Chapter 15. Memory Mapping and DMA

This chapter delves into the area of Linux memory management, with an emphasis on techniques that are useful to the device driver writer. Many types of driver programming require some understanding of how the virtual memory subsystem works; the material we cover in this chapter comes in handy more than once as we get into some of the more complex and performance-critical subsystems. The virtual memory subsystem is also a highly interesting part of the core Linux kernel and, therefore, it merits a look.

The material in this chapter is divided into three sections:

• The first covers the implementation of the mmap system call, which allows the mapping of device memory directly into a user process's address space. Not all devices require mmap support, but, for some, mapping device memory can yield significant performance improvements.

• We then look at crossing the boundary from the other direction with a discussion of direct access to user-space pages. Relatively few drivers need this capability; in many cases, the kernel performs this sort of mapping without the driver even being aware of it. But an awareness of how to map user-space memory into the kernel (with get_user_pages) can be useful.

• The final section covers direct memory access (DMA) I/O operations, which provide peripherals with direct access to system memory.

Of course, all of these techniques require an understanding of how Linux memory management works, so we start with an overview of that subsystem.