static int __init kernel_init(void * unused)

        init_post();       

init_post()： 这是内核启动应用程序的接口

static noinline int init_post(void)
{/* need to finish all async __init code before freeing the memory */async_synchronize_full();free_initmem();mark_rodata_ro();system_state = SYSTEM_RUNNING;numa_default_policy();current->signal->flags |= SIGNAL_UNKILLABLE;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.*///如果命令行被定义，则执行自定义的程序if (execute_command) {run_init_process(execute_command);printk(KERN_WARNING "Failed to execute %s.  Attempting ""defaults...\n", execute_command);}//若命令行为空，则顺序执行默认程序run_init_process("/sbin/init");//都是由busybox所提供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. ""See Linux Documentation/init.txt for guidance.");
}

execute_command在init_setup函数中被赋值

static int __init init_setup(char *str)
{unsigned int i;execute_command = str;/** In case LILO is going to boot us with default command line,* it prepends "auto" before the whole cmdline which makes* the shell think it should execute a script with such name.* So we ignore all arguments entered _before_ init=... [MJ]*/for (i = 1; i < MAX_INIT_ARGS; i++)argv_init[i] = NULL;return 1;
}
__setup("init=", init_setup);//在init.h中定义#define __setup(str, fn)					\__setup_param(str, fn, fn, 0)//__setup_param定义
#define __setup_param(str, unique_id, fn, early)			\static const char __setup_str_##unique_id[] __initconst	\__aligned(1) = str; \static struct obs_kernel_param __setup_##unique_id	\__used __section(.init.setup)			\__attribute__((aligned((sizeof(long)))))	\= { __setup_str_##unique_id, fn, early }struct obs_kernel_param {const char *str;int (*setup_func)(char *);int early;
};

obsolete_checksetup处理early为0的命令 

static int __init obsolete_checksetup(char *line)
{const struct obs_kernel_param *p;int had_early_param = 0;p =  _start;do {int n = strlen(p->str);if (parameqn(line, p->str, n)) {if (p->early) {/* Already done in parse_early_param?* (Needs exact match on param part).* Keep iterating, as we can have early* params and __setups of same names 8( */if (line[n] == '\0' || line[n] == '=')had_early_param = 1;} else if (!p->setup_func) {printk(KERN_WARNING "Parameter %s is obsolete,"" ignored\n", p->str);return 1;} else if (p->setup_func(line + n))return 1;}p++;} while (p < __setup_end);return had_early_param;
}

 do_early_param处理early为1的命令

/* Check for early params. */
static int __init do_early_param(char *param, char *val)
{const struct obs_kernel_param *p;for (p = __setup_start; p < __setup_end; p++) {if ((p->early && parameq(param, p->str)) ||(strcmp(param, "console") == 0 &&strcmp(p->str, "earlycon") == 0)) {if (p->setup_func(val) != 0)printk(KERN_WARNING"Malformed early option '%s'\n", param);}}/* We accept everything at this stage. */return 0;
}

execute_command：

        UBOOT传入的"init=linuxrc"

        execute_command=linuxrc

if (linuxrc) 
        run_init_process(linuxrc);//内核切换到了linuxrc程序执行

在linux中linuxrc指向了busybox