Screen backlight problem after suspend on Arch

Introduction

On Arch, it seems that there are often problems with the screen backlight. For example, I had a problem when suspending my computer with an almost discharged battery. When I plugged my computer, it was fully discharged. Trying to restart it from suspend didn’t display something on my screen. Let me explain all the details of this problem.

Environment

First of all, let me describe the environment where it happens. I am using a ASUS N750JV computer, with Arch Linux installed on it. My graphic card is an Intel; this is the result of lspci for the Intel card.

VGA compatible controller: Intel Corporation 4th Gen Core Processor Integrated Graphics Controller (rev 06)

Description of the problem

I was using my computer on battery and reaching around 5% of battery charge, I decided to suspend my computer by closing it. I don’t really know what kind of program do the suspend by default on Arch Linux with Gnome.

However, before I can plugged my computer, the battery fully discharge. In an ideal case, the computer would have detected that it has only a few minutes to shutdown in a clean way. But I am not really sure that Arch (or Gnome, I don’t really know which component should be in charge of this) is doing it right.

When I tried to reboot the computer, the screen never shows me anything. After a few try to hard reboot (holding the power button then reboot), I resigned.

The computer is started

However, I discovered something interesting, the computer seems to start; I had the first initializations steps displayed. After these steps, the screen shutdown. Based on the assumption that the computer is started and that only the screen didn’t work, I tried to blindly connect my session then run a terminal then try to reboot. And it works!

Temporary solution

That’s a good news, because I can try to fix it, even if it’s blindly. I tried a lot of things I found on Internet to switch on backlight with no success.

The only final idea I had was: “I may try to restart properly the suspend procedure”. So instead of trying to swith on the screen backlight, I tried to suspend my computer in CLI because I suspected than my problem came from a bad restart after the suspend.

sudo systemctl suspend

The computer effectively suspended, the hard drive stopped, it works! Now, typing space to restart it… and… magic! The screen came back to life.

However, first thing I tried is to close my computer then open it to check if it works well. That’s a deception because, I’m back to my initial problem. Trying to reboot it: same problem. This solution is only a temporary solution to debug but the problem is still there.

The solution

Now, I’m pretty sure that it’s a only a problem of my screen backlight. I then looked the specific Arch wiki page about backlight. And the solution is in the brightness file.

There is a brightness file under /sys/class/backlight. This brightness only contains a number with the intensity of the brightness of the screen. I have three different subdirectories in the backlight directory so I checked the brightness file in each of them (acpi_video0, acpi_video1 and intel_backlight). And the one in acpi_video1 was containing the value 0.

At this stage, be careful because you can’t just put any value in this file without knowing what the maximum value. In the same folder than your brightness file, you should find the max_brightness file which contains the maximum allowed value. For example, for my acpi_video1 directory, it’s 100 but for my intel_backlight directory, it’s 5273. Now, you can replace the value in the brightness file.

echo 100 > /sys/class/backlight/acpi_video1/brightness

Bonus

Check you computer is started

That’s something I didn’t tested, because I was lacking the tools. There is two ways I learned.

The first one is easy, you just plug another screen on VGA/DVI/DisplayPort. If it works, it’s a proof that your computer is started. However, it doesn’t work, it doesn’t proove anything; it’s possibly one of the endless problem of connecting a second screen on Linux!

A more tricky one is using a light. The hypothesis is that your screen is working, the only thing that is not working is the backlight of the screen. In a black room, using the light with the right angle on your screen, you should see the LCD pixels displaying things.

In the rabbit hole

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