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

#include "chardev.h"

#define SUCCESS		0
#define DEVICE_NAME	"chardev"
#define BUF_LEN		80

static int Major;
static int opened = 0;
static char msg[BUF_LEN];
static char* msg_ptr;

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

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 to cat and echo to the device file.\n");
	printk(KERN_INFO "Remove the device file and module when done.\n");

	return SUCCESS;
}

void cleanup_module(void)
{
	unregister_chrdev(Major, DEVICE_NAME);
}

static int device_open(struct inode* inode, struct file* filp)
{
	static int counter = 0;

	if (opened)
		return -EBUSY;

	opened++;
	if (counter++ == 0)
		sprintf(msg, "Come on, write something here :D\n");

	msg_ptr = msg;
	try_module_get(THIS_MODULE);

	return SUCCESS;
}

static int device_release(struct inode* inode, struct file* filp)
{
	opened--;

	module_put(THIS_MODULE);

	return 0;
}

static ssize_t device_read(struct file* filp, char* buffer, size_t len, loff_t* off)
{
	int bytes = 0;

	if (*msg_ptr == 0)
		return 0;

	while (len-- && *msg_ptr) {
		put_user(*msg_ptr++, buffer++);
		bytes++;
	}

	return bytes;
}

static ssize_t device_write(struct file* filp, const char* buff, size_t len, loff_t* off)
{
	int bytes = 0;

	if (len > BUF_LEN) {
		printk(KERN_ALERT "Error: dude you tried to overflow my buffer!\n");
		return -ENOBUFS;
	}

	while (len--) {
		get_user(msg[bytes], buff++);
		bytes++;
	}
	msg[bytes] = '\0';

	msg_ptr = msg;
	return bytes;
}