1、_start
_start是u-boot启动后的第一个执行地址，对于armv7来说，它就是简单的跳转到rest处执行和初始化异常向量表
vim arch/arm/lib/vectors.S
_start:2、reset
#ifdef CONFIG_SYS_DV_NOR_BOOT_CFG
.word CONFIG_SYS_DV_NOR_BOOT_CFG
#endif
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
vim arch/arm/cpu/armv7/start.S
reset:执行cpu底层的初始化，bl cpu_init_crit 调用了lowlevel_init（board/samsung/smdkv210/lowlever.S）
/* Allow the board to save important registers */
b save_boot_params
save_boot_params_ret:
#ifdef CONFIG_ARMV7_LPAE
/*
* check for Hypervisor support
*/
mrc p15, 0, r0, c0, c1, 1 @ read ID_PFR1
and r0, r0, #CPUID_ARM_VIRT_MASK @ mask virtualization bits
cmp r0, #(1 << CPUID_ARM_VIRT_SHIFT)
beq switch_to_hypervisor
switch_to_hypervisor_ret:
#endif
/*
* disable interrupts (FIQ and IRQ), also set the cpu to SVC32 mode,
* except if in HYP mode already
*/
mrs r0, cpsr
and r1, r0, #0x1f @ mask mode bits
teq r1, #0x1a @ test for HYP mode
bicne r0, r0, #0x1f @ clear all mode bits
orrne r0, r0, #0x13 @ set SVC mode
orr r0, r0, #0xc0 @ disable FIQ and IRQ
msr/*
* Setup vector:
* (OMAP4 spl TEXT_BASE is not 32 byte aligned.
* Continue to use ROM code vector only in OMAP4 spl)
*/
#if !(defined(CONFIG_OMAP44XX) && defined(CONFIG_SPL_BUILD))
/* Set V=0 in CP15 SCTLR register - for VBAR to point to vector */
mrc p15, 0, r0, c1, c0, 0 @ Read CP15 SCTLR Register
bic r0, #CR_V @ V = 0
mcr p15, 0, r0, c1, c0, 0 @ Write CP15 SCTLR Register
/* Set vector address in CP15 VBAR register */
ldr r0, =_start
mcr p15, 0, r0, c12, c0, 0 @Set VBAR
#endif
/* the mask ROM code should have PLL and others stable */
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
bl cpu_init_cp15
#ifndef CONFIG_SKIP_LOWLEVEL_INIT_ONLY
bl cpu_init_crit
#endif
#endif
bl _main
然后跳转到crt0.S中的main函数
3、_main
vim arch/arm/lib/crt0.S
ENTRY(_main)/* * Set up initial C runtime environment and call board_init_f(0). *//* modied by Sourcelink */#if !defined(CONFIG_SPL_BUILD) ldr sp, =(CONFIG_SYS_INIT_SP_ADDR) bic sp, sp, #7 /* 8-byte alignment for ABI compliance */ mov r0, sp bl board_init_f_alloc_reserve mov sp, r0 /* set up gd here, outside any C code */ mov r9, r0 bl board_init_f_init_reserve mov r0, #0#endif/* modied by Sourcelink */#ifdef CONFIG_SPL_BUILD bl copy_bl2_to_ram /* 拷贝BL2到DDR */ ldr pc, =CONFIG_SYS_SDRAM_BASE /* 跳转到DDR的起始地址执行BL2 */#else bl board_init_f#endif#if ! defined(CONFIG_SPL_BUILD)/* * Set up intermediate environment (new sp and gd) and call * relocate_code(addr_moni). Trick here is that we'll return * 'here' but relocated. */ ldr sp, [r9, #GD_START_ADDR_SP] /* sp = gd->start_addr_sp */#if defined(CONFIG_CPU_V7M) /* v7M forbids using SP as BIC destination */ mov r3, sp bic r3, r3, #7 mov sp, r3#else bic sp, sp, #7 /* 8-byte alignment for ABI compliance */#endif ldr r9, [r9, #GD_BD] /* r9 = gd->bd */ sub r9, r9, #GD_SIZE /* new GD is below bd */ adr lr, here ldr r0, [r9, #GD_RELOC_OFF] /* r0 = gd->reloc_off */ add lr, lr, r0#if defined(CONFIG_CPU_V7M) orr lr, #1 /* As required by Thumb-only */#endif ldr r0, [r9, #GD_RELOCADDR] /* r0 = gd->relocaddr */ b relocate_codehere:/* * now relocate vectors */ bl relocate_vectors/* Set up final (full) environment */ bl c_runtime_cpu_setup /* we still call old routine here */#endif#if !defined(CONFIG_SPL_BUILD) || defined(CONFIG_SPL_FRAMEWORK)# ifdef CONFIG_SPL_BUILD /* Use a DRAM stack for the rest of SPL, if requested */ bl spl_relocate_stack_gd cmp r0, #0 movne sp, r0 movne r9, r0# endif ldr r0, =__bss_start /* this is auto-relocated! */#ifdef CONFIG_USE_ARCH_MEMSET ldr r3, =__bss_end /* this is auto-relocated! */ mov r1, #0x00000000 /* prepare zero to clear BSS */ subs r2, r3, r0 /* r2 = memset len */ bl memset#else ldr r1, =__bss_end /* this is auto-relocated! */ mov r2, #0x00000000 /* prepare zero to clear BSS */clbss_l:cmp r0, r1 /* while not at end of BSS */#if defined(CONFIG_CPU_V7M) itt lo#endif strlo r2, [r0] /* clear 32-bit BSS word */ addlo r0, r0, #4 /* move to next */ blo clbss_l#endif#if ! defined(CONFIG_SPL_BUILD) bl coloured_LED_init bl red_led_on#endif /* call board_init_r(gd_t *id, ulong dest_addr) */ mov r0, r9 /* gd_t */ ldr r1, [r9, #GD_RELOCADDR] /* dest_addr */ /* call board_init_r */#if defined(CONFIG_SYS_THUMB_BUILD) ldr lr, =board_init_r /* this is auto-relocated! */ bx lr#else ldr pc, =board_init_r /* this is auto-relocated! */#endif /* we should not return here. */#endifENDPROC(_main)
1）设置c代码运行环境，为调用board_init_f接口做准备:
a) 先设置了堆栈指针。
b)调用board_init_f_alloc_reserve函数，从堆栈开始的地方，为u-boot中的global data 分配内存空间
c)调用board_init_f_init_reserve函数，对gd进行初始化
d)这两个函数都在common/init/board_init.c中
2）调用board_init_f函数，依次调用数组init_sequence_f中的函数完成前期的初始化工作:
a)主要调用init_baud_rate设置了串口波特率（common/board_f.c），接着调用serial_init初始化了串口（drivers/serial/serial.c）
b)调用display_options可以通过串口打印一些信息(lib/display_options.c)
c)调用printf_cpuinfo 答应cpu信息时钟和soc信号等（arch/arm/cpu/armv7/s5p-common/cpu_info.c）
3） 重新设置堆栈指针和gd -->重定位代码-->清除bss段-->点亮一个指示灯（自行添加 common/board_f.c）
4）调用board_init_r函数完成后续初始化工作（common/board_r.c）。