.

Here's the output of lspci

 ❯ lspci | grep VGA
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Venus XT [Radeon HD 8870M / R9 M270X/M370X] (rev 83)

I have installed the open source ATI driver as specified here: https://wiki.archlinux.org/index.php/ATI#Installation

Now when I try to use the screen brightness keyboard buttons, without surprise, they don't work. I read in the wiki on backlighting https://wiki.archlinux.org/index.php/backlight#ACPI.

 ❯ ls /sys/class/backlight
gmux_backlight  radeon_bl0

the max_brightness found in gmux_backlight = -1 and max_brightness for radeon_bl0 = 255.

I assumed I would be able to change the brightness for the radeon entry.

I tried changing the values using:

sudo tee /sys/class/backlight/radeon_bl0/brightness <<< 5

and nothing apparent changed. The value in the brightness file is changed however.

I also tried using utilities like xbacklight, however it doesn't seem to work either. Note that in the wiki, it's written that it shouldn't work with Radeon video cards.

Does anyone have an idea of what I could try now?

Thanks

Updates:

After doing more tests, it turns out that when I use xbacklight to decrease brightness multiple times, the actual_brightness value in /sys/class/backlight/radeon_bl0/actual_brightness decreases, with no visible change, until it gets close to 0, where the whole screen turns black.

I also tried adding each of the following kernel options in the bootloader

acpi_backlight=none
acpi_backlight=vendor
acpi_backlight=native
acpi_backlight=video

and that doesn't do it either.

After looking carefully, it seems like [acpi_backlight=native] makes the brightness to be around 50% by default. I currently have no way of changing it though.

Last edited by Pacane (2016-10-22 17:29:28)