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:
initrd 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
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); *************** }
④ start_kernel(init/main.c)函数中,
setup_arch(&command_line); //通过command_line返回参数列表,或者是编译之前配置的Boot Options,或者是bootloader传入的bootargs ... setup_command_line(command_line);// 其中,将command_line拷贝进static_command_line ... printk(KERN_NOTICE "Kernel command line: %s\n", boot_command_line); parse_early_param(); //该函数中定义了静态变量done,并且在setup_arch函数中调用之后,已经设置为1,所以此次调用直接返回 //解析并处理setup params,start和stop两个变量在链接脚本中定义,我传进的参数都被unknow_boototion处理。 parse_args("Booting kernel", static_command_line, __start___param,__stop___param - __start___param,&unknown_bootoption);
⑤ 参数处理结束
几点说明:
① ATAG, params(options, args)是两个不同的概念,ATAG以特定的数据结构的形式由bootloader存入,Linux Kernel取出; 而params以字符串的形式。可以说cmd_line只是bootloader传入的TAG之一。
#ifndef CONFIG_CMDLINE_FORCE static int __init parse_tag_cmdline(const struct tag *tag) { strlcpy(default_command_line, tag->u.cmdline.cmdline, COMMAND_LINE_SIZE); 如果u-boot传入“ATAG_CMDLINE” TAG,则使用其cmdline覆盖default_command_line return 0; } __tagtable(ATAG_CMDLINE, parse_tag_cmdline); #endif /* CONFIG_CMDLINE_FORCE */
② TAG处理函数,定义在arch/arm/kernel/setup.c中;init/main.c中,定义了一些parmas处理函数。
③ unknow_bootoption函数处理在params列表中找不到的boot option
static int __init unknown_bootoption(char *param, char *val) { DBGTRACE("Unkown boot option: %s: %s\n", param, val); /* Change NUL term back to "=", to make "param" the whole string. */ if (val) { 将参数还原成“=”连接的名值对 /* param=val or param="val"? */ if (val == param+strlen(param)+1) val[-1] = '='; else if (val == param+strlen(param)+2) { val[-2] = '='; memmove(val-1, val, strlen(val)+1); val--; } else BUG(); } /* Handle obsolete-style parameters */ if (obsolete_checksetup(param)) 如果是废弃的参数,则忽略 return 0; /* Unused module parameter. */ if (strchr(param, '.') && (!val || strchr(param, '.') < val)) 如果是不适用的模块参数,忽略 return 0; if (panic_later) return 0; if (val) { 如果参数的值不为空,则初始化成环境变量,存在envp_init中 /* Environment option */ unsigned int i; for (i = 0; envp_init[i]; i++) { if (i == MAX_INIT_ENVS) { panic_later = "Too many boot env vars at `%s'"; panic_param = param; } if (!strncmp(param, envp_init[i], val - param)) break; } envp_init[i] = param; } else { 如果参数值为空,则初始化成命令行参数,存放在argv_init中 /* Command line option */ unsigned int i; for (i = 0; argv_init[i]; i++) { if (i == MAX_INIT_ARGS) { panic_later = "Too many boot init vars at `%s'"; panic_param = param; } } argv_init[i] = param; } return 0; }