Two common methods and steps of character device registration

static int __init RK_irq_init(void)
{
    
	/* 1、 Build device number  */
	if (RK_irq.major) {
    
		RK_irq.devid = MKDEV(RK_irq.major, 0);
		register_chrdev_region(RK_irq.devid, RK_IRQ_CNT, RK_IRQ_NAME);
	} else {
    
		alloc_chrdev_region(&RK_irq.devid, 0, RK_IRQ_CNT, RK_IRQ_NAME);
		RK_irq.major = MAJOR(RK_irq.devid);
		RK_irq.minor = MINOR(RK_irq.devid);
	}

	/* 2、 Register character device  */
	cdev_init(&RK_irq.cdev, &RK_irq_fops);
	cdev_add(&RK_irq.cdev, RK_irq.devid, RK_IRQ_CNT);

	/* 3、 Create a class  */
	RK_irq.class = class_create(THIS_MODULE, RK_IRQ_NAME);
	if (IS_ERR(RK_irq.class)) {
    	
		return PTR_ERR(RK_irq.class);
	}

	/* 4、 Create device  */
	RK_irq.device = device_create(RK_irq.class, NULL, RK_irq.devid, NULL, RK_IRQ_NAME);
	if (IS_ERR(RK_irq.device)) {
    
		return PTR_ERR(RK_irq.device);
	}
		
	/* 5、 Start button  */
	atomic_set(&RK_irq.keyvalue, INVAKEY);
	atomic_set(&RK_irq.releasekey, 0);
	keyio_init();
	return 0;
}