/*
 *  chardev2.c - Create an input/output character device
 */

#include <linux/cdev.h>
#include <linux/delay.h>
#include <linux/device.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/irq.h>
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
#include <linux/poll.h>

#include "chardev.h"
#define SUCCESS 0
#define DEVICE_NAME "char_dev"
#define BUF_LEN 80

/*
 * Is the device open right now? Used to prevent
 * concurent access into the same device
 */
static int Device_Open = 0;

/*
 * The message the device will give when asked
 */
static char Message[BUF_LEN];

/*
 * How far did the process reading the message get?
 * Useful if the message is larger than the size of the
 * buffer we get to fill in device_read.
 */
static char *Message_Ptr;

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

/*
 * This is called whenever a process attempts to open the device file
 */
static int device_open(struct inode *inode, struct file *file)
{
    pr_info("device_open(%p)\n", file);

    /*
     * We don't want to talk to two processes at the same time
     */
    if (Device_Open)
        return -EBUSY;

    Device_Open++;
    /*
     * Initialize the message
     */
    Message_Ptr = Message;
    try_module_get(THIS_MODULE);
    return SUCCESS;
}

static int device_release(struct inode *inode, struct file *file)
{
    pr_info("device_release(%p,%p)\n", inode, file);

    /*
     * We're now ready for our next caller
     */
    Device_Open--;

    module_put(THIS_MODULE);
    return SUCCESS;
}

/*
 * This function is called whenever a process which has already opened the
 * device file attempts to read from it.
 */
static ssize_t device_read(struct file *file,   /* see include/linux/fs.h   */
                           char __user *buffer, /* buffer to be
                                                 * filled with data */
                           size_t length,       /* length of the buffer     */
                           loff_t *offset)
{
    /*
     * Number of bytes actually written to the buffer
     */
    int bytes_read = 0;

    pr_info("device_read(%p,%p,%ld)\n", file, buffer, length);

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

    /*
     * Actually put the data into the buffer
     */
    while (length && *Message_Ptr) {
        /*
         * Because the buffer is in the user data segment,
         * not the kernel data segment, assignment wouldn't
         * work. Instead, we have to use put_user which
         * copies data from the kernel data segment to the
         * user data segment.
         */
        put_user(*(Message_Ptr++), buffer++);
        length--;
        bytes_read++;
    }

    pr_info("Read %d bytes, %ld left\n", bytes_read, length);

    /*
     * Read functions are supposed to return the number
     * of bytes actually inserted into the buffer
     */
    return bytes_read;
}

/*
 * This function is called when somebody tries to
 * write into our device file.
 */
static ssize_t device_write(struct file *file,
                            const char __user *buffer,
                            size_t length,
                            loff_t *offset)
{
    int i;

    pr_info("device_write(%p,%s,%ld)", file, buffer, length);

    for (i = 0; i < length && i < BUF_LEN; i++)
        get_user(Message[i], buffer + i);

    Message_Ptr = Message;

    /*
     * Again, return the number of input characters used
     */
    return i;
}

/*
 * This function is called whenever a process tries to do an ioctl on our
 * device file. We get two extra parameters (additional to the inode and file
 * structures, which all device functions get): the number of the ioctl called
 * and the parameter given to the ioctl function.
 *
 * If the ioctl is write or read/write (meaning output is returned to the
 * calling process), the ioctl call returns the output of this function.
 *
 */
long device_ioctl(struct file *file,      /* ditto */
                  unsigned int ioctl_num, /* number and param for ioctl */
                  unsigned long ioctl_param)
{
    int i;
    char *temp;
    char ch;

    /*
     * Switch according to the ioctl called
     */
    switch (ioctl_num) {
    case IOCTL_SET_MSG:
        /*
         * Receive a pointer to a message (in user space) and set that
         * to be the device's message.  Get the parameter given to
         * ioctl by the process.
         */
        temp = (char *) ioctl_param;

        /*
         * Find the length of the message
         */
        get_user(ch, temp);
        for (i = 0; ch && i < BUF_LEN; i++, temp++)
            get_user(ch, temp);

        device_write(file, (char *) ioctl_param, i, 0);
        break;

    case IOCTL_GET_MSG:
        /*
         * Give the current message to the calling process -
         * the parameter we got is a pointer, fill it.
         */
        i = device_read(file, (char *) ioctl_param, 99, 0);

        /*
         * Put a zero at the end of the buffer, so it will be
         * properly terminated
         */
        put_user('\0', (char *) ioctl_param + i);
        break;

    case IOCTL_GET_NTH_BYTE:
        /*
         * This ioctl is both input (ioctl_param) and
         * output (the return value of this function)
         */
        return Message[ioctl_param];
        break;
    }

    return SUCCESS;
}

/* Module Declarations */

/*
 * This structure will hold the functions to be called
 * when a process does something to the device we
 * created. Since a pointer to this structure is kept in
 * the devices table, it can't be local to
 * init_module. NULL is for unimplemented functions.
 */
struct file_operations Fops = {
    .read = device_read,
    .write = device_write,
    .unlocked_ioctl = device_ioctl,
    .open = device_open,
    .release = device_release, /* a.k.a. close */
};

/*
 * Initialize the module - Register the character device
 */
int init_module()
{
    int ret_val;
    /*
     * Register the character device (atleast try)
     */
    ret_val = register_chrdev(MAJOR_NUM, DEVICE_NAME, &Fops);

    /*
     * Negative values signify an error
     */
    if (ret_val < 0) {
        pr_alert("%s failed with %d\n",
                 "Sorry, registering the character device ", ret_val);
        return ret_val;
    }

    Major = ret_val;

    cls = class_create(THIS_MODULE, DEVICE_FILE_NAME);
    device_create(cls, NULL, MKDEV(Major, MAJOR_NUM), NULL, DEVICE_FILE_NAME);

    pr_info("Device created on /dev/%s\n", DEVICE_FILE_NAME);

    return 0;
}

/*
 * Cleanup - unregister the appropriate file from /proc
 */
void cleanup_module()
{
    device_destroy(cls, MKDEV(Major, 0));
    class_destroy(cls);

    /*
     * Unregister the device
     */
    unregister_chrdev(Major, DEVICE_NAME);
}

MODULE_LICENSE("GPL");