example of kernel module: one-shot timer

timer_kern_module.c

// ****************************************
// timer_kern_module.c
// based on: https://windhole.booth.pm/items/1169009
// ****************************************

#include <linux/module.h>
#include <linux/debugfs.h>

MODULE_LICENSE("GPL v2");
MODULE_AUTHOR("John Doe");
MODULE_DESCRIPTION("A simple example of interval timer");

static struct dentry *topdir;
static struct dentry *testfile;

// buffer for reading/writing data on kernel space
static char testbuf[128];

// one-shot timer
struct timer_list mytimer;

static DEFINE_MUTEX(mytimer_mutex);

// timeout[ms]
static unsigned long mytimer_timeout_msecs = 1000 * 1000;

// display message on timeout
static void mytimer_fn(struct timer_list *timer)
{
    printk(KERN_ALERT "%lu secs passed.\n", mytimer_timeout_msecs / 1000);
}

// show remaining time
static ssize_t mytimer_remain_msecs_read(struct file *f, char __user *buf, size_t len, loff_t *ppos)
{
    unsigned long diff_msecs, now = jiffies;

    mutex_lock(&mytimer_mutex);
    if (time_after(mytimer.expires, now)) {
        diff_msecs = (mytimer.expires - now) * 1000 / HZ;
    } else {
        diff_msecs = 0;
    }
    mutex_unlock(&mytimer_mutex);

    snprintf(testbuf, sizeof(testbuf), "%lu\n", diff_msecs);

    return simple_read_from_buffer(buf, len, ppos, testbuf, strlen(testbuf));
}

// change timeout[ms]
static ssize_t mytimer_remain_msecs_write(struct file *f, const char __user *buf, size_t len, loff_t *ppos)
{
    ssize_t ret;

    mutex_lock(&mytimer_mutex);
    ret = simple_write_to_buffer(testbuf, sizeof(testbuf), ppos, buf, len);
    if (ret < 0) {
        mutex_unlock(&mytimer_mutex);
        return ret;
    }

    sscanf(testbuf, "%20lu", &mytimer_timeout_msecs);
    mod_timer(&mytimer, jiffies + mytimer_timeout_msecs * HZ / 1000);
    mutex_unlock(&mytimer_mutex);

    return ret;
}

// debugfs interfaces
// read remaining time: $ sudo cat /sys/kernel/debug/mytimer/remain_msecs
// change timeout:      $ sudo echo [timeout] > /sys/kernel/debug/mytimer/remain_msecs
static struct file_operations test_fops = {
    .owner = THIS_MODULE,
    .read  = mytimer_remain_msecs_read,
    .write = mytimer_remain_msecs_write,
};

static int mymodule_init(void)
{
    timer_setup(&mytimer, mytimer_fn, 0);
    mytimer.expires = jiffies + mytimer_timeout_msecs * HZ / 1000;
    add_timer(&mytimer);

    topdir = debugfs_create_dir("mytimer", NULL);
    if (!topdir) {
        return -ENOMEM;
    }

    testfile = debugfs_create_file("remain_msecs", 0660, topdir, NULL, &test_fops);
    if (!testfile) {
        return -ENOMEM;
    }

    return 0;
}

static void mymodule_exit(void)
{
    debugfs_remove_recursive(topdir);
    del_timer(&mytimer);
}

module_init(mymodule_init);
module_exit(mymodule_exit);