Linux内核启动：setup_arch

BootLoader 可以向Linux 传递参数，编译内核时也可以配置boot options 。

调试中使用的U-Boot bootargs如下：

noinitrd root=/dev/mtdblock3 rw console=ttySAC0,115200 init=/linuxrc mem=64M

内核版本：

2.6.35.7

内核的处理参数的整体过程如下：

① u-boot将配置参数地址通过寄存器传递给内核

② 内核(arch/arm/kernel/head-common.S中的 __mmap_switched)将这个地址存入__atags_pointer(定义于arch/arm/kernel/setup.c)

③ setup_arch() 函数

<span style="font-size: 14px;">void __init setup_arch(char **cmdline_p)

{
         struct tag *tags = (struct tag *)&init_tags;
         struct machine_desc *mdesc;
         char *from = default_command_line; 编译内核时配置的Boot Options


         unwind_init();


         setup_processor();
         mdesc = setup_machine(machine_arch_type);
         machine_name = mdesc->name;


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


         if (__atags_pointer)                                               检查BootLoader是否传入参数
                   tags = phys_to_virt(__atags_pointer);
         else if (mdesc->boot_params)
                   tags = phys_to_virt(mdesc->boot_params);  machine descriptor中传入的启动参数地址(arch/arm/mach-s3c2440/mach-mini2440.c)


         /*
          * 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;       使用default init_tags，其中内存的定义是: 起始地址：0x30000000，大小是16M


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


         if (tags->hdr.tag == ATAG_CORE) {
                   if (meminfo.nr_banks != 0)                如果内存已经初始化，则忽略mem TAG
                            squash_mem_tags(tags);
                   save_atags(tags);
                   parse_tags(tags);             解析TAGS，其中如果U-boot传入ATAG_CMDLINE，则使用U-boot传入的bootargs覆盖default_command_line
         }


         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;


         /* parse_early_param needs a boot_command_line */
         strlcpy(boot_command_line, from, COMMAND_LINE_SIZE);         将defualt_command_line拷入boot_command_line


         /* populate cmd_line too for later use, preserving boot_command_line */
         strlcpy(cmd_line, boot_command_line, COMMAND_LINE_SIZE);
         *cmdline_p = cmd_line;


         parse_early_param();               大部分参数的early属性为0，即大部分参数在早期不处理，如noinitrd，console等


         paging_init(mdesc);
         request_standard_resources(&meminfo, mdesc);


         ***************
</span>
}

void __init setup_arch(char **cmdline_p)

{
         struct tag *tags = (struct tag *)&init_tags;
         struct machine_desc *mdesc;
         char *from = default_command_line; 编译内核时配置的Boot Options


         unwind_init();


         setup_processor();
         mdesc = setup_machine(machine_arch_type);
         machine_name = mdesc->name;

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

 
         if (__atags_pointer)                                               检查BootLoader是否传入参数
                   tags = phys_to_virt(__atags_pointer);
         else if (mdesc->boot_params)
                   tags = phys_to_virt(mdesc->boot_params);  machine descriptor中传入的启动参数地址(arch/arm/mach-s3c2440/mach-mini2440.c)

 
         /*
          * 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;       使用default init_tags，其中内存的定义是: 起始地址：0x30000000，大小是16M

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

 
         if (tags->hdr.tag == ATAG_CORE) {
                   if (meminfo.nr_banks != 0)                如果内存已经初始化，则忽略mem TAG
                            squash_mem_tags(tags);
                   save_atags(tags);
                   parse_tags(tags);             解析TAGS，其中如果U-boot传入ATAG_CMDLINE，则使用U-boot传入的bootargs覆盖default_command_line
         }

 
         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;

 
         /* parse_early_param needs a boot_command_line */
         strlcpy(boot_command_line, from, COMMAND_LINE_SIZE);         将defualt_command_line拷入boot_command_line
 

         /* populate cmd_line too for later use, preserving boot_command_line */
         strlcpy(cmd_line, boot_command_line, COMMAND_LINE_SIZE);
         *cmdline_p = cmd_line;

 
         parse_early_param();               大部分参数的early属性为0，即大部分参数在早期不处理，如noinitrd，console等

 
         paging_init(mdesc);
         request_standard_resources(&meminfo, mdesc);

        
         ***************

}

一．u-boot传递TAG到kernel的解析

       在setup_arch函数的parse_tags中对传递过来的TAGLIST进行了解析

       对每一项的tag使用parse_tag分析，

       for (t = &__tagtable_begin; t < &__tagtable_end; t++)

              if (tag->hdr.tag == t->tag) {

                     t->parse(tag);

                     break;

              }

       其中__tagtable_begin,__tagtable_end在vmlinux.ld中也有定义，这里看tagtable的建立过程

       #define __tagtalbe(tag,fn)/

       Static struct tagtable __tagtable_##fn __tag={tag,fn}

       #define __tag __userd __attribute__((__section__(“.taglist.init”)))

对于上述宏中的fn,就是tagtable结构中的parse指针所指向的函数。

       而在setup.c中，已经通过__tagtalbe(ATAG_XXX,XXX)建立起所有可能的tagtable,所以可以通过遍历__tagtable_begin~__tagtable_end找到对应的tagtable,并调用对应的parse进行解析并配置