reference - https://wiki.kldp.org/KoreanDoc/html/EmbeddedKernel-KLDP/device-understanding.html

hddinfo.c

/* hddinfo.c */
#ifndef __KERNEL__
#define __KERNEL__
#endif
#ifndef MODULE
#define MODULE
#endif

#define __NO_VERSION__
#include <linux/module.h>
#include <linux/version.h>
#include <linux/errno.h>
#include <linux/fs.h>
#include <linux/ide.h>
#include <asm/uaccess.h>

#define HDA			0x0300
#define HARDDISK	HDA

char kernel_version[] = UTS_RELEASE;

static int hddinfo_open(struct inode *node, struct file *f)
{
	return 0;
}

static int hddinfo_release(struct inode *node, struct file *f)
{
	return 0;
}

static ssize_t hddinfo_read(struct file *f, char *buf, size_t nbytes, loff_t *ppos)
{
	return nbytes;
}

static ssize_t read_serial(char *dst)
{
	ide_drive_t *drv;

	drv = get_info_ptr(HARDDISK);
	if (drv)
		copy_to_user(dst, drv->id->serial_no, 20);
	else
	{
		;//PDEBUG("HDDINFO : Cannot get the drive information\n");
		return 0;
	}

	return 20;
}

int hddinfo_ioctl(struct inode *node, struct file *f, unsigned int ioctl_num, unsigned long ioctl_param)
{
	switch (ioctl_num)
	{
		case 0 :
			read_serial((char *)(ioctl_param));
			break;
	}

	return 0;
}

struct file_operations Fops = {
	NULL,
	NULL,
	hddinfo_read,
	NULL,
	NULL,
	NULL,
	hddinfo_ioctl,
	NULL,
	hddinfo_open,
	NULL,
	hddinfo_release
};

int init_module()
{
	if (register_chrdev(212, "hddinfo", &Fops) < 0)
	{
		//PDEBUG("HDDINFO : Unable to register driver.\n");
		return -EIO;
	}

	return 0;
}

void cleanup_module()
{
	if (unregister_chrdev(212, "hddinfo") < 0)
		;//PDEBUG("HDDINFO : Unable to unregister\n");
}