/*
 * http://linux.die.net/lkmpg/x569.html
 * http://appusajeev.wordpress.com/2011/06/18/writing-a-linux-character-device-driver/
 */

/*
 *  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 "cdev_rw"    /* 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_opened = 0;    /* Is device open?
				 * Used to prevent multiple access to device */
static char my_data[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);

    printk(KERN_INFO "Unregistering char device");
}

/*
 * 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) {
    if (device_opened)
        return -EBUSY;

    device_opened++;
    msg_ptr = my_data;

    try_module_get(THIS_MODULE);
    printk(KERN_INFO "cdev->device_opened");
    return SUCCESS;
}

/*
 * Called when a process closes the device file.
 */
static int device_release(struct inode *inode, struct file *file) {
    device_opened--;        /* 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);
    printk(KERN_INFO "cdev->device_release");
    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 len,    /* 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 (len && *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++);

        len--;
        bytes_read++;
    }

    printk(KERN_INFO "cdev->device_read: %s", my_data);

    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) {
    short idx = 0;
    while (idx < len) {
        my_data[idx] = buff[idx]; //copy the given string to the driver but in reverse
        idx++;
    }
    my_data[idx] = '\0';
    printk(KERN_INFO "cdev->device_write: %s", my_data);
    return len;
}