Linux Device Drivers, 3rd Edition

Chapter 7. Time, Delays, and Deferred Work

At this point, we know the basics of how to write a full-featured char module. Real-world drivers, however, need to do more than implement the operations that control a device; they have to deal with issues such as timing, memory management, hardware access, and more. Fortunately, the kernel exports a number of facilities to ease the task of the driver writer. In the next few chapters, we’ll describe some of the kernel resources you can use. This chapter leads the way by describing how timing issues are addressed. Dealing with time involves the following tasks, in order of increasing complexity:

• Measuring time lapses and comparing times

• Knowing the current time

• Delaying operation for a specified amount of time

• Scheduling asynchronous functions to happen at a later time

Measuring Time Lapses

The kernel keeps track of the flow of time by means of timer interrupts. Interrupts are covered in detail in Chapter 10.

Timer interrupts are generated by the system’s timing hardware at regular intervals; this interval is programmed at boot time by the kernel according to the value of HZ, which is an architecture-dependent value defined in <linux/param.h> or a subplatform file included by it. Default values in the distributed kernel source range from 50 to 1200 ticks per second on real hardware, down to 24 for software simulators. Most platforms run at 100 or 1000 interrupts per second; the popular x86 PC defaults to 1000, although it used to be 100 ...