Glo Controlling the Glo's Frontlight

[demondog]

ok thanks yoq

[sergeyvl12]

Originally Posted by KevinShort

I was looking at a strace of nickel today, and found that the Glo's frontlight is controlled by ioctl calls to /dev/ntx_io. I hacked together some quick C code to control the light:

#include <stdio.h>
#include <fcntl.h>
#include <sys/ioctl.h>

int main(int argc, char *argv[])
{
    if ( argc != 2 ) {
        printf("Usage: %s brightness \n", argv[0]);
        return 1;
    }
    
    int light;

    // Open the file for reading and writing
    if ((light = open("/dev/ntx_io", O_RDWR)) == -1) {
        printf("Error opening ntx_io device");	
    }

    int brightness = atoi ( (argv[1]) );
    ioctl(light, 241, brightness);

    return 0;
}

I've attached the source code and the compiled program below.

Thanks, Kevin

Is it possible to get the current frontlight level?

[giorgio130]

Originally Posted by sergeyvl12

Thanks, Kevin

Is it possible to get the current frontlight level?

That has been investigated for koreader use, but we couldn't find any way to do that reliably.

[sergeyvl12]

Originally Posted by giorgio130

That has been investigated for koreader use, but we couldn't find any way to do that reliably.

Thanks, I didn't find it also. Probably nickel keeps this value deep inside

The one solution I see for now is intercepting ioctls (LD_PRELOAD) but I don't like it...

[yoq]

Originally Posted by sergeyvl12

Is it possible to get the current frontlight level?

The easiest way, (but rather cpu heavy) is to scan dmesg as every change is printed to the log.

But there are other options:
The kernel source for the backlight driver is at arch/arm/mach-mx5/mx50_ntx_io.c, but there is no ioctl to get the current level. It shouldn't be too hard to implement a new one, but how many users will want to change their kernel for that

A more hacky way: Since we are root, we can simply fetch the content of the last_FL_duty variable from kernel memory:

hexdump -s 0x803ba614 -n1 -v -e '"%d"' /dev/kmem

Of course, the address will be off as soon as a new kernel build is released. Does anyone know how often they do this? I know they did when they changed the toolchain, but other than that?

[yoq]

not helpful?

[sergeyvl12]

Thanks!

I decided to not use the nickel frontlight level. It's difficult to get it from nickel and it's completely impossible to tell nickel the level that I selected in the KoboLauncher...

[giorgio130]

The same for Koreader, having to register a memory address for each device and possibly changing with firmware updates is a nightmare to maintain.

[png]

Hey Yoq! I've been lurking this forum since I got my Kobo glo and your resistor hack worked perfectly. Thanks for taking the time to share your knowledge. I agree with you that the 30k resistor works best for this purpose. I've tried a 25k, 27k and 30k resistor. You can get a very dim screen with the 27k at 1% brightness but it tends to quite flicker a bit. I don't recommend 25k because the light is off at the lowest brightness. I'm using the night mode hack along with the resistor hack and its great for reading at night.

[ksiflhjla8]

I used a big resistor and it works. Thank you

[Cecell3000]

[Stremon]

Originally Posted by yoq

I would add a wired resistor like this:

It's not the right one though, it should be 3x orange (33k or a bit higher)

Thanks for this great mod, I did it with a normal size resistor, and it is working really well!
There is just one strange thing; now when the backlight is on, the kobo emit a hiss sound from the place the resistor is... No hiss when the backlight is off.
I tried removing the resistor and putting it back again, but the hiss is still here.
Do you have a hiss too on your device? If no, is there anything I can check to find what's wrong?
thank you

[PeterT]

Must be leaking electrons causing the hiss.

[Stremon]

Originally Posted by PeterT

Must be leaking electrons causing the hiss.

sorry if that's a stupid question, but what does it mean?

[PeterT]

Was a joke.....