uboot启动流程及内核参数传递

一. Bootloader启动流程

开发板中的Bootloader启动过程一般分为三个流程。当开发板上电后，系统首先执行存储在iROM中的Bootloader0(BL0)程序，做一些基本的初始化工作；随后根据选择的存储器(mmc/sd/nand flash/nor flash)中读取Bootloader1(BL1)程序到stepping stone，BL1可以进行系统时钟初始化等工作。最后一步才是真正的uboot程序引导，BL1拷贝uboot程序(称为BL2)到sdram，BL2可以初始化堆栈、内存以及引导os等功能。

二. uboot启动流程(BL2阶段)

在完成了前面两部bootloader的工作后，真正进入了uboot的工作范围。uboot的启动也分为两阶段，以s3c6410提供的uboot代码为例：

2.1 第一阶段（汇编代码部分）

（1）设置中断向量

（2）设置SVC模式

（3）设置控制寄存器

（4）关闭看门狗

（5）关中断

（6）拷贝第二阶段代码

（7）设置堆栈

（8）清除bss段

（9）跳转到第二阶段代码

	ldr	pc, _start_armboot

_start_armboot:
	.word start_armboot

2.2 第二阶段（C代码部分）

（1）函数start_armboot在lib_arm/Board.c中，各种初始化工作：

void start_armboot (void)

{

flash_init ()

mem_malloc_init()

nand_init()

env_relocate()

devices_init()

jumptable_init()

console_init_r()

enable_interrupts()

board_late_init()

eth_initialize()

最后循环方式，等待接受命令

   for (;;) {
          main_loop ();
   }

}

（2）主循环负责命令的输入以及调用相应的command

void main_loop (void)
{
#ifndef CFG_HUSH_PARSER
	static char lastcommand[CFG_CBSIZE] = { 0, };
	int len;
	int rc = 1;
	int flag;
#endif

#if defined(CONFIG_BOOTDELAY) && (CONFIG_BOOTDELAY >= 0)
	char *s;
	int bootdelay;
#endif
#ifdef CONFIG_PREBOOT
	char *p;
#endif
#ifdef CONFIG_BOOTCOUNT_LIMIT
	unsigned long bootcount = 0;
	unsigned long bootlimit = 0;
	char *bcs;
	char bcs_set[16];
#endif /* CONFIG_BOOTCOUNT_LIMIT */

#if defined(CONFIG_VFD) && defined(VFD_TEST_LOGO)
	ulong bmp = 0;		/* default bitmap */
	extern int trab_vfd (ulong bitmap);

#ifdef CONFIG_MODEM_SUPPORT
	if (do_mdm_init)
		bmp = 1;	/* alternate bitmap */
#endif
	trab_vfd (bmp);
#endif	/* CONFIG_VFD && VFD_TEST_LOGO */

#ifdef CONFIG_BOOTCOUNT_LIMIT
	bootcount = bootcount_load();
	bootcount++;
	bootcount_store (bootcount);
	sprintf (bcs_set, "%lu", bootcount);
	setenv ("bootcount", bcs_set);
	bcs = getenv ("bootlimit");
	bootlimit = bcs ? simple_strtoul (bcs, NULL, 10) : 0;
#endif /* CONFIG_BOOTCOUNT_LIMIT */

#ifdef CONFIG_MODEM_SUPPORT
	debug ("DEBUG: main_loop:   do_mdm_init=%d\n", do_mdm_init);
	if (do_mdm_init) {
		char *str = strdup(getenv("mdm_cmd"));
		setenv ("preboot", str);  /* set or delete definition */
		if (str != NULL)
			free (str);
		mdm_init(); /* wait for modem connection */
	}
#endif  /* CONFIG_MODEM_SUPPORT */

#ifdef CONFIG_VERSION_VARIABLE
	{
		extern char version_string[];

		setenv ("ver", version_string);  /* set version variable */
	}
#endif /* CONFIG_VERSION_VARIABLE */

#ifdef CFG_HUSH_PARSER
	u_boot_hush_start ();
#endif

#ifdef CONFIG_AUTO_COMPLETE
	install_auto_complete();
#endif

#ifdef CONFIG_PREBOOT
	if ((p = getenv ("preboot")) != NULL) {
# ifdef CONFIG_AUTOBOOT_KEYED
		int prev = disable_ctrlc(1);	/* disable Control C checking */
# endif

