5.1. System Initialization Overview

It's important to know where and how the main network-related subsystems are initialized, including device drivers. However, because this book is concerned only with the networking aspect of such initializations, I will not cover device drivers in general, or generic kernel services (e.g., memory management). For an understanding of that background, I recommend that you read Linux Device Drivers and Understanding the Linux Kernel, both published by O'Reilly.

Figure 5-1 shows briefly where, and in what sequence, some of the kernel subsystems are initialized at boot time (see init/main.c).

Figure 5-1. Kernel initialization

When the kernel boots up, it executes start_kernel, which initializes a bunch of subsystems, as partially shown in Figure 5-1. Before start_kernel terminates, it invokes the init kernel thread, which takes care of the rest of the initializations. Most of the initialization activities related to this chapter happen to be inside do_basic_setup.

Among the various initialization tasks, we are mainly interested in three:

Boot-time options

Two calls to parse_args, one direct and one indirect via parse_early_param, handle configuration parameters that a boot loader such as LILO or GRUB has passed to the kernel at boot time. We will see how this task is handled in the section "Boot-Time Kernel Options."

Interrupts and timers

Hardware and software interrupts are initialized with init_IRQ and softirq_init, respectively. Interrupts are covered in Chapter 9. In this chapter, we will see just how device drivers register a handler with an IRQ and how IRQ handlers are organized in memory. Timers are also initialized early in the boot process so that later tasks can use them.

Initialization routines

Kernel subsystems and built-in device drivers are initialized by do_initcalls. free_init_mem frees a piece of memory that holds unneeded code. This optimization is possible thanks to smart routine tagging. See Chapter 7 for more details.

run_init_process determines the first process run on the system, the parent of all other processes; it has a PID of 1 and never halts until the system is done. Normally the program run is init, part of the SysVinit package. However, the administrator can specify a different program through the init= boot time option. When no such option is provided, the kernel tries to execute the init command from a set of well-known locations, and panics if it cannot find any. The user can also provide boot-time options that will be passed to init (see the section "Boot-Time Kernel Options").