Simple Example of a Real-Time Clock (RTC) Linux Driver: A Beginner’s Guide

Real-Time Clock (RTC) Linux drivers are essential for managing system time and maintaining accurate timestamps across reboots and power cycles. This guide provides a straightforward example of an RTC Linux driver, aimed at helping beginners understand the basics of RTC drivers and their implementation in the Linux kernel.

What is an RTC Driver?

An RTC driver in Linux manages the Real-Time Clock hardware, which keeps track of the current time even when the system is powered off. It allows the operating system to read and set the RTC, ensuring that timekeeping functions are accurate and reliable.

Why Implement an RTC Driver?

• Accurate Timekeeping: Ensures system time is preserved during power cycles.
• System Synchronization: Maintains consistent time across reboots.
• Scheduled Tasks: Supports time-based scheduling and event logging.

Simple Example of an RTC Linux Driver

This example demonstrates a basic RTC driver implementation in Linux. It covers the essential components required to create a functional RTC driver.

1. Setting Up the Driver Skeleton

Create a basic driver skeleton with the necessary kernel modules and header files. Save the following code in a file named simple_rtc.c.

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

#define DEVICE_NAME "simple_rtc"

static struct rtc_device *rtc_dev;
static struct class *rtc_class;
static struct device *rtc_device;

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 *pos)
{
    struct rtc_time tm;
    int ret;

    ret = rtc_read_time(rtc_dev, &tm);
    if (ret < 0)
        return ret;

    ret = snprintf(buf, count, "%4d-%02d-%02d %02d:%02d:%02d\n",
                   tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
                   tm.tm_hour, tm.tm_min, tm.tm_sec);

    return ret;
}

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

static int __init simple_rtc_init(void)
{
    int ret;

    ret = register_chrdev(0, DEVICE_NAME, &rtc_fops);
    if (ret < 0) {
        printk(KERN_ERR "Unable to register RTC device\n");
        return ret;
    }

    rtc_class = class_create(THIS_MODULE, "rtc");
    if (IS_ERR(rtc_class)) {
        unregister_chrdev(ret, DEVICE_NAME);
        return PTR_ERR(rtc_class);
    }

    rtc_device = device_create(rtc_class, NULL, MKDEV(ret, 0), NULL, DEVICE_NAME);
    if (IS_ERR(rtc_device)) {
        class_destroy(rtc_class);
        unregister_chrdev(ret, DEVICE_NAME);
        return PTR_ERR(rtc_device);
    }

    rtc_dev = rtc_class_open("rtc0");
    if (IS_ERR(rtc_dev)) {
        device_destroy(rtc_class, MKDEV(ret, 0));
        class_destroy(rtc_class);
        unregister_chrdev(ret, DEVICE_NAME);
        return PTR_ERR(rtc_dev);
    }

    printk(KERN_INFO "Simple RTC driver initialized\n");
    return 0;
}

static void __exit simple_rtc_exit(void)
{
    rtc_class_close(rtc_dev);
    device_destroy(rtc_class, MKDEV(0, 0));
    class_destroy(rtc_class);
    unregister_chrdev(0, DEVICE_NAME);

    printk(KERN_INFO "Simple RTC driver removed\n");
}

module_init(simple_rtc_init);
module_exit(simple_rtc_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("A simple RTC Linux driver example");

2. Building the Driver

Create a Makefile to build the driver:

obj-m += simple_rtc.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

Run make to compile the driver:

make

3. Installing and Testing the Driver

Load the driver using insmod:

sudo insmod simple_rtc.ko

Create the device file:

sudo mknod /dev/simple_rtc c <major_number> 0

Replace <major_number> with the major number assigned to the driver, which you can find using dmesg or cat /proc/devices.

4. Using the Driver

Read the RTC time using the following command:

cat /dev/simple_rtc

You should see the current date and time printed to the console.

5. Cleaning Up

Remove the driver module:

sudo rmmod simple_rtc

Delete the device file:

sudo rm /dev/simple_rtc

Conclusion

This guide provides a simple example of an RTC Linux driver, demonstrating how to implement basic functionalities for managing Real-Time Clock hardware. By following these steps, you can build, test, and understand a basic RTC driver, which is a foundational skill for Linux kernel development.