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. |
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