# Kernel upgrade 3.15.3 -> 3.16.1 screen dimming broke

## chias

I use XFCE, and I just updated my kernel from 3.15.3 to 3.16.1. Since updating, the screen dimming / brightening keys no longer dim the screen, which stays at full brightness (although the notice thing works, and indicates that the screen is dimming even though it isn't). The only effect the buttons do have is that when brightness gets to zero, the screen switches off entirely (which did *not* happen under 3.15.3).

The only way I've found of actually changing screen brightness is to unplug the laptop, at which point it dims to 20% brightness, but plugging it back in restores it to 100%. Booting into the 3.15.3 kernel makes the problem go away and dimming/brightening works as expected.

I configured 3.16.1 by simply copying my 3.15.3 .config file over and running "make menuconfig". Has anyone else seen this / know how to go about diagnosing what's wrong and fixing it?

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## Sulman

What's the hardware platform? Which gpu driver? I've found screen brightness hotkeys to be the one thing that periodically break whatever flavour of Linux you use.

It sounds like XFCE is recognising the keystrokes correctly but they're not driving anything anymore. Do you know what they map to? Perhaps that behaviour has changed, either in ACPI or for e.g. xbacklight.

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## chias

Thanks for the tips

Hardware platform is a "Samsung ATIV book 6", and I believe I'm using the Intel graphics drivers although the laptop does also have a Radeon card.

Since this issue arose both xfce4-power-manager and the kernel have both received an upgrade, and the problem remains  :Sad:  I don't know what the keys map to (I have not changed any keymappings from the defaults) and I don't have xbacklight installed.

EDIT: after installing xbacklight, I can now dim the screen by using xbacklight. The brightness keys continue to display the brightness changing notification without changing the brightness, though

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## albright

Did you try the kernel boot parameter:

 *Quote:*   

> acpi_backlight=vendor

 

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## chias

Correction to my previous post: xbacklight works with the old kernel only.

Thanks for the tip about the kernel boot parameter! I just tried it though, and screen dimming still does not work. My boot parameters are:

```
quiet root=/dev/ram0 crypt_root=UUID=5f1018f8-8007-43ea-9032-9b1bf1abb3c8 real_root=/dev/mapper/linux-gentoo root_keydev=UUID=399F-3122 root_key=S1AXNEAD702790F_1.bin.gpg rootfstype=ext4 key_timeout=0 dolvm acpi_backlight=vendor
```

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## chias

So this is interesting. I had a look in '/sys/class/backlight/intel_backlight':

```
human intel_backlight # cat max_brightness 

4648

human intel_backlight # cat brightness 

4648

human intel_backlight # cat actual_brightness

4648

```

Then I used the screen-dimming buttons to attempt to change the brightness. The brightness-change notification came up as always, just like volume change notifications do, and then:

```
human intel_backlight # cat max_brightness 

4648

human intel_backlight # cat brightness 

472

human intel_backlight # cat actual_brightness

472

```

But, and here's the kicker, the actual brightness of the screen did not change. The only way I am able to change the brightness of the screen is by unplugging or plugging in my laptop: when it's plugged in it shines with the fury of a thousand suns; when it's unplugged I can hardly see a thing.

Writing to the 'brightness' file acts exactly the same as using the brightness keys: the only time anything happens is if I set a brightness value of zero, at which point the screen turns off entirely. Anything non-zero is "on", where "on" is either full-brightness if plugged in or super-dim if not plugged in.

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## albright

I'm curious if you boot from another linux distro cd or usb stick, do

the backlight keys work?

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## chias

Good question! Just booted using an Xubuntu live-usb, and yes the brightness keys work perfectly.

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## chias

Because 100% and 1% are both fairly unusable, my current workaround is to use kernel 3.15.3 and use xbacklight to adjust the screen brightness. I'd really like to be able to update though... does anyone have any other ideas what might solve things?

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## chias

I still haven't managed to make any headway on this... does anyone have any other suggestions?

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## Sakaki

Hi chias,

It may be that you are affected by this i915 issue?

If so, you might try setting:

```
video.use_native_backlight=0
```

in your kernel command line, and see if that fixes things.

I had a similar issue on my Panasonic CF-AX3, and setting this option resolved it.

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## chias

Hm, that didn't seem to help either.

Further investigation though shows that this issue continues to vex me even with a fresh Debian install on the same hardware. Which says to me that it probably isn't an issue to be fixed on the Gentoo forums :<

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