Linux Kernel Module Cheat

23.6.3. LEDs

TODO: broken when arm moved to -M virt, same as GPIO.

Hack QEMU’s hw/misc/arm_sysctl.c with a printf:

static void arm_sysctl_write(void *opaque, hwaddr offset,
                            uint64_t val, unsigned size)
{
    arm_sysctl_state *s = (arm_sysctl_state *)opaque;

    switch (offset) {
    case 0x08: /* LED */
        printf("LED val = %llx\n", (unsigned long long)val);

and then rebuild with:

./build-qemu --arch arm
./build-linux --arch arm --config-fragment linux_config/leds

But beware that one of the LEDs has a heartbeat trigger by default (specified on dts), so it will produce a lot of output.

And then activate it with:

cd /sys/class/leds/versatile:0
cat max_brightness
echo 255 >brightness

Relevant QEMU files:

hw/arm/versatilepb.c
hw/misc/arm_sysctl.c

Relevant kernel files:

arch/arm/boot/dts/versatile-pb.dts
drivers/leds/led-class.c
drivers/leds/leds-sysctl.c