18.2 of的函数集中的of是Open Firmware的缩写
内核中操作dtb的一套函数都是of开头，这个of是open firmware。dts的方法来源于open Firmware
On Sun SPARC systems, the Open Firmware interface is displayed on the console terminal before the bootstrapping of the system software.
include/linux/of.h
#ifndef _Linux_OF_H
/* Definitions for talking to the Open Firmware PORM on Power Mcintosh and other computer.*/