MACHINE_START(SMDK2410, "SMDK2410") /* @TODO: request a new identifier and switch
* to SMDK2410 */
/* Maintainer: Jonas Dietsche */
.phys_io = S3C2410_PA_UART,
.io_pg_offst = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
.boot_params = S3C2410_SDRAM_PA + 0x100,
.map_io = smdk2410_map_io,
.init_irq = s3c24xx_init_irq,
.init_machine = smdk_machine_init,
.timer = &s3c24xx_timer,
MACHINE_END

由此可見在.arch.info.init段內(nèi)存放了__desc_mach_desc_SMDK2410結構體。初始化了相應的初始化函數(shù)指針。問題又來了， 這些初始化指針函數(shù)是什么時候被調(diào)用的呢？
分析發(fā)現(xiàn)，不一而同。
如 s3c24xx_init_irq()函數(shù)是通過start_kernel()里的init_IRQ()函數(shù)調(diào)用init_arch_irq()實現(xiàn)的。 因為在MACHINE_START結構體中 .init_irq = s3c24xx_init_irq，而在setup_arch()函數(shù)中init_arch_irq = mdesc->init_irq， 所以調(diào)用init_arch_irq()就相當于調(diào)用了s3c24xx_init_irq()。
又如smdk_machine_init()函數(shù) 的初始化。在MACHINE_START結構體中，函數(shù)指針賦值，.init_machine = smdk_machine_init。而init_machine()函數(shù)被linux/arch/arm/kernel/setup.c文件中的 customize_machine()函數(shù)調(diào)用并被arch_initcall(Fn)宏處 理，arch_initcall(customize_machine)。 被arch_initcall(Fn)宏處理過函數(shù)將linux/init/main.c
do_initcalls()函數(shù)調(diào)用。 具體參看下邊的部分。

void __init setup_arch(char cmdline_p)
{
struct tag *tags = (struct tag *)&init_tags;
struct machine_desc *mdesc;
char *from = default_command_line;

setup_processor();
mdesc = setup_machine(machine_arch_type);//machine_arch_type =SMDK2410 by edwin
machine_name = mdesc->name;

if (mdesc->soft_reboot)
reboot_setup("s");

if (mdesc->boot_params)
tags = phys_to_virt(mdesc->boot_params);

/*
* If we have the old style parameters, convert them to
* a tag list.
*/
if (tags->hdr.tag != ATAG_CORE)
convert_to_tag_list(tags);
if (tags->hdr.tag != ATAG_CORE)
tags = (struct tag *)&init_tags;

if (mdesc->fixup)
mdesc->fixup(mdesc, tags, &from, &meminfo);

if (tags->hdr.tag == ATAG_CORE) {
if (meminfo.nr_banks != 0)
squash_mem_tags(tags);
parse_tags(tags);
}

init_mm.start_code = (unsigned long) &_text;
init_mm.end_code = (unsigned long) &_etext;
init_mm.end_data = (unsigned long) &_edata;
init_mm.brk = (unsigned long) &_end;

memcpy(saved_command_line, from, COMMAND_LINE_SIZE);

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saved_command_line[COMMAND_LINE_SIZE-1] = /0;
parse_cmdline(cmdline_p, from);
paging_init(&meminfo, mdesc);
request_standard_resources(&meminfo, mdesc);

#ifdef CONFIG_SMP
smp_init_cpus();
#endif

cpu_init();

/*
* Set up various architecture-specific pointers
*/
init_arch_irq = mdesc->init_irq;
system_timer = mdesc->timer;
init_machine = mdesc->init_machine;

