Kernel Module 1 Hello World - FrankBau/meta-marsboard-bsp GitHub Wiki

My module version 1: "Hello world"

Kernel modules are mostly written in C, but the usual library functions like malloc, fopen, close, must not be used because their implementation is targeted to user space.

A working yocto development environment should be installed and the kernel already built (bitbake virtual/kernel).

Create a new, empty folder my_module and remember the absolute path to this folder, something like /home/user/my_module.

In that folder create two files:

a source code file my_module.c
a Makefile

my_module.c

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

static int __init my_init(void)
{
    printk(KERN_INFO "hello, my module\n");
    return 0;
}

static void __exit my_exit(void)
{
    printk(KERN_INFO "good bye, my module\n" );
}

module_init(my_init);
module_exit(my_exit);

MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("F.B.");
MODULE_DESCRIPTION("My First Driver");

Makefile

This can be a one liner:

obj-m += my_module.o

Build module

On the build host, setup yocto build environmentd and open a kernel development shell:

$ . ./setup-environment build
$ bitbake -c devshell virtual/kernel

The last command opens a new shell. This is the kernel development shell (devshell) which is used for the next steps. The kernel development shell has a number of environment variables set for cross-compilation. You can test this by invoking the gcc cross-compiler for the board:

root:kernel-source$ arm-poky-linux-gnueabi-gcc --version
arm-poky-linux-gnueabi-gcc (GCC) 4.9.2
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

The module is now built "out-of-tree" in its own folder, say /home/user/my_module as created above. But, make is executed in the kernel source directory, so don't change the current devshell directory!

root:kernel-source$ make M=/home/user/my_module modules

The make command generates several files in the module directory. The resulting kernel module is my_module.ko (.ko = "kernel object").

Test the kernel module

Copy my_module.ko to the target board, see Exchanging files with MarS-Board.

On the target board, load the module into the currently running Linux kernel space. Useful commands:

Kernel Module 2 - Char Device shows how to communicate with the kernel module from a user mode process.