Arch Linux

porphyry5

Member

Registered: 2014-06-04

Posts: 88

backlight adjustment lost after system update

I installed arch on an Acer c720 Chromebook, and used a command suggested by https://wiki.archlinux.org/index.php/backlight  to alter the screen brightness, e.g.

echo 30 > /sys/class/backlight/intel_backlight/brightness

  After updating the system, this command would no longer work, because

~ # ls -al /sys/class/backlight
total 0
drwxr-xr-x  2 root root 0 Jul 23 18:47 ./
drwxr-xr-x  43 root root 0 Jul 23 11:46 ../
~ # mkdir /sys/class/backlight/intel_backlight
mkdir: cannot create directory ... Operation not permitted
~ # whoami
root

I started X and did

~ $ xbacklight -dec 50
No outputs have backlight property

So, the directory concerned no longer exists, and I cannot create it.  How should I proceed with this problem

ewaller

Administrator

From: Pasadena, CA

Registered: 2009-07-13

Posts: 19,804

Re: backlight adjustment lost after system update

As a sanity check, is your running kernel the same as the one you have installed?  Perhaps the kernel cannot find its modules.
Compare the output of uname -a and pacman -Qi linux and ensure that the version numbers match.

Edit:

ewaller$@$odin /sys/class/backlight/acpi_video0 1018 %cd
ewaller$@$odin ~ 1019 %ls /sys/class/backlight/acpi_video0
actual_brightness  bl_power  brightness  device  max_brightness  power  subsystem  type  uevent
ewaller$@$odin ~ 1020 %cat /sys/class/backlight/acpi_video0/brightness
0
ewaller$@$odin ~ 1021 %lspci | grep VGA
00:02.0 VGA compatible controller: Intel Corporation Mobile 4 Series Chipset Integrated Graphics Controller (rev 07)
ewaller$@$odin ~ 1022 %uname -a
Linux odin 3.15.6-1-ARCH #1 SMP PREEMPT Fri Jul 18 07:06:42 CEST 2014 x86_64 GNU/Linux
ewaller$@$odin ~ 1023 %

BTW, the backlight is at 0 because the lid is closed.  This output is from a ssh session

Last edited by ewaller (2014-07-24 16:09:28)

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porphyry5

Member

Registered: 2014-06-04

Posts: 88

Re: backlight adjustment lost after system update

As a sanity check, is your running kernel the same as the one you have installed?  Perhaps the kernel cannot find its modules.
Compare the output of uname -a and pacman -Qi linux and ensure that the version numbers match.

Both show version 3.15.5-2
On my system, there is nothing in /sys/class/backlight/ at all, and, as root, I am not permitted to create anything in it

ewaller

Administrator

From: Pasadena, CA

Registered: 2009-07-13

Posts: 19,804

Re: backlight adjustment lost after system update

... and, as root, I am not permitted to create anything in it

Don't get too hung up on that point.  /sys is a pseudo file system that provides a communication conduit to the kernel.  Only the kernel can create 'files' in /sys. (I think).

What is your specific video system?

---

Nothing is too wonderful to be true, if it be consistent with the laws of nature -- Michael Faraday
Sometimes it is the people no one can imagine anything of who do the things no one can imagine. -- Alan Turing
---
How to Ask Questions the Smart Way

porphyry5

Member

Registered: 2014-06-04

Posts: 88

Re: backlight adjustment lost after system update

What is your specific video system?

 lspci | grep VGA
00:02.0 VGA compatible controller: Intel Corporation Haswell-ULT Integrated Graphics Controller (rev 09)

SolarBoyMatt

Member

Registered: 2012-01-07

Posts: 263

Re: backlight adjustment lost after system update

This is a known problem with some Chromebooks and the 3.15 Kernel https://bugs.freedesktop.org/show_bug.cgi?id=79813

Currently, linux-chromebook and linux-mainline-chromebook in the AUR provide patches for it.

porphyry5

Member

Registered: 2014-06-04

Posts: 88

Re: backlight adjustment lost after system update

This is a known problem with some Chromebooks and the 3.15 Kernel https://bugs.freedesktop.org/show_bug.cgi?id=79813

Currently, linux-chromebook and linux-mainline-chromebook in the AUR provide patches for it.

Many thanks.