#ifdef CONFIG_VT
#if defined(CONFIG_VGA_CONSOLE)
conswitchp = &vga_con;
#elif defined(CONFIG_DUMMY_CONSOLE)
conswitchp = &dummy_con;
#endif
#endif
}
5.3. rest_init()函數(shù)分析
下面我們來分析下rest_init()函數(shù)。
Start_kernel() 函數(shù)負責初始化內(nèi)核各子系統(tǒng)，最后調(diào)用reset_init()，啟動一個叫做init的內(nèi)核線程，繼續(xù)初始化。在init內(nèi)核線程中，將執(zhí)行下列 init()函數(shù)的程序。Init()函數(shù)負責完成根文件系統(tǒng)的掛接、初始化設備驅(qū)動程序和啟動用戶空間的init進程等重要工作。

static void noinline rest_init(void)
__releases(kernel_lock)
{
kernel_thread(init, NULL, CLONE_FS | CLONE_SIGHAND);
numa_default_policy();
unlock_kernel();

/*
* The boot idle thread must execute schedule()
* at least one to get things moving:
*/
preempt_enable_no_resched();
schedule();
preempt_disable();

/* Call into cpu_idle with preempt disabled */
cpu_idle();
}

static int init(void * unused)
{
lock_kernel();
/*
* init can run on any cpu.
*/
set_cpus_allowed(current, CPU_MASK_ALL);
/*
* Tell the world that were going to be the grim
* reaper of innocent orphaned children.
*
* We dont want people to have to make incorrect
* assumptions about where in the task array this
* can be found.
*/
child_reaper = current;

smp_prepare_cpus(max_cpus);

do_pre_smp_initcalls();

smp_init();
sched_init_smp();

cpuset_init_smp();

/*
* Do this before initcalls, because some drivers want to access
* firmware files.
*/
populate_rootfs(); //掛接根文件系統(tǒng)

do_basic_setup(); //初始化設備驅(qū)動程序

/*
* check if there is an early userspace init. If yes, let it do all
* the work //啟動用戶空間的init進程

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*/

if (!ramdisk_execute_command)
ramdisk_execute_command = "/init";

if (sys_access((const char __user *) ramdisk_execute_command, 0) != 0) {
ramdisk_execute_command = NULL;
prepare_namespace();
}

/*
* Ok, we have completed the initial bootup, and
* were essentially up and running. Get rid of the
* initmem segments and start the user-mode stuff..
*/
free_initmem();
unlock_kernel();
mark_rodata_ro();
system_state = SYSTEM_RUNNING;
numa_default_policy();

if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
printk(KERN_WARNING "Warning: unable to open an initial console./n");

(void) sys_dup(0);
(void) sys_dup(0);

if (ramdisk_execute_command) {
run_init_process(ramdisk_execute_command);
printk(KERN_WARNING "Failed to execute %s/n",
ramdisk_execute_command);
}

/*
* We try each of these until one succeeds.
*
* The Bourne shell can be used instead of init if we are
* trying to recover a really broken machine.
*/
if (execute_command) {
run_init_process(execute_command);
printk(KERN_WARNING "Failed to execute %s. Attempting "
"defaults.../n", execute_command);
}
run_init_process("/sbin/init");
run_init_process("/etc/init");
run_init_process("/bin/init");
run_init_process("/bin/sh");

panic("No init found. Try passing init= option to kernel.");
}

5.3.1. 掛接根文件系統(tǒng)
Linux/init/ramfs.c
void __init populate_rootfs(void)
{
char *err = unpack_to_rootfs(__initramfs_start,
__initramfs_end - __initramfs_start, 0);
if (err)
panic(err);
#ifdef CONFIG_BLK_DEV_INITRD
if (initrd_start) {
#ifdef CONFIG_BLK_DEV_RAM
int fd;
printk(KERN_INFO "checking if image is initramfs...");
err = unpack_to_rootfs((char *)initrd_start,
initrd_end - initrd_start, 1);
if (!err) {
printk(" it is/n");
unpack_to_rootfs((char *)initrd_start,
initrd_end - initrd_start, 0);
free_initrd();
return;
}
printk("it isnt (%s); looks like an initrd/n", err);