Simple example of Real Time Clock ( RTC ) Linux Driver

A simple RTC driver that reads the time from the CMOS (Real Time Clock) registers.

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/uaccess.h>
#include <linux/rtc.h>

MODULE_LICENSE("GPL");

static dev_t rtc_dev;
static struct cdev rtc_cdev;

static int rtc_open(struct inode *inode, struct file *file)
{
    return 0;
}

static ssize_t rtc_read(struct file *file, char __user *buf, size_t count, loff_t *offset)
{
    struct rtc_time tm;
    rtc_time_to_tm(get_seconds(), &tm);

    if (copy_to_user(buf, &tm, sizeof(struct rtc_time)))
    {
        pr_err("Failed to copy data to user\n");
        return -EFAULT;
    }

    return sizeof(struct rtc_time);
}

static const struct file_operations rtc_fops = {
    .owner = THIS_MODULE,
    .open = rtc_open,
    .read = rtc_read,
};

static int __init rtc_init(void)
{
    if (alloc_chrdev_region(&rtc_dev, 0, 1, "rtc-cmos") < 0)
    {
        pr_err("Failed to allocate character device region\n");
        return -1;
    }

    cdev_init(&rtc_cdev, &rtc_fops);
    rtc_cdev.owner = THIS_MODULE;

    if (cdev_add(&rtc_cdev, rtc_dev, 1) < 0)
    {
        pr_err("Failed to add character device\n");
        unregister_chrdev_region(rtc_dev, 1);
        return -1;
    }

    pr_info("RTC CMOS Driver loaded\n");
    return 0;
}

static void __exit rtc_exit(void)
{
    cdev_del(&rtc_cdev);
    unregister_chrdev_region(rtc_dev, 1);
    pr_info("RTC CMOS Driver unloaded\n");
}

module_init(rtc_init);
module_exit(rtc_exit);

This example initializes a simple character device using the cdev structure. The rtc_read function reads the current system time using the get_seconds function and converts it to an rtc_time structure, which is then copied to the user.