Arch Linux

dakane

Member

Registered: 2014-08-18

Posts: 7

Backlight very dim and won't respond to attempts to change brightness

A few weeks ago my sister ran pacman -Syu on her Toshiba laptop running Arch. The backlight is fine in Grub, and a disk check completes before the screen goes almost completely blank (I believe that the backlight is completely off but the screen is still barely visible) I've dutifully followed the instructions on the backlight page on the wiki. I can access the system via VNC or SSH. The video chipset is a Radeon Xpress 1200. Here's most of what I've tried:

Changing the value of  /sys/class/backlight/acpi_video0/brightness - I can change it just fine to any value less than or equal to max_brightness, but this does not affect the brightness. There are no other directories in /sys/class/backlight/.

Setting the brightness with xrandr has no effect.

xbacklight returns "No outputs have backlight property"

Adding each of these kernel options has no effect:

acpi_backlight=video
acpi_backlight=vendor
acpi_backlight=native
acpi_backlight=none
acpi_osi="Windows 2012"
acpi_osi="Windows 2006"
acpi_osi=Linux
systemd.restore_state=0

systemd.restore_state=0 would keep the brightness from being restored to what it was after the previous boot, but the problem persists.

lsmod reveals that toshiba_acpi is loaded. I tried blacklisting it. The problem was unaffected.

setpci -s 00:02.0 F4.B=0 (using the proper PCI ID) has no effect.

Many other forum posts mention other problems which occur only when resuming from a suspended state. I haven't found anything that seems to be this exact problem, although some other forum posts gave me things to try.

What other information should I post? What should I try from here?

techwiz

Member

Registered: 2012-01-26

Posts: 48

Re: Backlight very dim and won't respond to attempts to change brightness

This thread might be helpful: https://bbs.archlinux.org/viewtopic.php?id=134972

dakane

Member

Registered: 2014-08-18

Posts: 7

Re: Backlight very dim and won't respond to attempts to change brightness

In that thread, the problem turned out to be that the permissions on some things in /sys/class/backlight/acpi_video0/ were wrong. In this case, I am able to set /sys/class/backlight/acpi_video0/brightness to any value less than or equal to max_brightness (7) but doing so has no effect.

One thing that thread did make me realize though is that in

setpci -s 00:02.0 F4.B=0

the final 0 should be replaced by the desired brightness. However, this can be set to any value from 0 to 100 with no effect.

lordnaikon

Member

Registered: 2014-09-30

Posts: 34

Re: Backlight very dim and won't respond to attempts to change brightness

Hi dakane,

i have the exact same problem with a mobile radeon x1600 after updating the system. I have entries in /sys/class/backlight that make sense and everything seem to be running well. No errors in xorg log (at least for what i can tell) can set the backlight in various ways without errors but nothing do really affect the brightness. i also tried various kernel parameters to no avail. have you investigated anything further regarding this problem?