Although some devices can be controlled using nothing but their I/O regions, most real devices are a bit more complicated than that. Devices have to deal with the external world, which often includes things such as spinning disks, moving tape, wires to distant places, and so on. Much has to be done in a time frame that is different from, and far slower than, that of the processor. Since it is almost always undesirable to have the processor wait on external events, there must be a way for a device to let the processor know when something has happened.
That way, of course, is interrupts. An interrupt is simply a signal that the hardware can send when it wants the processor's attention. Linux handles interrupts in much the same way that it handles signals in user space. For the most part, a driver need only register a handler for its device's interrupts, and handle them properly when they arrive. Of course, underneath that simple picture there is some complexity; in particular, interrupt handlers are somewhat limited in the actions they can perform as a result of how they are run.
It is difficult to demonstrate the use of interrupts without a real hardware device to generate them. Thus, the sample code used in this chapter works with the parallel port. Such ports are starting to become scarce on modern hardware, but, with luck, most people are still able to get their hands on a system with an available port. We'll be working with the short module from the previous chapter; with some small additions it can generate and handle interrupts from the parallel port. The module's name, short, actually means short int (it is C, isn't it?), to remind us that it handles interrupts.
Before we get into the topic, however, it is time for one cautionary note. Interrupt handlers, by their nature, run concurrently with other code. Thus, they inevitably raise issues of concurrency and contention for data structures and hardware. If you succumbed to the temptation to pass over the discussion in Chapter 5, we understand. But we also recommend that you turn back and have another look now. A solid understanding of concurrency control techniques is vital when working with interrupts.