# ifndef CFG_HUSH_PARSER
		run_command (p, 0);
# else
		parse_string_outer(p, FLAG_PARSE_SEMICOLON |
				    FLAG_EXIT_FROM_LOOP);
# endif

# ifdef CONFIG_AUTOBOOT_KEYED
		disable_ctrlc(prev);	/* restore Control C checking */
# endif
	}
#endif /* CONFIG_PREBOOT */

#if defined(CONFIG_BOOTDELAY) && (CONFIG_BOOTDELAY >= 0)
	s = getenv ("bootdelay");
	bootdelay = s ? (int)simple_strtol(s, NULL, 10) : CONFIG_BOOTDELAY;

	debug ("### main_loop entered: bootdelay=%d\n\n", bootdelay);

# ifdef CONFIG_BOOT_RETRY_TIME
	init_cmd_timeout ();
# endif	/* CONFIG_BOOT_RETRY_TIME */

#ifdef CONFIG_BOOTCOUNT_LIMIT
	if (bootlimit && (bootcount > bootlimit)) {
		printf ("Warning: Bootlimit (%u) exceeded. Using altbootcmd.\n",
		        (unsigned)bootlimit);
		s = getenv ("altbootcmd");
	}
	else
#endif /* CONFIG_BOOTCOUNT_LIMIT */
		s = getenv ("bootcmd");

	debug ("### main_loop: bootcmd=\"%s\"\n", s ? s : "<UNDEFINED>");

	if (bootdelay >= 0 && s && !abortboot (bootdelay)) {
# ifdef CONFIG_AUTOBOOT_KEYED
		int prev = disable_ctrlc(1);	/* disable Control C checking */
# endif

# ifndef CFG_HUSH_PARSER
		run_command (s, 0);
# else
		parse_string_outer(s, FLAG_PARSE_SEMICOLON |
				    FLAG_EXIT_FROM_LOOP);
# endif

# ifdef CONFIG_AUTOBOOT_KEYED
		disable_ctrlc(prev);	/* restore Control C checking */
# endif
	}

# ifdef CONFIG_MENUKEY
	if (menukey == CONFIG_MENUKEY) {
	    s = getenv("menucmd");
	    if (s) {
# ifndef CFG_HUSH_PARSER
		run_command (s, 0);
# else
		parse_string_outer(s, FLAG_PARSE_SEMICOLON |
				    FLAG_EXIT_FROM_LOOP);
# endif
	    }
	}
#endif /* CONFIG_MENUKEY */
#endif	/* CONFIG_BOOTDELAY */

#ifdef CONFIG_AMIGAONEG3SE
	{
	    extern void video_banner(void);
	    video_banner();
	}
#endif

	/*
	 * Main Loop for Monitor Command Processing
	 */
#ifdef CFG_HUSH_PARSER
	parse_file_outer();
	/* This point is never reached */
	for (;;);
#else
	for (;;) {
#ifdef CONFIG_BOOT_RETRY_TIME
		if (rc >= 0) {
			/* Saw enough of a valid command to
			 * restart the timeout.
			 */
			reset_cmd_timeout();
		}
#endif
		len = readline (CFG_PROMPT);

		flag = 0;	/* assume no special flags for now */
		if (len > 0)
			strcpy (lastcommand, console_buffer);
		else if (len == 0)
			flag |= CMD_FLAG_REPEAT;
#ifdef CONFIG_BOOT_RETRY_TIME
		else if (len == -2) {
			/* -2 means timed out, retry autoboot
			 */
			puts ("\nTimed out waiting for command\n");
# ifdef CONFIG_RESET_TO_RETRY

