Our simple kernel timer module – code view 1

Without further ado, let's dive into the code of a simple kernel timer, written using the Loadable Kernel Module (LKM) framework (this can be found at ch5/timer_simple). As with most drivers, we keep a context or private data structure containing the information required while running; here, we call it st_ctx. We instantiate it as the ctx variable. We also specify the time to expire (as 420 ms) in a global named exp_ms:

// ch5/timer_simple/timer_simple.c
#include <linux/timer.h>
[ ... ]
static struct st_ctx {
    struct timer_list tmr;
    int data;
} ctx;
static unsigned long exp_ms = 420;

Now, let's check out the first portion of our init code:

static int __init timer_simple_init(void)
{
    ctx.data = INITIAL_VALUE;

    /* Initialize our kernel timer */
    ctx.tmr.expires = jiffies + msecs_to_jiffies(exp_ms);
    ctx.tmr.flags = 0;
    timer_setup(&ctx.tmr, ding, 0);

This is pretty straightforward. First...