Linux Device Drivers, Second Edition

Chapter 6. Flow of Time

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 necessary operations; they have to deal with issues such as timing, memory management, hardware access, and more. Fortunately, the kernel makes a number of facilities available to ease the task of the driver writer. In the next few chapters we’ll fill in information on some of the kernel resources that are available, starting with how timing issues are addressed. Dealing with time involves the following, in order of increasing complexity:

• Understanding kernel timing

• Knowing the current time

• Delaying operation for a specified amount of time

• Scheduling asynchronous functions to happen after a specified time lapse

Time Intervals in the Kernel

The first point we need to cover is the timer interrupt, which is the mechanism the kernel uses to keep track of time intervals. Interrupts are asynchronous events that are usually fired by external hardware; the CPU is interrupted in its current activity and executes special code (the Interrupt Service Routine, or ISR) to serve the interrupt. Interrupts and ISR implementation issues are covered in Chapter 9.

Timer interrupts are generated by the system’s timing hardware at regular intervals; this interval is set by the kernel according to the value of HZ, which is an architecture-dependent value defined in <linux/param.h>. Current Linux versions define HZ to be ...