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);

linux内核commandline参数解析过程(原创)  

http://6xudonghai.blog.163.com/blog/static/3364062920093165038776/

最近研究了一下linux内核启动时ramdisk的加载过程,本来是用的是JIFFS文件系统因为调试的需要,现在要通过u-boot直接将内核镜像和ramdisk文件系统镜像下载到SRAM中，首先需要添加并修改command
line中的几个参数:

initrd=0x30800040,0x400000 root=/dev/ram init=/linuxrc rootfstype=ext2

 

linux版本：linux-2.6.14

硬件：TE2410 开发板

 

先看看这个commandline是在什么地方解析的

1.start_kernel(void) --> setup_arch(&command_line) --> parse_tags(tags)

parse_tags根据u-boot传过来的tag list解析,调用下面几个函数:

static int __init parse_tag_mem32(const struct tag *tag)

static int __init parse_tag_cmdline(const struct tag *tag)//拷贝commandline

{

strlcpy(default_command_line,tag->u.cmdline.cmdline, COMMAND_LINE_SIZE);

       printk(tag->u.cmdline.cmdline);

       return 0;

}

static int __init parse_tag_initrd2(const struct tag *tag)

{

       phys_initrd_start = tag->u.initrd.start;

       phys_initrd_size = tag->u.initrd.size;

       return 0;

}

 

2.setup_arch(&command_line) -->parse_cmdline(cmdline_p, from)

解析commandline中的2个参数"initrd=" "mem="

 

parse_cmdline调用下面两个函数:

static void __init early_initrd(char **p)

{

       unsigned long start, size;

       start = memparse(*p, p);

       if (**p == ',') {

              size = memparse((*p) + 1, p);

              phys_initrd_start = start;

              phys_initrd_size = size;

       }

}

__early_param("initrd=", early_initrd);

 

static void __init early_mem(char **p)

__early_param("mem=", early_mem);

 

3.start_kernel(void) --> setup_arch(&command_line) -->paging_init(&meminfo, mdesc)

-->bootmem_init(mi)

根据 phys_initrd_start  phys_initrd_size 设置全局变量initrd_start initrd_end

这两个变量在函数populate_rootfs()中会用到

static void __init bootmem_init(struct meminfo *mi)

{

. . .

#ifdef CONFIG_BLK_DEV_INITRD

       if (phys_initrd_size && initrd_node >= 0) {

              reserve_bootmem_node(NODE_DATA(initrd_node), phys_initrd_start,

                                 phys_initrd_size);

              initrd_start = __phys_to_virt(phys_initrd_start);

              initrd_end = initrd_start + phys_initrd_size;

       }

#endif

. . .

}

4.start_kernel(void)-->parse_early_param()-->

parse_args("early options", tmp_cmdline, NULL, 0, do_early_param);

解析commandline的其余参数"root=" "rootfstype=" "init=" "ro"
"rw" "console="等

static int __init root_dev_setup(char *line)

{

       strlcpy(saved_root_name, line, sizeof(saved_root_name));

       return 1;

}

__setup("root=", root_dev_setup);

 

static int __init fs_names_setup(char *str)

{

       root_fs_names = str;

       return 1;

}

__setup("rootfstype=", fs_names_setup);

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);

 

static int __init readonly(char *str)

{

       if (*str)

              return 0;

       root_mountflags |= MS_RDONLY;

       return 1;

}

 

static int __init readwrite(char *str)

{

       if (*str)

              return 0;

       root_mountflags &= ~MS_RDONLY;

       return 1;

}

 

__setup("ro", readonly);

__setup("rw", readwrite);

ramdisk加载过程

在init进程中populate_rootfs()

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) {/*判断是否加载了initrd*/

              int fd;

              printk(KERN_INFO "checking if image is initramfs...");

              err = unpack_to_rootfs((char *)initrd_start,

                     initrd_end - initrd_start, 1);

              if (!err) {/*判断加载的是不是cpio-initrd*/

                     printk(" it is\n");

                     unpack_to_rootfs((char *)initrd_start,

                            initrd_end - initrd_start, 0);

                     free_initrd();

                     return;

              }

              printk("it isn't (%s); looks like an initrd\n", err);

/*如果不是cpio-initrd,则认为是一个image-initrd，将其内容保存到*/ /*initrd.image中,在后面的image-initrd的处理代码中会读取/initrd.image*/

              fd = sys_open("/initrd.image", O_WRONLY|O_CREAT, 700);

              if (fd >= 0) {

                     sys_write(fd, (char *)initrd_start,

                                   initrd_end - initrd_start);

                     sys_close(fd);

                     free_initrd();

              }

       }

#endif

 

后面执init()-->prepare_namespace()-->initrd_load()--> rd_load_image("/initrd.image")

-->identify_ramdisk_image(in_fd, rd_image_start)

 

identify_ramdisk_image对Ramdisk的文件系统类型做检查

 

prepare_namespace()-->mount_root()-->

mount_block_root("/dev/root", root_mountflags);

 

mount_block_root加载文件系统

关于Ramdisk加载过程详细分析参考以下几篇

Linux内核Ramdisk(initrd)机制 http://blog.csdn.net/ruixj/archive/2009/01/14/3772752.aspx

Initrd 流程分析 http://hi.baidu.com/mczyh/blog/item/86153139527ed422b8998f17.html

Linux2.6 内核的 Initrd 机制解析http://www.ibm.com/developerworks/cn/linux/l-k26initrd/index.html