Arch Linux

CarterCox

Member

From: Argentina

Registered: 2018-02-24

Posts: 116

[SOLVED]Finding more information on backlight control

Hi guys. I'm having a bit of an issue with my backlight.

Whenever my laptop is connected to AC, the backlight value (in /sys/class/acpi_video0/brightness) is 24. When I unplug the charger, the backlight goes down, but the brightness value is still 24. When I plug it back in, the brightness doesn't go up.

I'm not using a DE, and I'm using only TLP and logind.conf as power management.

May this be a hardware issue (I'm using HP EliteBook 6930p, latest BIOS)?

Thanks!

Last edited by CarterCox (2018-06-08 01:01:28)

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And neither the angels in Heaven above
   Nor the demons down under the sea
Can ever dissever my soul from the soul
   Of the beautiful Annabel Lee;

piratebill

Member

From: Sol System

Registered: 2011-10-20

Posts: 145

Re: [SOLVED]Finding more information on backlight control

does the brightness change if you run xbacklight?

CarterCox

Member

From: Argentina

Registered: 2018-02-24

Posts: 116

Re: [SOLVED]Finding more information on backlight control

does the brightness change if you run xbacklight?

I'm not using X. I'm on the CLI alone.

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And neither the angels in Heaven above
   Nor the demons down under the sea
Can ever dissever my soul from the soul
   Of the beautiful Annabel Lee;

seth

Member

From: Won't reply 2 private help req

Registered: 2012-09-03

Posts: 78,849

Re: [SOLVED]Finding more information on backlight control

/sys/class/acpi_video0/brightness

Literally? Not /sys/class/backlight/* ?

The typical cause for the wrong reading is that you're looking at the wrong device (eg. a keyboard backlight or sth. superseeded by some module-specific device)
No idea about the bad behavior, so I just blame TLP :-P
(Ie. what happens w/ tlp disabled/not running?)

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V1del

Forum Moderator

Registered: 2012-10-16

Posts: 25,435

Re: [SOLVED]Finding more information on backlight control

TLP explicitly does not handle backlight in any form. FWIW (not on my machine so I can't verify and might be talking out of my ass here) AFAIK the acpi* brightness devices are deprecated and they are on some device specific path, i.e. likely intel_backlight instead no? What I'm gathering from the bits and pieces of the backlight wiki article seems to support that.

CarterCox

Member

From: Argentina

Registered: 2018-02-24

Posts: 116

Re: [SOLVED]Finding more information on backlight control

Ok, I kinda described the problem the wrong way. Here is some
more detailed information: 

ls -hl /sys/class/backlight/acpi video0/          
total 0                       
-r--r--r-- 1 root root 4.0K Jun  7 14:19 actual brightness      
-rw-r--r-- 1 root root 4.0K Jun  7 14:19 bl power         
-rw-r--r-- 1 root root 4.0K Jun  7 14:16 brightness         
lrwxrwxrwx 1 root root    0 Jun  7 14:19 device -> ../../../0000:00:02.0
-r--r--r-- 1 root root 4.0K Jun  7 14:16 max brightness       
drwxr-xr-x 2 root root    0 Jun  7 14:19 power          
lrwxrwxrwx 1 root root    0 Jun  7 14:16 subsystem ->         
../../../../../class/backlight                
-r--r--r-- 1 root root 4.0K Jun  7 14:16 type           
-rw-r--r-- 1 root root 4.0K Jun  7 14:16 uevent

       
The brightness file there has only 2 numbers ("24") and that controlls
brightness. So if I do "echo 12 >               
/sys/class/backlight/acpi video0/brightness" my brightness will go down. 
Also, I also have another instance of this:           

ls /sys/class/backlight/                
acpi video0  intel backlight

               
and in intel backlight there is a file called brightness that has the value
2368695, and this one affects my brightness as well. When I remove my
charger, it goes down to 473739. If I plug it, it goes up to 642180, which 
is certainly the desired behaviour, but I would like to change those   
values.                       
                         
In a related/unrelated note, V1del, shouldn't there be only one symlink (I 
read the exact same thing on the Wiki)? Why are there two? and why do both 
control the brightness? And why do both have the same files, with different
values, yet do the same? Why does only one react to the charger     
reconnecting?

---

And neither the angels in Heaven above
   Nor the demons down under the sea
Can ever dissever my soul from the soul
   Of the beautiful Annabel Lee;

seth

Member

From: Won't reply 2 private help req

Registered: 2012-09-03

Posts: 78,849

Re: [SOLVED]Finding more information on backlight control

No idea, I control AC/BAT events using acpid, https://wiki.archlinux.org/index.php/Acpid

If TLP doesn't maintain the backlight either something else you installed ("ps aux") or the UEFI/BIOS will.
Udev would be a candidate, see /etc/udev/rules.d and /usr/lib/udev/rules.d

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How to upload text · How to boot w/o GUI · Disable Windows Fast-Start! · Fix your xinitrc

CarterCox

Member

From: Argentina

Registered: 2018-02-24

Posts: 116

Re: [SOLVED]Finding more information on backlight control

No idea, I control AC/BAT events using acpid, https://wiki.archlinux.org/index.php/Acpid

If TLP doesn't maintain the backlight either something else you installed ("ps aux") or the UEFI/BIOS will.
Udev would be a candidate, see /etc/udev/rules.d and /usr/lib/udev/rules.d

Yeah acpid seems overly complicated, although maybe more robust. I don't
think I have the skill to set it up. Maybe if there were some pre-configured
templates somewhere.

By the way, I have never configured any of this (as I said, I don't use acpid,
TLP doesn't control screen and I never set any udev rules, and there are no
default rules set (at least in the rules.d directory, as mentioned here)), so you
think the BIOS is responsible?

Anyways, I'll read  a bit on udev rules and see what I can do. Doesn't seem
too difficult.

I'll mark as solved. Thank you all!

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And neither the angels in Heaven above
   Nor the demons down under the sea
Can ever dissever my soul from the soul
   Of the beautiful Annabel Lee;