Linux启动参数及实现 __setup与early_param

#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 }
        
#define __setup(str, fn)                    /
    __setup_param(str, fn, fn, 0)
    
#define early_param(str, fn)                    /

    __setup_param(str, fn, fn, 1)

__setup与early_param不同的是，early_param 宏注册的内核选项必须要在其他内核选项之前被处理。

在函数start_kernel中，parse_early_param处理early_param定义的参数，parse_args处理__setup定义的参数。

 parse_early_param();
 parse_args("Booting kernel", static_command_line, __start___param,
     __stop___param - __start___param,
     &unknown_bootoption);

    
1,所有的系统启动参数都是由形如 static int __init foo(char *str);的函数来支持的
注:#define __init        __attribute__ ((__section__ (".init.text")))
申明所有的启动参数支持函数都放入.init.text段

2.1,用__setup宏来导出参数的支持函数
__setup("foo=" , foo);
展开后就是如下的形式

static char __setup_str_foo[] __initdata = "foo=";    
static struct obs_kernel_param __setup_foo    
        __attribute_used__                
        __attribute__((__section__(".init.setup")))    
        __attribute__((aligned((sizeof(long)))))    
        = { __setup_str_foo, foo, 0 };//"foo=",foo,0
也就是说,启动参数(函数指针)被封装到obs_kernel_param结构中,
所有的内核启动参数形成内核映像.init.setup段中的一个
obs_kernel_param数组

2.2用early_param宏来申明需要'早期'处理的启动参数,例如在
arch/i386/kernel/setup.c就有如下的申明:
early_param("mem", parse_mem);
展开后和__setup是一样的只是early参数不一样,因此会在do_early_param
中被处理

3,内核对启动参数的解析:下面函数历遍obs_kernel_param数组,调用
支持函数
static int __init do_early_param(char *param, char *val)
{
    struct obs_kernel_param *p;

    for (p = __setup_start; p < __setup_end; p++) {
        if (p->early && strcmp(param, p->str) == 0) {
            if (p->setup_func(val) != 0)
                printk(KERN_WARNING
                       "Malformed early option '%s'/n", param);
        }
    }
    /* We accept everything at this stage. */
    return 0;
}
这个函数在parse_early_param中被调用,而parse_early_param在start_kernel
中被调用,parse_early_param之后的parse_args会调用下面函数
static int __init obsolete_checksetup(char *line)
{
    struct obs_kernel_param *p;
    int had_early_param = 0;

    p = __setup_start;
    do {
        int n = strlen(p->str);
        if (!strncmp(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))//调用支持函数
                return 1;
        }
        p++;
    } while (p < __setup_end);

    return had_early_param;
}

init/main.c中启动参数申明列表:
__setup("nosmp", nosmp);
__setup("maxcpus=", maxcpus);
__setup("reset_devices", set_reset_devices);

__setup("debug", debug_kernel);
__setup("quiet", quiet_kernel);

__setup("loglevel=", loglevel);

__setup("init=", init_setup);
__setup("rdinit=", rdinit_setup);

__setup("initcall_debug", initcall_debug_setup);