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#1 2016-08-13 10:03:30

jf25
Member
From: Germany
Registered: 2015-07-11
Posts: 35

Can't change backlight brightness since linux kernel 4.7

Hello,
I was wondering if I'm the only one with this problem? Since yesterday's updates, I can no longer adjust the brightness of my laptop display. I usually use either the keys Fn+arrows on my keyboard or the energy settings gui of Cinnamon. None of them work any more. I also installed xorg-xbacklight and tried to set the brightness through xbacklight, but that doesn't work either (it does affect Cinnamon's brightness slider though).
Edit: /sys/class/backlight/intel_backlight does exist.

The following packages got an update:

2016-08-12 09:17:24    evince
2016-08-12 09:17:25    gd
2016-08-12 09:17:25    gvfs
2016-08-12 09:17:25    gvfs-nfs
2016-08-12 09:17:25    gvfs-smb
2016-08-12 09:17:25    iproute2
2016-08-12 09:17:33    linux
2016-08-12 09:17:50    linux-headers
2016-08-12 09:17:50    python2-pyparsing
2016-08-12 09:17:50    python-pyparsing

I think all of that gnome and python stuff is unrelated. So the only real suspect here is the linux kernel which got upgraded from 4.6.4-1 to 4.7-1. Unfortunately I couldn't find anything suspicious in the system logs (journalctl, dmesg). I can post them anyway, if somebody is interested.

So is there anybody else with this problem or does someone know a solution approach?

Thank you in advance!

Last edited by jf25 (2016-08-13 10:15:43)

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#2 2016-08-13 10:15:12

Head_on_a_Stick
Member
From: London
Registered: 2014-02-20
Posts: 7,680
Website

Re: Can't change backlight brightness since linux kernel 4.7

Does your backlight work again if you downgrade the kernel?

If it does, you may have suffered a kernel regression -- you can use a git bisect to find the cause and fix it:
https://wiki.archlinux.org/index.php/Bisecting_bugs

Alternatively, try the LTS kernel and see if that works:
https://www.archlinux.org/packages/core … linux-lts/

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#3 2016-08-13 10:38:06

jf25
Member
From: Germany
Registered: 2015-07-11
Posts: 35

Re: Can't change backlight brightness since linux kernel 4.7

Yep, downgrading linux (and linux-headers) to 4.6.4-1, "fixes" the issue. I think bisecting the kernel would be too time-consuming though. I have a thesis to write.

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#4 2016-08-13 10:39:07

Head_on_a_Stick
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From: London
Registered: 2014-02-20
Posts: 7,680
Website

Re: Can't change backlight brightness since linux kernel 4.7

Switch to the LTS kernel then, you shouldn't use an outdated kernel.

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#5 2016-08-13 15:39:32

mrDoctorWho
Member
From: TARDIS
Registered: 2014-07-05
Posts: 7

Re: Can't change backlight brightness since linux kernel 4.7

Hello,

I'm actually having the same problems.

First, after the linux 4.7 update the laptop screen could go off by a timer and then it was not possible to power it on again (though, I found that if I change the tty then the screen powers on again).

Then, after the kernel was updated to 4.7-1, the backlight became broken too.

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#6 2016-08-13 17:00:00

Head_on_a_Stick
Member
From: London
Registered: 2014-02-20
Posts: 7,680
Website

Re: Can't change backlight brightness since linux kernel 4.7

@mrDoctorWho -- have you filed a bug report?

Does the LTS kernel work?

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#7 2016-08-14 02:49:02

mrDoctorWho
Member
From: TARDIS
Registered: 2014-07-05
Posts: 7

Re: Can't change backlight brightness since linux kernel 4.7

@Head_on_a_Stick, no I actually didn't fill a bug report yet.

The LTS kernel works just fine.

Update: the screen on-off problem still exists with the LTS kernel, so I guess it's not about the kernel.

Last edited by mrDoctorWho (2016-08-14 04:28:51)

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#8 2016-08-14 10:20:05

linuxchr1s
Member
From: World
Registered: 2016-08-14
Posts: 1

Re: Can't change backlight brightness since linux kernel 4.7

I got the same issue. After upgrading from 4.6 to 4.7 I found out that changing the backlight brightness doesn’t work anymore. However, the values are set correctly:

$ cat /sys/class/backlight/intel_backlight/max_brightness
4882
$ cat /sys/class/backlight/intel_backlight/brightness    
4882

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#9 2016-08-14 14:18:09

jf25
Member
From: Germany
Registered: 2015-07-11
Posts: 35

Re: Can't change backlight brightness since linux kernel 4.7

That's interesting. With kernel 4.7, I have the same file paths and values as linuxchr1s. With kernel 4.6 however:

$ ls /sys/class/backlight
acpi_video0    acpi_video1
$ cat /sys/class/backlight/acpi_video0/max_brightness
8
$ cat /sys/class/backlight/acpi_video0/brightness
8
$ cat /sys/class/backlight/acpi_video1/max_brightness
8
$ cat /sys/class/backlight/acpi_video1/brightness
(depends on the actual brightness)

Could it be, that this has something to do with dual gpu? I have both an Nvidia (running with open-source drivers and PRIME) and Intel GPU in my system.

