/*
 *  iir.c: Creates a read-only char device that always gives you a random number
 *         iir stands for: infinite improbable randomness
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/string.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 "iir"	/* Dev name as it appears in /proc/devices   */

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

static int Major;		/* Major number assigned to our device driver */

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 "infinite improbable randomness loaded (major number %d)\n", Major);

	return SUCCESS;
}

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

	printk(KERN_INFO "infinite improbable randomness unloaded\n");
}

/*
 * 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)
{
	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)
{
	/* 
	 * 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)
{
	/* 
	 * Actually put the data into the buffer 
	 */
  size_t read = 0;
	while (length) {

		/* 
		 * 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('9', buffer++);

		length--;
    read++;
	}

	/* 
	 * Most read functions return the number of bytes put into the buffer
	 */
	return 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)
{
  if(!strncmp(buff, "are you really that random?", len)) { 
    printk(KERN_INFO "That's the problem with randomness, you can never be sure.\n");
    return len;
  }
	return -EINVAL;
}