二、uboot_源码分析

一、uboot启动原理

1、第一阶段初始化（汇编）

（1）关闭看门狗

（2）初始化时钟

（3）初始化SDRAM

（4）设置堆栈

（5）拷贝第二阶段代码

2、第二阶段初始化（c语言）

（1）实现读写flash

（2）串口通信

（3）网络通信

（4）读取内核，引导内核

二、源码分析

1、第一阶段，硬件相关初始化（arch/arm/cpu/arm920t/start.S）

1、向量表
.globl _start
_start:	b       reset
	ldr	pc, _undefined_instruction
	ldr	pc, _software_interrupt
	ldr	pc, _prefetch_abort
	ldr	pc, _data_abort
	ldr	pc, _not_used
	ldr	pc, _irq
	ldr	pc, _fiq
 
2、上电跳转reset启动
 
（1）设置CPU为SVC32模式
reset:
	/*
	 * set the cpu to SVC32 mode
	 */
	mrs	r0,cpsr
	bic	r0,r0,#0x1f
	orr	r0,r0,#0xd3
	msr	cpsr,r0
（2）关闭看门狗
/* turn off the watchdog */
#if defined(CONFIG_S3C2400)
# define pWTCON		0x15300000
# define INTMSK		0x14400008	/* Interupt-Controller base addresses */
# define CLKDIVN	0x14800014	/* clock divisor register */
#elif defined(CONFIG_S3C2410)
# define pWTCON		0x53000000
# define INTMOD     0X4A000004
# define INTMSK		0x4A000008	/* Interupt-Controller base addresses */
# define INTSUBMSK	0x4A00001C
# define CLKDIVN	0x4C000014	/* clock divisor register */
#endif
 
#if defined(CONFIG_S3C2400) || defined(CONFIG_S3C2410)
	ldr     r0, =pWTCON
	mov     r1, #0x0
	str     r1, [r0]
（3）屏蔽所有中断
/*
	 * mask all IRQs by setting all bits in the INTMR - default
	 */
	mov	r1, #0xffffffff
	ldr	r0, =INTMSK
	str	r1, [r0]
# if defined(CONFIG_S3C2410)
	ldr	r1, =0x3ff
	ldr	r0, =INTSUBMSK
	str	r1, [r0]
# endif
（4）设置CPU相关设置（cpu_init_crit： 清除I/O caches，初始化存储管理器）
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
	adr	r0, _start		/* r0 <- current position of code   */
	ldr	r1, _TEXT_BASE		/* test if we run from flash or RAM */
	cmp     r0, r1                  /* don't reloc during debug         */
	blne	cpu_init_crit
#endif
（5）设置栈
	/* Set up the stack						    */
stack_setup:
	ldr	r0, _TEXT_BASE		/* upper 128 KiB: relocated uboot   */
	sub	r0, r0, #CFG_MALLOC_LEN	/* malloc area                      */
	sub	r0, r0, #CFG_GBL_DATA_SIZE /* bdinfo   
#ifdef CONFIG_USE_IRQ
	sub	r0, r0, #(CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ)
#endif
	sub	sp, r0, #12		/* leave 3 words for abort-stack    */
（6）初始化时钟
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
    bl clock_init
#endif  
（7）重定位，将代码拷贝至内存（SDRAM）
#ifndef CONFIG_SKIP_RELOCATE_UBOOT
relocate:				/* relocate U-Boot to RAM	    */
	adr	r0, _start		/* r0 <- current position of code   */
	ldr	r1, _TEXT_BASE		/* test if we run from flash or RAM */
	cmp     r0, r1                  /* don't reloc during debug         */
	beq     clear_bss
	
	ldr	r2, _armboot_start
	ldr	r3, _bss_start
	sub	r2, r3, r2		/* r2 <- size of armboot            */
#if 1
	bl  CopyCode2Ram	/* r0: source, r1: dest, r2: size */
#else
	add	r2, r0, r2		/* r2 <- source end address         */
 
copy_loop:
	ldmia	r0!, {r3-r10}		/* copy from source address [r0]    */
	stmia	r1!, {r3-r10}		/* copy to   target address [r1]    */
	cmp	r0, r2			/* until source end addreee [r2]    */
	ble	copy_loop
#endif
#endif	/* CONFIG_SKIP_RELOCATE_UBOOT */
（8）清除bss段
clear_bss:
	ldr	r0, _bss_start		/* find start of bss segment        */
	ldr	r1, _bss_end		/* stop here                        */
	mov 	r2, #0x00000000		/* clear 
clbss_l:str	r2, [r0]		/* clear loop...                    */
	add	r0, r0, #4
	cmp	r0, r1
	ble	clbss_l
（9）跳转至第二阶段
    ldr	pc, _start_armboot
_start_armboot:	.word start_armboot

