Linux Device Drivers

Chapter 8. Hardware Management

While playing with scull and similar toys can be a pleasant way to become familiar with the software interface of a Linux device driver, testing a real device requires hardware. The driver is the abstraction layer between software concepts and hardware circuitry; as such, it needs to talk with both of them. Up to now, we have examined the internals of software concepts; this chapter should complete the picture by showing you how a driver can access I/O ports and I/O memory, while being portable across Linux platforms.

As usual, I won’t bind the sample code to a particular device. However, we can no longer use a memory-based device like scull. Instead, the examples in this chapter use the parallel port to show I/O instructions and the standard video buffer of text-mode VGA boards to show memory-mapped I/O.

I chose the parallel port because it offers direct input and output of several bits of information. Data bits written to the device appear on the output pins, and voltage levels on the input pins are directly accessible by the processor. In practice, you have to connect LEDs to the port to actually see the results of an I/O operation. The parallel port is easy to program, much easier than the serial port, and almost every computer (even the Alpha) has a parallel port that works like the one in the PC.

As far as memory-mapped I/O is concerned, text-mode VGA is the most standardized memory-mapped device, and almost every computer has a VGA-compatible ...