This blog will contain the code snippets that are needed to implement particular feature in kernel. I’ll mainly focus on code snippet rather than the explanation. Again! this blog may not be fully correct. You’re expected to “check the facts/code” before applying. Please correct me, if i’m wrong. I’ll be more happy to accept the changes. This Blog is not beginner friendly.

Kernel MODULE MACRO

Handling module parameters

#include <linux/moduleparam.h>  // No need to include this. 'linux/module.h' included it.

module_param(name, type, perm);
MODULE_PARM_DESC(myarr,"this is my array of int");

inmod module.ko param=value

Char Device

There are 2 way to create char device. Creating the device node through ‘kernel module’ or using mknod

We will go through both way.

Note: you have to populate the strcut file_operations before binding it struct cdev. See example

Kernel Module

TODO: Add header files.


static dev_t device_number;
struct cdev pcd_cdev;
struct class *class_pcd;
struct device *device_pcd;

struct file_operations pcd_fops;

/* Init function */
static int __init func_init(void)
{
	int ret;

	/*1. Dynamically allocate a device number */
	ret = alloc_chrdev_region(&device_number,0,1,"pcd_devices");
	if(ret < 0){
		pr_err("Alloc chrdev failed\n");
		goto out;
	}

	pr_info("Device number <major>:<minor> = %d:%d\n",MAJOR(device_number),MINOR(device_number));

	cdev_init(&pcd_cdev,&pcd_fops);
	pcd_cdev.owner = THIS_MODULE;

	/* 3. Register a device (cdev structure) with VFS */
	ret = cdev_add(&pcd_cdev,device_number,1);
	if(ret < 0){
		pr_err("Cdev add failed\n");
		goto unreg_chrdev;
	}
	/*4. create device class under /sys/class/ */
	class_pcd = class_create(THIS_MODULE,"pcd_class");
	if(IS_ERR(class_pcd)){
		pr_err("Class creation failed\n");
		ret = PTR_ERR(class_pcd);
		goto cdev_del;
	}

	/*5.  populate the sysfs with device information */
	device_pcd = device_create(class_pcd,NULL,device_number,NULL,"pcd");
	if(IS_ERR(device_pcd)){
		pr_err("Device create failed\n");
		ret = PTR_ERR(device_pcd);
		goto class_del;
	}

	pr_info("Module init was successful\n");

	return 0;

class_del:
	class_destroy(class_pcd);
cdev_del:
	cdev_del(&pcd_cdev);	
unreg_chrdev:
	unregister_chrdev_region(device_number,1);
out:
	pr_info("Module insertion failed\n");
	return ret;
}

/* Cleanup function */
static void __exit func_cleanup(void)
{
	device_destroy(class_pcd,device_number);
	class_destroy(class_pcd);
	cdev_del(&pcd_cdev);
	unregister_chrdev_region(device_number,1);
	pr_info("module unloaded\n");
}

Using MKNOD

See example. TODO: Add header files.