/*
    List of printk Log Levels:
    Log Level   | Macro	      | Priority Code	|  Purpose
    0	        | KERN_EMERG  | <0>	            |  System is unstable (panic-level message).
    1	        | KERN_ALERT  | <1>	            |  Immediate action required.
    2	        | KERN_CRIT	  | <2>	            |  Critical errors (serious issues).
    3	        | KERN_ERR	  | <3>	            |  General errors (non-critical).
    4	        | KERN_WARNING|	<4>	            |  Warnings (might cause issues).
    5	        | KERN_NOTICE |	<5>	            |  Normal but notable conditions.
    6	        | KERN_INFO	  | <6>	            |  Informational messages (default for general logs).
    7	        | KERN_DEBUG  | <7>	            |  Debugging messages (used for development).
*/
#include <linux/module.h>
#include <linux/fs.h>
#include <asm/uaccess.h>
#include <linux/init.h>
#include <linux/cdev.h>
#include <linux/sched.h>
#include <linux/errno.h>
#include <asm/current.h>
#include <linux/uaccess.h>
#include <linux/slab.h>

#define CHAR_DEV_NAME "demo_cdev"
#define MAX_LENGTH 4000
#define SUCCESS 0

static char *char_device_buf;
struct cdev *demo_cdev;
dev_t mydev;    //device tree as an instance
                //We are using this because all character
                //drivers belongs to device tree.

static int char_dev_open(struct inode *inode,
                            struct file *file){

    printk(KERN_INFO "chr_drv_dynamic :: open operation invoked \n");
    return SUCCESS;
}

static int char_dev_release(struct inode *inode,
                            struct file *file){

    printk(KERN_INFO "chr_drv_dynamic :: release opration invoked\n");
    return SUCCESS;
}

static ssize_t char_dev_read(struct file *file,
                            char *buf,
                            size_t lbuf,
                            loff_t *ppos){

    printk(KERN_INFO "chr_dev_dynamic :: read operation invoked\n");
    return 0;
}

static ssize_t char_dev_write(struct file *file,
                            const char *buf,
                            size_t lbuf,
                            loff_t *ppos){
    
    printk(KERN_INFO "chr_drv_dynamic :: write operation invoked \n");
    return lbuf;
}


/* This syntax is called "Designated initialization"*/
/* This is allowed in C after C99*/
static struct file_operations char_dev_fops = {
    .owner = THIS_MODULE,       //To define the ownership of this struct
    .read = char_dev_read,      //When file is read
    .write = char_dev_write,    //When file is written on
    .open = char_dev_open,      //When file is open
    .release = char_dev_release //When file is closed
};

static int __init char_dev_init(void){
    int ret, count = 1;

    //this alloc_chrdev_region will acquire
    //major number dynamically.
    if(alloc_chrdev_region(&mydev, 0, count, CHAR_DEV_NAME) < 0){
        printk(KERN_ERR "failed to reserve major/minor range\n");
        return -1;
    }

    //cdev_alloc function is used to allocate
    //a character device.
    if( !(demo_cdev = cdev_alloc())){
        printk(KERN_ERR "cdev_alloc() failed\n");
        unregister_chrdev_region(mydev, count);
        return -1;
    }
    //It is used to initialize one 
    //character device allocated.
    //Also to mention the capabilities
    //of this character device, we also
    //mention char_dev_fops.
    cdev_init(demo_cdev, &char_dev_fops);

    //cdev_add is used to add character device
    //to character device list.
    ret = cdev_add(demo_cdev, mydev, count);
    if( ret < 0 ){
        printk(KERN_INFO "Error registering device driver\n");
        cdev_del( demo_cdev );
        unregister_chrdev_region(mydev, count);
        return -1;
    }
    printk(KERN_INFO"\nDevice Registered: %s\n", CHAR_DEV_NAME);
    printk(KERN_INFO"Major number = %d, Minor number = %d\n", MAJOR (mydev), MINOR (mydev));
    
    char_device_buf = (char *)kmalloc(MAX_LENGTH, GFP_KERNEL);
    return 0;
}


static void __exit char_dev_exit(void){
    cdev_del(demo_cdev);
    unregister_chrdev_region(mydev,1);
    kfree(char_device_buf);
    printk(KERN_INFO "\n Driver unregistered \n");
}

module_init(char_dev_init);
module_exit(char_dev_exit);

MODULE_AUTHOR("Junet Hossain");
MODULE_DESCRIPTION("Character Device Driver - Test");
MODULE_LICENSE("GPL");