Linux Kernel Development 1.0

Some things before you start:-

• check you kernel version => uname -r
• you will get an version name and local version. e.g.... (3.2.0-4-amd64)

• then search for your source tree ... it should be in /usr/src/ ... if not found there
• download you Linux source tree with exact version . in my case it will be Linux 3.2.0. Google it and you will get a zip file. Extract it under the folder /usr/src/

• now you need to run some commands
• first go into the kernel source directory then follow these steps 
• make menuconfig (if you want to customise your kernel)
• make -j<number of cpus>
• make install 
• make modules_install
• Now your kernel is ready

For more digging on above section go to "http://unix.stackexchange.com/questions/20864/what-happens-in-each-step-of-the-linux-kernel-building-process" . This will help you.

Here it starts:-

• You need two things first is your driver code(c file) and next is Makefile.
• This is a hello world program for kernel. Save it in hello.c
  #include <linux/module.h>
  #include <linux/kernel.h>
  
  int init_module(void)       //this function runs when your module is loaded
  {
     printk("Hello world");
     return 0;
  }
  void cleanup_module(void)   //this function runs when your module is unloaded
  {
    printk("Goodbye world \n");
  } 
  MODULE_LICENSE("GPL");  //compulsory part in every driver code

• Now in Makefile .... be sure to name the file as Makefile (capital M don't forget):-
  obj-m := hello.o           //same as your module name with an .o extension

• Something you must not do:-
• compile the program to get (.o)  . Makefile will take care of that.
• Now write this command on the terminal.
  make -C /usr/src/"your Linux version" M=`pwd` modules

• If every thing goes write then you will see 6 files generated . You can inspect all of them and dig them vigorously . The file which is of your use is the one with .ko extension. it is a dynamic loadable module. go and read about it !! 
• Since you have a dynamic module its time you load it !! Here is the command 
• insmod <dynamic module>
• you might require root privileges so use sudo 
• open dmesg and check whether all you wrote in the printk is there or not. you can use grep to find it . dmesg is the kernel ring buffer which gives us the details provided by the kernel messages.
• Since you have loaded it now its time to remove it.
• rmmod <dynamic module>
• This is it !! you have created a kernel module. 
• Amount of efforts put in to write a hello world  for kernel is high but if you love hardware then this is how it is going to be !!