linux内核启动过程中__set_up的作用！

__set_up是一个宏

#define __setup(str, fn)                    
    __setup_param(str, fn, fn, 0)

#define __setup_param(str, unique_id, fn, early)            
    static char __setup_str_##unique_id[] __initdata = str;    
    static struct obs_kernel_param __setup_##unique_id    
        __attribute_used__                
        __attribute__((__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;
};

#define __init        __attribute__ ((__section__ (".init.text")))
#define __initdata    __attribute__ ((__section__ (".init.data")))
#define __exitdata    __attribute__ ((__section__(".exit.data")))

举例：

static int __init init_setup(char *str)
{
    unsigned int i;

    execute_command = str;

    for (i = 1; i < MAX_INIT_ARGS; i++)
        argv_init[i] = NULL;
    return 1;
}
__setup("init=", init_setup);
//转换为
    static char __setup_str_init_setup[] __attribute__ ((__section__ (".init.data"))) ="init=";    
    static struct obs_kernel_param __setup_init_setup    
        __attribute_used__                
        __attribute__((__section__(".init.setup")))    
        __attribute__((aligned((sizeof(long)))))    
        = { __setup_str_init_setup, init_setup, 0 }

/*vmlinux.lds的.init段中
{
　　...
　　__setup_start = .;
   *(.init.setup)
   __setup_end = .;
   ...
}*/
static int __init obsolete_checksetup(char *line)/* Handle obsolete-style parameters */
{
    struct obs_kernel_param *p;
    int had_early_param = 0;

    p = __setup_start;                  /*从.init.setup段的开始分析*/
    do {
        int n = strlen(p->str);         /*计算出.init.setup中字符串的长度*/
        if (!strncmp(line, p->str, n)) {
            if (p->early) {             /*这里的都是early属性为0的*/
                /* 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] == '' || line[n] == '=')
                    had_early_param = 1;
            } else if (!p->setup_func) {
                printk(KERN_WARNING "Parameter %s is obsolete,"
                       " ignored
", p->str);
                return 1;
            } else if (p->setup_func(line + n))/*调用参数中对应函数，参数为偏移n个（初始化是参数长度）后的字符串*/
                return 1;
        }
        p++;                                   /*继续处理，直到结束*/
    } while (p < __setup_end);

    return had_early_param;
}

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;
}
//作用就是把“=”后边的传给execute_command。
//比如设置了init=/linuxrc   则execute_command=&“/linuxrc” 表示不正确 就是那个结构体的str参数偏移n个的地址

static int noinline init_post(void)
{
        ...
    if (ramdisk_execute_command) {
        run_init_process(ramdisk_execute_command);
        printk(KERN_WARNING "Failed to execute %s
",
                ramdisk_execute_command);
    }
        ...
    if (execute_command) {
        run_init_process(execute_command);
        printk(KERN_WARNING "Failed to execute %s.  Attempting "
                    "defaults...
", 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.");
}