Timer in kernel modules

Timers are quite straightforward to implement.

They need a couple of libraries to include,

#include <linux/jiffies.h>
#include <linux/timer.h>

a global timer_list structure that holds the timer,

static struct timer_list my_timer;

and a timer callback.

void timer_callback(struct timer_list * data);

The timer should be initialised in the __init function of the module through this function:

timer_setup(timer, callback, flags);

In our example, if we don't need any particular flag, it would look like timer_setup(&my_timer, timer_callback, 0);.

When removing the module from the kernel, we also need to delete the created timer:

del_timer(&my_timer);

The duration of the timer proper is set through the function mod_timer(struct timer_list *timer, unsigned long expires). The expires argument is in jiffies, i.e. the number of ticks since the system booted. <linux/jiffies.h> defines a bunch of handy functions to convert usual time scales into jiffies:

• msecs_to_jiffies(const unsigned int m)
• usecs_to_jiffies(const unsigned int u)
• nsecs_to_jiffies(u64 n) Note that the documentation states that this function "is designed for scheduler, not for use in device drivers to calculate timeout value."

jiffies itself is a global variable that holds the current tick count. For example, jiffies+usecs_to_jiffies(1) would be 1 µs from now.

Once the mod_timer() function has been called, the timer starts and calls its callback function at the end of its period.

Resources

• <linux/timer.h
• <linux/jiffies.h
• Let's code a Linux Driver - 8: Timer in a Linux Kernel Module