Hack to control the keyboard backlight level on a Thinkpad Lenovo X1 Carbon Touch

tmp.c

/* gcc -o tmp `pkg-config --libs --cflags glib-2.0` tmp.c
 *
 * # modprobe ec_sys
 *
 * # watch -n 0.1 hexdump -C /sys/kernel/debug/ec/ec0/io
 *
 * 00000000  a7 05 a0 e2 00 86 05 00  00 00 47 00 00 03 00 10  |..........G.....|
 * 00000000  a7 05 a0 e2 00 86 05 00  00 00 47 00 00 43 00 10  |..........G..C..|
 * 00000000  a7 05 a0 e2 00 86 05 00  00 00 47 00 00 83 00 10  |..........G.....|
 *
 * # ./tmp 0
 * # ./tmp 1
 * # ./tmp 2
 *
 */

#include <glib.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

char levels[] = {
	0x03,
	0x43,
	0x83
};

static void
usage (char **argv)
{
	g_print ("%s [level]\n", argv[0]);
	g_print ("Where level is between 1 and 3\n");
}

int main (int argc, char **argv)
{
	int fd;
	int level;

	if (argc < 2) {
		usage (argv);
		return 1;
	}
	level = atoi(argv[1]);
	if (level < 0 || level > 3) {
		usage (argv);
		return 1;
	}

	fd = open ("/sys/kernel/debug/ec/ec0/io", O_RDWR);
	if (fd < 0) {
		g_print ("open: %s\n", g_strerror (errno));
		return 1;
	}
	if (lseek (fd, 0xd, SEEK_CUR) < 0) {
		g_print ("seek: %s\n", g_strerror (errno));
		return 1;
	}
	if (write (fd, &levels[level], 1) < 0) {
		g_print ("write: %s\n", g_strerror (errno));
		return 1;
	}

	return 0;
}

Here's a version that doesn't need libglib:

/* gcc -Wall -Werror -o backlight backlight.c
 *
 * # modprobe ec_sys
 *
 * # watch -n 0.1 hexdump -C /sys/kernel/debug/ec/ec0/io
 *
 * 00000000  a7 05 a0 e2 00 86 05 00  00 00 47 00 00 03 00 10  |..........G.....|
 * 00000000  a7 05 a0 e2 00 86 05 00  00 00 47 00 00 43 00 10  |..........G..C..|
 * 00000000  a7 05 a0 e2 00 86 05 00  00 00 47 00 00 83 00 10  |..........G.....|
 *
 * # ./backlight 0
 * # ./backlight 1
 * # ./backlight 2
 *
 */

#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

static char levels[] = {
    0x03,
    0x43,
    0x83
};

static void
usage(char **argv)
{
    printf("%s [level]\n", argv[0]);
    printf("Where level is between 1 and 3\n");
}

int
main(int argc, char **argv)
{
    int fd;
    int level;

    if (argc < 2) {
        usage(argv);
        return 1;
    }

    level = atoi(argv[1]);

    if (level < 0 || level > 3) {
        usage(argv);
        return 1;
    }

    fd = open("/sys/kernel/debug/ec/ec0/io", O_RDWR);

    if (fd < 0) {
        printf("open: %s\n", strerror(errno));
        return 1;
    }

    if (lseek(fd, 0xd, SEEK_CUR) < 0) {
        printf("seek: %s\n", strerror(errno));
        return 1;
    }

    if (write(fd, &levels[level], 1) < 0) {
        printf("write: %s\n", strerror(errno));
        return 1;
    }

    return 0;
}

@maltinho, bruno066, just sudo modprobe ec_sys to get that (if your kernel is configured to ship it as a module)

Here is a bash script version which does not require compilation. Works on my X260.

https://gist.github.com/vzaliva/0adba7bc40e2f31a0b5f802af2a63267

Thanks for figuring out registers!

On Fedora at least, compiled versions need chown root:root <filename>; chmod +s <filename> to gain root permissions.

The ec_sys kernel module is not loaded automatically on my system (X230 Tablet) so needs # echo ec_sys > /etc/modules_load.d to load, and #echo options ec_sys write-support=1 > /etc/modprobe.d/ec_sys.conf to be able to write to the relevant device persistently after reboot.

Once that's done, works like a charm!

Can someone tell me how to set this up on X1 Ubuntu 20 so that backlight is persistent? Not sure what steps to take so if someone can write step by step guide.