By the way, switching the display on and off works like a charm for me with any kernel. So this is indeed a separate issue.

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#10 2016-08-14 16:00:55

Head_on_a_Stick
Member
From: London
Registered: 2014-02-20
Posts: 7,680
Website

Re: Can't change backlight brightness since linux kernel 4.7

Have any of you tried the various parameters listed in the ArchWiki?

https://wiki.archlinux.org/index.php/Ba … ne_options

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#11 2016-08-14 16:41:58

jf25
Member
From: Germany
Registered: 2015-07-11
Posts: 35

Re: Can't change backlight brightness since linux kernel 4.7

Great tip! I tried acpi_backlight=video and everything works again. Now I have:

$ ls /sys/class/backlight/
acpi_video0  acpi_video1  intel_backlight

Again, only the values in acpi_video1 seem to have any effect.

Thank you so much!

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#12 2016-08-18 12:25:33

lviggiani
Member
Registered: 2014-02-28
Posts: 141

Re: Can't change backlight brightness since linux kernel 4.7

Head_on_a_Stick wrote:

Have any of you tried the various parameters listed in the ArchWiki?

https://wiki.archlinux.org/index.php/Ba … ne_options

I have similar issue here. Since I've upgraded to 4.7, if I set the screen brightness to any value other than 100% it starts flashing from the brightness level I've set to 100% and back.
I've tried with the kernel parameters suggested in the wiki above and with acpi_backlight = video or vendor the backlifght control does not work, whereas with acpi_backlight = native I have the same behavior as described above (flashing).
Any ideas?
This happens on a Dell XPS 13 (old ivybridge model).

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#13 2016-08-19 16:27:25

Head_on_a_Stick
Member
From: London
Registered: 2014-02-20
Posts: 7,680
Website

Re: Can't change backlight brightness since linux kernel 4.7

lviggiani wrote:

Any ideas?

LTS kernel, perhaps?

Does `xbacklight` work?

I set keybinds for that with my HD4600

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#14 2016-08-20 06:18:29

kjslag
Member
Registered: 2009-11-03
Posts: 43

Re: Can't change backlight brightness since linux kernel 4.7

I had the same issue. Adding acpi_backlight=video to the kernel parameters solved the issue for me also.

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#15 2016-09-12 21:14:00

carbohydrates
Member
Registered: 2014-07-19
Posts: 12

Re: Can't change backlight brightness since linux kernel 4.7

I am having the same problem.  ThinkPad x230. 

With 4.4.6, the backlight 'just works', absolutely perfectly.  There's an acpi0 and intel_backlight present in /sys/class/backlight.

With 4.7.3, it doesn't.  The only device present is an intel_backlight.  xbacklight works, barely.  The numbers are off - there are crazy floating point errors.  It's impossible to inc or dec in good, round numbers.  Same with acpi_backlight=video.  It gives me an acpi0 device again, but it takes dozens of button presses to change the brightness even one step.

I'll take a crack at bisecting the changes to figure out what caused this regression.

Does anyone know if a bug was ever filed?  It's way less usable like this...

Last edited by carbohydrates (2016-09-12 21:26:08)

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#16 2016-09-12 21:38:12

lviggiani
Member
Registered: 2014-02-28
Posts: 141

Re: Can't change backlight brightness since linux kernel 4.7

Hi, I've reported the issue here
https://bugzilla.kernel.org/show_bug.cgi?id=153401
I'm currently trying to help find the commit that broken the thing.
On my Dell, as a workaround I've disabled the "Load legacy boot ROM" option from the BIOS boot menu and the backlight can now be adjusted in 4.7.2

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#17 2016-09-13 01:17:29

carbohydrates
Member
Registered: 2014-07-19
Posts: 12

Re: Can't change backlight brightness since linux kernel 4.7

So setting your system to be EFI-only changes the behavior?  Interesting, I'll have to try that after this enormous git repo finshes cloning smile

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#18 2016-09-13 06:01:58

lviggiani
Member
Registered: 2014-02-28
Posts: 141

Re: Can't change backlight brightness since linux kernel 4.7

Yes... I'll check the patch mentioned in the bug report discussion and see if recompiling without it restores the old behaviour

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#19 2016-09-13 07:00:30

carbohydrates
Member
Registered: 2014-07-19
Posts: 12

Re: Can't change backlight brightness since linux kernel 4.7

Sounds good - and I have a patch that works for me now smile 

Hopefully it'll get committed so it can help everyone!

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