/*
 * 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;

		/*
		 * Actually put the data into the buffer
		 */
		while (length && *msg_Ptr) {

				/*
				 * The buffer is in the user data segment, not the kernel
				 * segment so "*" assignment won't work.  We have to use
				 * put_user which copies data from the kernel data segment to
				 * the user data segment.
				 */
				put_user(*(msg_Ptr++), buffer++);

				length--;
				bytes_read++;
		}

		/*
		 * Most read functions return the number of bytes put into the buffer
		 */
		return bytes_read;
}

/*  
 * Called when a process writes to dev file: echo "hi" > /dev/hello
 */
static ssize_t
device_write(struct file *filp, const char *buff, size_t len, loff_t * off)
{
		printk(KERN_ALERT "Sorry, this operation isn't supported.\n");
		return -EINVAL;
}