			/* Reinit board to run initialization code again */
			do_reset (NULL, 0, 0, NULL);
# else
			return;		/* retry autoboot */
# endif
		}
#endif

		if (len == -1)
			puts ("<INTERRUPT>\n");
		else
			rc = run_command (lastcommand, flag);

		if (rc <= 0) {
			/* invalid command or not repeatable, forget it */
			lastcommand[0] = 0;
		}
	}
#endif /*CFG_HUSH_PARSER*/
}

main_loop函数中，readline()函数先判断有没有超时，如果超时就执行do_reset函数，自动启动；否则就调用run_command函数

（3）run_command函数负责命令的执行

结构体cmd_tbl_s存储一个命令的相关信息

struct cmd_tbl_s {
	char		*name;		/* Command Name			*/
	int		maxargs;	/* maximum number of arguments	*/
	int		repeatable;	/* autorepeat allowed?		*/
					/* Implementation function	*/
	int		(*cmd)(struct cmd_tbl_s *, int, int, char *[]);
	char		*usage;		/* Usage message	(short)	*/
#ifdef	CFG_LONGHELP
	char		*help;		/* Help  message	(long)	*/
#endif
#ifdef CONFIG_AUTO_COMPLETE
	/* do auto completion on the arguments */
	int		(*complete)(int argc, char *argv[], char last_char, int maxv, char *cmdv[]);
#endif
};

每执行一个命令都会从command table中查找相关的命令

cmd_tbl_t *find_cmd (const char *cmd)
{
	cmd_tbl_t *cmdtp;
	cmd_tbl_t *cmdtp_temp = &__u_boot_cmd_start;	/*Init value */
	const char *p;
	int len;
	int n_found = 0;

	/*
	 * Some commands allow length modifiers (like "cp.b");
	 * compare command name only until first dot.
	 */
	len = ((p = strchr(cmd, '.')) == NULL) ? strlen (cmd) : (p - cmd);

	for (cmdtp = &__u_boot_cmd_start;
	     cmdtp != &__u_boot_cmd_end;
	     cmdtp++) {
		if (strncmp (cmd, cmdtp->name, len) == 0) {
			if (len == strlen (cmdtp->name))
				return cmdtp;	/* full match */

			cmdtp_temp = cmdtp;	/* abbreviated command ? */
			n_found++;
		}
	}
	if (n_found == 1) {			/* exactly one match */
		return cmdtp_temp;
	}

	return NULL;	/* not found or ambiguous command */
}

（4）命令的注册

在连接脚本u-boot.lds中定义了“.u_boot_cmd”段

__u_boot_cmd_start = .;
	.u_boot_cmd : { *(.u_boot_cmd) }
	__u_boot_cmd_end = .;

意思是只要在结构体cmd_tbl_t带上了.u_boot_cmd的属性，就可以被编译到“.u_boot_cmd”段中，而命令的查找在__u_boot_cmd_start与__u_boot_cmd_end之间查找就可以了。在command.h中定义了两个宏

#define U_BOOT_CMD(name,maxargs,rep,cmd,usage,help) \
cmd_tbl_t __u_boot_cmd_##name Struct_Section = {#name, maxargs, rep, cmd, usage}
#define Struct_Section  __attribute__ ((unused,section (".u_boot_cmd")))

宏U_BOOT_CMD负责命令的注册，而带有宏Struct_Section的任何变量都会被编译到.u_boot_cmd段中。因此宏U_BOOT_CMD扩展后就完成了命令的注册。

三. 内核参数传递

bootm命令用于启动经过处理的内核镜像，这里经过处理是指使用mkimage创建的镜像，比原本的内核镜像bzImage或zImage多了0x40个字节的头部信息字节。参考cmd_bootm.c中的do_bootm函数可以知道内核参数的传递过程。

char *commandline = getenv ("bootargs");   /* 获取环境变量bootargs */
theKernel = (void (*)(int, int, uint))images->ep; /* 获取内核入口地址 */
theKernel (0, machid, bd->bi_boot_params);      /* 启动内核 */

最后注册bootm，便可以调用内核启动函数

U_BOOT_CMD(
 	bootm,	CFG_MAXARGS,	1,	do_bootm,
 	"bootm   - boot application image from memory\n",
 	"[addr [arg ...]]\n    - boot application image stored in memory\n"
 	"\tpassing arguments 'arg ...'; when booting a Linux kernel,\n"
 	"\t'arg' can be the address of an initrd image\n"
#ifdef CONFIG_OF_FLAT_TREE
	"\tWhen booting a Linux kernel which requires a flat device-tree\n"
	"\ta third argument is required which is the address of the of the\n"
	"\tdevice-tree blob. To boot that kernel without an initrd image,\n"
	"\tuse a '-' for the second argument. If you do not pass a third\n"
	"\ta bd_info struct will be passed instead\n"
#endif
);

;

uboot启动流程及内核参数传递

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