Device Driver

Prepare the System for Building LKMs

The system must be prepared to build kernel code, and to do this you must have the Linux headers installed on your device. On a typical Linux desktop machine you can use your package manager to locate the correct package to install. For example, under 64-bit Debian you can use:

molloyd@DebianJessieVM:~$ sudo apt-get update

molloyd@DebianJessieVM:~$ apt-cache search linux-headers-$(uname -r)

linux-headers-3.16.0-4-amd64 - Header files for Linux 3.16.0-4-amd64

molloyd@DebianJessieVM:~$ sudo apt-get install linux-headers-3.16.0-4-amd64

molloyd@DebianJessieVM:~$ cd /usr/src/linux-headers-3.16.0-4-amd64/

molloyd@DebianJessieVM:/usr/src/linux-headers-3.16.0-4-amd64$ ls

arch include Makefile Module.symvers scripts

Hello World module

hello_world.c:

#include <linux/init.h> // Macros used to mark up functions e.g., __init __exit#include <linux/module.h> // Core header for loading LKMs into the kernel#include <linux/kernel.h> // Contains types, macros, functions for the kernel MODULE_LICENSE("GPL"); ///< The license type -- this affects runtime behaviorMODULE_AUTHOR("Derek Molloy"); ///< The author -- visible when you use modinfoMODULE_DESCRIPTION("A simple Linux driver for the BBB."); ///< The description -- see modinfoMODULE_VERSION("0.1"); ///< The version of the module static char *name = "world"; ///< An example LKM argument -- default value is "world"module_param(name, charp, S_IRUGO); ///< Param desc. charp = char ptr, S_IRUGO can be read/not changedMODULE_PARM_DESC(name, "The name to display in /var/log/kern.log"); ///< parameter description /** @brief The LKM initialization function* The static keyword restricts the visibility of the function to within this C file. The __init* macro means that for a built-in driver (not a LKM) the function is only used at initialization* time and that it can be discarded and its memory freed up after that point.* @return returns 0 if successful*/static int __init helloBBB_init(void){ printk(KERN_INFO "EBB: Hello %s from the BBB LKM!\n", name); return 0;} /** @brief The LKM cleanup function* Similar to the initialization function, it is static. The __exit macro notifies that if this* code is used for a built-in driver (not a LKM) that this function is not required.*/static void __exit helloBBB_exit(void){ printk(KERN_INFO "EBB: Goodbye %s from the BBB LKM!\n", name);} /** @brief A module must use the module_init() module_exit() macros from linux/init.h, which* identify the initialization function at insertion time and the cleanup function (as* listed above)*/module_init(helloBBB_init);module_exit(helloBBB_exit);

Makefile

obj-m += helloworld.o
all: make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean: make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

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