Linux Device Drivers, Second Edition

Delaying Execution

Device drivers often need to delay the execution of a particular piece of code for a period of time—usually to allow the hardware to accomplish some task. In this section we cover a number of different techniques for achieving delays. The circumstances of each situation determine which technique is best to use; we’ll go over them all and point out the advantages and disadvantages of each.

One important thing to consider is whether the length of the needed delay is longer than one clock tick. Longer delays can make use of the system clock; shorter delays typically must be implemented with software loops.

Long Delays

If you want to delay execution by a multiple of the clock tick or you don’t require strict precision (for example, if you want to delay an integer number of seconds), the easiest implementation (and the most braindead) is the following, also known as busy waiting:

 unsigned long j = jiffies + jit_delay * HZ;

 while (jiffies < j)
     /* nothing */;

This kind of implementation should definitely be avoided. We show it here because on occasion you might want to run this code to understand better the internals of other code.

So let’s look at how this code works. The loop is guaranteed to work because jiffies is declared as volatile by the kernel headers and therefore is reread any time some C code accesses it. Though “correct,” this busy loop completely locks the processor for the duration of the delay; the scheduler never interrupts a process that ...