How to do it...

Let's see how to do it by following these steps:

1. First of all, we have to add our new chrdev_fasync() method in the struct file_operations of our driver:

static const struct file_operations chrdev_fops = {
    .owner   = THIS_MODULE,
    .fasync  = chrdev_fasync,
    .poll    = chrdev_poll,
    .llseek  = no_llseek,
    .read    = chrdev_read,
    .open    = chrdev_open,
    .release = chrdev_release
};

2. The implementation is as follows:

static int chrdev_fasync(int fd, struct file *filp, int on)
{
    struct chrdev_device *chrdev = filp->private_data;

    return fasync_helper(fd, filp, on, &chrdev->fasync_queue);
}

3. Finally, we have to add the kill_fasync() call into our (simulated) interrupt handler to signal that the signal SIGIO can be sent due to the fact that new data is ready to be read: 

static enum hrtimer_restart chrdev_timer_handler(struct hrtimer *ptr)
{
    struct chrdev_device *chrdev = container_of(ptr,
                                    struct chrdev_device, timer);

...
    /* Wake up any possible sleeping process */
    wake_up_interruptible(&chrdev->queue);
    kill_fasync(&chrdev->fasync_queue, SIGIO, POLL_IN);

    /* Now forward the expiration time and ask to be rescheduled */
    hrtimer_forward_now(&chrdev->timer, ns_to_ktime(delay_ns));
    return HRTIMER_RESTART;
}