Linux Device Drivers

Chapter 13. Mmap and DMA

This chapter introduces the internals of Linux memory management and memory mapping. It also describes how ``Direct Memory Access'' (DMA) is used by device drivers. Although you might object that DMA belongs more to hardware handling than to the software interface, I feel it more related to memory management than to hardware control.

This chapter is quite advanced; most driver writers won’t need to go so deep into the system internals. Nonetheless, understanding how memory works will help you design a driver that makes effective use of the system’s capabilities.

Memory Management in Linux

Rather than describing the theory of memory management in operating systems, this section tries to pinpoint the main features of the Linux implementation of the theory. This section is mainly informative and skipping over it shouldn’t prevent you from understanding the later topics that are more implementation-oriented.

Page Tables

When a program looks up a virtual address, the processor splits the address into bitfields. Each bitfield is used as an index into an array, called a page table, to retrieve either the address of the next table or the address of the physical page that holds the virtual address.

The Linux kernel manages three levels of page tables in order to map virtual addresses to physical addresses. This might appear strange at first. As most PC programmers are aware, the x86 hardware implements only two levels of page table. In fact, most 32-bit processors ...