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#1 2020-05-04 02:34:38

Micronaut
Member
Registered: 2019-07-04
Posts: 91  

Backlight control on laptops

After installing and using Devuan Ascii on two different (pretty old) laptops, I discovered that I had not quite gotten everything working. The install process had caught nearly everything, like the touchpad, the internal camera, etc. But when I thought to try to adjust the screen brightness, if just did not work. This is not the 'power saving' option where the screen is dimmed automatically when the machine is idle. This is the deliberate manual change of screen brightness when using the machine, in a low light situation where you want a less bright screen for example.

Searching around, I found that something called 'xbacklight' had to be installed, and then when it complained 'No outputs have backlight property' I set out to solve that problem. The recommended solutions such as adding command options to GRUB did not work until I ran across the page: 'https://shreve.io/posts/fixing-xbacklight' which recommends adding a config file inside the X11 directory tree. '/usr/share/X11/xorg.conf.d/10-backlight.conf' Placing the recommended file with the recommended contents in the recommended place finally made the manual backlight/brightness controls work on my laptops.

Now, here are the questions:
He got the info on how to do this from a 'bug report' -- does this mean that the Ascii installer (and therefore the Debian install it was based on) does not recognize certain hardware correctly and installs the wrong X11 drivers? Or does this mean that the default system configuration was botched when designing this version of the Debian/Devuan distro, not taking laptops properly into account? In either case, is this fixed, or will it be fixed, in Beowulf -- or will I have to remember to make manual adjustments to my systems when I upgrade?

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#2 2020-05-04 02:48:56

ralph.ronnquist
Administrator
From: Clifton Hill, Victoria, AUS
Registered: 2016-11-30
Posts: 452  

Re: Backlight control on laptops

I can't really answer your questions, but thought to mention that I tend to use xrandr for changing brighness, eg

$ xrandr --output eDP-1 --gamma 1:0.8:0.7 --brightness 0.7

(The simultaneous gamma adjustment suits my display)

Use xrandr without arguments to work out the name(s) of your monitor(s). And of course, man xrandr for details.

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#3 2020-05-04 11:19:37

Head_on_a_Stick
Member
From: London
Registered: 2019-03-24
Posts: 976  
Website

Re: Backlight control on laptops

Micronaut wrote:

He got the info on how to do this from a 'bug report' -- does this mean that the Ascii installer (and therefore the Debian install it was based on) does not recognize certain hardware correctly and installs the wrong X11 drivers?

Not really, it's just that the system defaults to the X server's builtin modesetting DDX driver for Intel cards and that driver doesn't work with xbacklight. The modesetting driver is reported to be a better option otherwise, see the advice on the xserver-xorg-video-intel package page:

The use of this driver is discouraged if your hw is new enough (ca. 2007 and newer). You can try uninstalling this driver and let the server use it's builtin modesetting driver instead.

https://packages.debian.org/stretch/xse … ideo-intel

ralph.ronnquist wrote:

I tend to use xrandr for changing brighness

That only changes the brightness at the software level, it works by reducing the available dynamic range for the display and so should only be used as a last resort.

man xrandr wrote:

       --brightness brightness
              Multiply the gamma values on the crtc currently attached to the output to
              specified floating value. Useful for overly bright or overly dim outputs.
              However,  this is a software only modification, if your hardware has sup‐
              port to actually change the brightness, you will probably prefer  to  use
              xbacklight.


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#4 2020-05-04 17:48:27

Magnus
Member
Registered: 2020-03-14
Posts: 15  

Re: Backlight control on laptops

I use Ceres and i have this line in /etc/rc.local to set the screen brightnes for my laptop

sh -c "echo 466 >/sys/class/backlight/intel_backlight/brightness"

466 is the setting

To see the screens max brightness I use

cat /sys/class/backlight/intel_backlight/max_brightness

And to see that setting the screen use I use

cat /sys/class/backlight/intel_backlight/actual_brightness

Last edited by Magnus (2020-05-04 17:49:49)

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#5 2020-05-04 19:27:16

Micronaut
Member
Registered: 2019-07-04
Posts: 91  

Re: Backlight control on laptops

Hrm, both of these laptops are much newer than 2007. They date from 2011-2012 or so. By that standard, they should work with the DDX driver. But they don't. It still sounds like there is a config problem at a very deep  level. They have not accounted for laptops properly.

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#6 2020-05-04 20:49:54

Head_on_a_Stick
Member
From: London
Registered: 2019-03-24
Posts: 976  
Website

Re: Backlight control on laptops

Micronaut wrote:

It still sounds like there is a config problem at a very deep  level

It's just that xbacklight doesn't work with the modesetting driver. That wouldn't matter at all if the kernel supported the hardware properly and provided the correct interface for the keyboard backlight controls. If the laptops are very old then this might be due to regressions, or perhaps the kernel just never worked properly with those machines.


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