Linux Device Drivers

Using Resources

A module can’t accomplish its task without using system resources, such as memory, I/O ports, and interrupt lines, as well as DMA channels if you use the mainboard’s DMA controller.

As a programmer, you are already accustomed to managing memory allocation, and writing kernel code is no different in this regard. Your program obtains a memory area using kmalloc and releases it using kfree. These functions behave like malloc and free, except that kmalloc takes an additional argument, the priority. Most of the time, a priority of GFP_KERNEL will do. The GFP acronym stands for ``Get Free Page.''

Requesting I/O ports and interrupt lines, on the other hand, looks strange at first, because normally a programmer accesses them with explicit instructions in the code, without telling the operating system about it. ``Allocating'' ports and interrupts is different from memory allocation in that memory is allocated from a pool, and every address behaves the same; I/O ports have individual roles, and a driver needs to work with specific ports, not just some ports.

Ports

The job of a typical driver is, for the most part, writing and reading ports. This is true both at initialization time and during normal work. A device driver must be guaranteed exclusive access to its ports in order to prevent interference from other drivers—if a module probing for its hardware should happen to write to ports owned by another device, weird things would undoubtedly happen.

The developers ...