/*
 * http://linux.die.net/lkmpg/x569.html
 */

/*
 *  chardev.c: Creates a read-only char device that says how many times
 *  you've read from the dev file
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <asm/uaccess.h>        /* for put_user */

/*  
 *  Prototypes - this would normally go in a .h file
 */
int init_module(void);
void cleanup_module(void);
static int device_open(struct inode *, struct file *);
static int device_release(struct inode *, struct file *);
static ssize_t device_read(struct file *, char *, size_t, loff_t *);
static ssize_t device_write(struct file *, const char *, size_t, loff_t *);

#define SUCCESS 0
#define DEVICE_NAME "chardev"   /* Dev name as it appears in /proc/devices   */
#define BUF_LEN 80              /* Max length of the message from the device */

/* 
 * Global variables are declared as static, so are global within the file. 
 */

static int Major;               /* Major number assigned to our device driver */
static int Device_Open = 0;     /* Is device open?  
                                 * Used to prevent multiple access to device */
static char msg[BUF_LEN];       /* The msg the device will give when asked */
static char *msg_Ptr;

static struct file_operations fops = {
        .read = device_read,
        .write = device_write,
        .open = device_open,
        .release = device_release
};

/*
 * This function is called when the module is loaded
 */
int init_module(void)
{
        Major = register_chrdev(0, DEVICE_NAME, &fops);

        if (Major < 0) {
          printk(KERN_ALERT "Registering char device failed with %d\n", Major);
          return Major;
        }

        printk(KERN_INFO "I was assigned major number %d. To talk to\n", Major);
        printk(KERN_INFO "the driver, create a dev file with\n");
        printk(KERN_INFO "'mknod /dev/%s c %d 0'.\n", DEVICE_NAME, Major);
        printk(KERN_INFO "Try various minor numbers. Try to cat and echo to\n");
        printk(KERN_INFO "the device file.\n");
        printk(KERN_INFO "Remove the device file and module when done.\n");

        return SUCCESS;
}

/*
 * This function is called when the module is unloaded
 */
void cleanup_module(void)
{
        /* 
         * Unregister the device 
         */
        unregister_chrdev(Major, DEVICE_NAME);
}

/*
 * Methods
 */

/* 
 * Called when a process tries to open the device file, like
 * "cat /dev/mycharfile"
 */
static int device_open(struct inode *inode, struct file *file)
{
        static int counter = 0;

        if (Device_Open)
                return -EBUSY;

        Device_Open++;
        sprintf(msg, "I already told you %d times Hello world!\n", counter++);
        msg_Ptr = msg;
        try_module_get(THIS_MODULE);

        return SUCCESS;
}

/* 
 * Called when a process closes the device file.
 */
static int device_release(struct inode *inode, struct file *file)
{
        Device_Open--;          /* We're now ready for our next caller */

        /* 
         * Decrement the usage count, or else once you opened the file, you'll
         * never get get rid of the module. 
         */
        module_put(THIS_MODULE);

        return 0;
}

/* 
 * Called when a process, which already opened the dev file, attempts to
 * read from it.
 */
static ssize_t device_read(struct file *filp,   /* see include/linux/fs.h   */
                           char *buffer,        /* buffer to fill with data */
                           size_t length,       /* length of the buffer     */
                           loff_t * offset)
{
        /*
         * Number of bytes actually written to the buffer 
         */
        int bytes_read = 0;

        /*
         * If we're at the end of the message, 
         * return 0 signifying end of file 
         */
        if (*msg_Ptr == 0)
                return 0;

        /*