2、第二阶段，c语言程序（lib_arm/board.c）

（1）lib_arm/board.c

1、设置运行环境
/* Pointer is writable since we allocated a register for it */
	gd = (gd_t*)(_armboot_start - CFG_MALLOC_LEN - sizeof(gd_t));
	/* compiler optimization barrier needed for GCC >= 3.4 */
	__asm__ __volatile__("": : :"memory");
 
	memset ((void*)gd, 0, sizeof (gd_t));
	gd->bd = (bd_t*)((char*)gd - sizeof(bd_t));
	memset (gd->bd, 0, sizeof (bd_t));
 
typedef struct global_data {
	bd_t *bd;
	unsigned long flags;
	unsigned long board_type;
	unsigned long baudrate;
	unsigned long have_console;	/* serial_init() was called */
	unsigned long ram_size;		/* RAM size */
	unsigned long reloc_off;	/* Relocation Offset */
	unsigned long env_addr;		/* Address  of Environment struct */
	unsigned long env_valid;	/* Checksum of Environment valid? */
	void **jt;			/* jump table */
} gd_t;
（2）进一步初始化硬件
	monitor_flash_len = _bss_start - _armboot_start;
 
	for (init_fnc_ptr = init_sequence; *init_fnc_ptr; ++init_fnc_ptr) {
		if ((*init_fnc_ptr)() != 0) {
			hang ();
		}
	}
init_fnc_t *init_sequence[] = {
	cpu_init,		/* basic cpu dependent setup */
	board_init,		/* basic board dependent setup */
	dram_init,		/* configure available RAM banks */
	mem_malloc_init,        /* dependant on dram_init */
	interrupt_init,		/* set up exceptions */
	timer_init,
	serial_init,
	env_init,		/* initialize environment */
	init_baudrate,		/* initialze baudrate settings */
	serial_init,		/* serial communications setup */
	display_banner,
	display_dram_config,
 
	NULL,
};
 
int board_init (void)    // board/mini2440/mini2440.c
{
    S3C24X0_CLOCK_POWER * const clk_power = S3C24X0_GetBase_CLOCK_POWER();
    S3C24X0_GPIO * const gpio = S3C24X0_GetBase_GPIO();
 
    /* set up the I/O ports */
    gpio->GPACON = 0x007FFFFF;
    gpio->GPBCON = 0x00044555;
    gpio->GPBUP = 0x000007FF;
    gpio->GPCCON = 0xAAAAAAAA;
    gpio->GPCUP = 0x0000FFFF;
    gpio->GPDCON = 0xAAAAAAAA;
    gpio->GPDUP = 0x0000FFFF;
    gpio->GPECON = 0xAAAAAAAA;
    gpio->GPEUP = 0x0000FFFF;
    gpio->GPFCON = 0x000055AA;
    gpio->GPFUP = 0x000000FF;
    gpio->GPGCON = 0xFF95FFBA;
    gpio->GPGUP = 0x0000FFFF;
    gpio->GPHCON = 0x002AFAAA;
    gpio->GPHUP = 0x000007FF;
 
    /* support both of S3C2410 and S3C2440, by www.arm9.net */
    if ((gpio->GSTATUS1 == 0x32410000) || (gpio->GSTATUS1 == 0x32410002))
    {
        /* arch number of SMDK2410-Board */
        gd->bd->bi_arch_number = MACH_TYPE_SMDK2410;
    }
    else
    {
        /* arch number of SMDK2440-Board */
        gd->bd->bi_arch_number = MACH_TYPE_S3C2440;    //机器ID，内核启动时会使用
    }
 
    /* adress of boot parameters */
    gd->bd->bi_boot_params = 0x30000100;
    icache_enable();
#if 0
    dcache_enable();
#endif
    return 0;
}
 
（3）环境变量初始化（uboot命令行的printenv）
	/* initialize environment */
	env_relocate ();
（4）uboot主循环
	/* main_loop() can return to retry autoboot, if so just run it again. */
	for (;;) {
		main_loop ();
	}

（2）common/main.c

1、获取bootcmd，如果没有命令则启动内核
		s = getenv ("bootcmd");
 
	debug ("### main_loop: bootcmd=\"%s\"\n", s ? s : "");
 
	if (bootdelay >= 0 && s && !abortboot (bootdelay)) {
# ifdef CONFIG_AUTOBOOT_KEYED
		int prev = disable_ctrlc(1);	/* disable Control C checking */
# endif
 
# ifndef CFG_HUSH_PARSER
#ifdef CONFIG_SURPORT_WINCE
        if (!TOC_Read())
        {
            /* Launch wince */
            char cmd_buf[16];
            printf("Booting wince ...\n");            
            strcpy(cmd_buf, "wince");
            run_command(cmd_buf, 0);
        }
        else
#endif
        {
            printf("Booting Linux ...\n");            
    	    run_command (s, 0);
        }
# else
		parse_string_outer(s, FLAG_PARSE_SEMICOLON |
				    FLAG_EXIT_FROM_LOOP);
# endif
 
（2）获取命令行命令，执行命令
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 ("\n");
		else
			rc = run_command (lastcommand, flag);
 
		if (rc <= 0) {
			/* invalid command or not repeatable, forget it */
			lastcommand[0] = 0;
		}
	}