Using hrtimers in the Linux Kernel

Question

After searching online and on Stackoverflow for a great deal of time, I have come to realize there are not a lot of concrete examples of using hrtimers in the Linux Kernel. Any example I have found is vague and does not explain the functionality of their program or does not explain how the hrtimers are working well enough for me to understand.

I know there is documentation at /include/linux/hrtimer.h, but that documentation is not clear and seems to assume I am already familiar with them.

Can anyone give a basic example of using this timer?

Answer 1

Simple example, callback every 100ms:

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/hrtimer.h>
#include <linux/ktime.h>

static struct hrtimer test_hrtimer;
static u64 sampling_period_ms = 100; // 100ms
static u32 loop = 0;

static enum hrtimer_restart test_hrtimer_handler(struct hrtimer *timer)
{
    pr_info("test_hrtimer_handler: %u\n", ++loop);
    hrtimer_forward_now(&test_hrtimer, ms_to_ktime(sampling_period_ms));
    return HRTIMER_RESTART;
}

static int __init test_hrtimer_init(void)
{
    hrtimer_init(&test_hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
    test_hrtimer.function = &test_hrtimer_handler;
    hrtimer_start(&test_hrtimer, ms_to_ktime(sampling_period_ms), HRTIMER_MODE_REL);
    pr_info("init test_hrtimer.\n");

    return 0;
}

static void __exit test_hrtimer_exit(void)
{
    hrtimer_cancel(&test_hrtimer );
    pr_info("exit test_hrtimer.\n");
    return;
}

module_init(test_hrtimer_init);
module_exit(test_hrtimer_exit);

MODULE_LICENSE("GPL");