How to turn off backlight of a laptop display?

I booted Official OpenWrt 21.02.0(Sep 2 2021) from USB on HP Pavilion x2 laptop. Everything works fine and I found a way to set the terminal blank from another [forum post].(How to turn off the screen of a laptop?) It does turn the screen blank, however the backlight is still on.

I found a potential way to turn the display off fully via a software called "vbetool" and using the "vbetool dpms off" command. However there is no available openwrt package for it, and the binary I copied from arch and debian releases do not work (I tried running it in bash instead of ash).

Does anyone know the solution to this problem? How can I compile this software on openwrt? Is there any other way to mitigate the problem? Any help will be appreciated.

check if /sys/class/backlight/..... exists.

it does. I even tried to write it to 0 but it doesn't seem to have any effect.

Please have a look at this. I found this in my kernel log. Could this be the issue?

After some digging I found that this is a very common problem on Bay Trail Laptops/Tablets running Linux. Backlight/Brightness control does not work correctly.

I found a way to address this, by setting “video=LVDS-1:d” flag in grub config. Will try this and update when I wake up tomorrow.

probably asking a dumb question:
many laptops turn off the screen and backlight at hardware level (so this works regardless of the OS) if you close the lid. Did you check if your laptop does this? Does the screen remain on if you close the laptop screen on the keyboard?

Yes it stays on when i close the lid unfortunately. I can even see the light leaking out of the gap at night. :frowning:

OK. So this didn't work. Seems like all those software lvl changes rely on the driver to work properly, but it wasn't working properly, it failed to grab GPIO control for the backlight.

What I ended up doing is prying the enclosure open and disconnecting the display cable. That worked like a charm.

Thanks everyone.

For anyone trying to turn their Bay Trail Tablet/Laptop into a router running openwrt, and wanting to turn off the display, you need to:
1.pry your device open not disconnect the battery cable
3.disconnect the display cable
4.assemble your device

Reason why u can't disconnect the battery:
Because when you disconnect the battery the BIOS settings will be cleared automatically, and you need to access the bios to turn off secure boot, which is very difficult with out a display.
Even if u manage to do that, on the next boot it will ask u to enter a password to ensure you want to disable secure boot. And that password is randomly generated, so u can't see it without a screen.


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