Linux Device Drivers, Second Edition

Kernel Timers

The ultimate resources for time keeping in the kernel are the timers. Timers are used to schedule execution of a function (a timer handler) at a particular time in the future. They thus work differently from task queues and tasklets in that you can specify when in the future your function will be called, whereas you can’t tell exactly when a queued task will be executed. On the other hand, kernel timers are similar to task queues in that a function registered in a kernel timer is executed only once—timers aren’t cyclic.

There are times when you need to execute operations detached from any process’s context, like turning off the floppy motor or finishing another lengthy shutdown operation. In that case, delaying the return from close wouldn’t be fair to the application program. Using a task queue would be wasteful, because a queued task must continually reregister itself until the requisite time has passed.

A timer is much easier to use. You register your function once, and the kernel calls it once when the timer expires. Such a functionality is used often within the kernel proper, but it is sometimes needed by the drivers as well, as in the example of the floppy motor.

The kernel timers are organized in a doubly linked list. This means that you can create as many timers as you want. A timer is characterized by its timeout value (in jiffies) and the function to be called when the timer expires. The timer handler receives an argument, which is stored in the ...