onefinity-firmware/docs/cross_compile_rpi_kernel_module.md

Install the cross compiler

sudo apt-get update
sudo apt-get install -y gcc-arm-linux-gnueabihf

Determine the correct kernel version on the target pi

dpkg-query -l raspberrypi-kernel

Get the kernel source for the correct kernel version

wget https://github.com/raspberrypi/linux/archive/raspberrypi-kernel_1.20171029-1.tar.gz
tar xf raspberrypi-kernel_1.20171029-1.tar.gz
cd linux-raspberrypi-kernel_1.20171029-1

Prep the kernel source

KERNEL=kernel7
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- bcm2709_defconfig
make -j 8 ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- modules_prepare

Create hello.c module

#include <linux/init.h>
#include <linux/module.h>

MODULE_LICENSE("GPL");

static int hello_init(void) {
  printk(KERN_ALERT "Hello world!\n");
  return 0;
}

static void hello_exit(void) {
  printk(KERN_ALERT "Goodbye cruel world!\n");
}

module_init(hello_init);
module_exit(hello_exit);

Create a Makefile

CROSS := arm-linux-gnueabihf-
DIR:=$(shell dirname $(realpath $(lastword $(MAKEFILE_LIST))))
obj-m := hello.o

all:
    $(MAKE) ARCH=arm CROSS_COMPILE=$(CROSS) -C kernel M=$(DIR) modules

clean:
    $(MAKE) ARCH=arm CROSS_COMPILE=$(CROSS) -C kernel M=$(DIR) clean

Compile the module

ln -sf path/to/rpi-kernel kernel
make

Copy module to rpi

scp hello.ko pi.local:

Load it on the pi

sudo insmod hello.ko

Look for message in syslog

tail /var/log/syslog