Linux Device Drivers, Second Edition

Using Resources

A module can’t accomplish its task without using system resources such as memory, I/O ports, I/O memory, and interrupt lines, as well as DMA channels if you use old-fashioned DMA controllers like the Industry Standard Architecture (ISA) one.

As a programmer, you are already accustomed to managing memory allocation; 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. Usually, a priority of GFP_KERNEL or GFP_USER will do. The GFP acronym stands for “get free page.” (Memory allocation is covered in detail in Chapter 7.)

Beginning driver programmers may initially be surprised at the need to allocate I/O ports, I/O memory,^[11] and interrupt lines explicitly. After all, it is possible for a kernel module to simply access these resources without telling the operating system about it. Although system memory is anonymous and may be allocated from anywhere, I/O memory, ports, and interrupts have very specific roles. For instance, a driver needs to be able to allocate the exact ports it needs, not just some ports. But drivers cannot just go about making use of these system resources without first ensuring that they are not already in use elsewhere.

I/O Ports and I/O Memory

The job of a typical driver is, for the most part, writing and reading I/O ports and I/O memory. Access ...