BootLoader分析：启动流程分析（以uboot为例）

代码入口：_start

从链接脚本中可知，uboot的入口地址为_start

# u-boot.lds

OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
OUTPUT_ARCH(arm)
ENTRY(_start)

_start如下

// arch/arm/lib/vectors.S

	.section ".vectors", "ax"
_start:
#ifdef CONFIG_SYS_DV_NOR_BOOT_CFG
	.word	CONFIG_SYS_DV_NOR_BOOT_CFG
#endif
	ARM_VECTORS
#endif /* !defined(CONFIG_ENABLE_ARM_SOC_BOOT0_HOOK) */

/*
 *************************************************************************
 *
 * Indirect vectors table
 *
 * Symbols referenced here must be defined somewhere else
 *
 *************************************************************************
 */

	.globl  _reset
	.globl	_undefined_instruction
	.globl	_software_interrupt
	.globl	_prefetch_abort
	.globl	_data_abort
	.globl	_not_used
	.globl	_irq
	.globl	_fiq

由上可知，这是arm32定义的规范的异常向量表，宏ARM_VECTORS定义如下

// arch/arm/lib/vectors.S

/*
 * A macro to allow insertion of an ARM exception vector either
 * for the non-boot0 case or by a boot0-header.
 */
        .macro ARM_VECTORS
#ifdef CONFIG_ARCH_K3
	ldr     pc, _reset
#else
	b	reset
#endif
	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
	.endm

查看reset内容

// arch/arm/cpu/armv7/start.S

reset:
	/* Allow the board to save important registers */
	b	save_boot_params

// arch/arm/cpu/armv7/start.S
ENTRY(save_boot_params)
	b	save_boot_params_ret		@ back to my caller
ENDPROC(save_boot_params)

// arch/arm/cpu/armv7/start.S

save_boot_params_ret:
	/*
	 * 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	cpsr,r0

save_boot_params_ret内容如上，由cpsr寄存器内容可知，r1获取处理器的当前工作模式：M[4:0]，并且设置SVC用户模式，同时屏幕FIQ和IRQ中断，写回cpsr寄存器

M[4:0]为10011表示SVC模式

设置SCTLR寄存器中的VBAR异常向量表基址

• V[13] = 0：异常向量表基址为0x0，该地址可以被re-mapped（重映射）
• V[13] = 1：异常向量表基址为0xffff0000，此时该地址不能被重映射

/*
 * 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

#ifdef CONFIG_HAS_VBAR
	/* Set vector address in CP15 VBAR register */
	ldr	r0, =_start
	mcr	p15, 0, r0, c12, c0, 0	@Set VBAR
#endif
#endif

接着是cpu_init_cp15和cpu_init_crit两部分

	/* the mask ROM code should have PLL and others stable */
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
#ifdef CONFIG_CPU_V7A
	bl	cpu_init_cp15
#endif
#ifndef CONFIG_SKIP_LOWLEVEL_INIT_ONLY
	bl	cpu_init_crit
#endif
#endif

cpu_init_cp15干了什么？

• 关闭MMU和清理缓存

/*************************************************************************
 *
 * cpu_init_cp15
 *
 * Setup CP15 registers (cache, MMU, TLBs). The I-cache is turned on unless
 * CONFIG_SYS_ICACHE_OFF is defined.
 *
 *************************************************************************/
ENTRY(cpu_init_cp15)
	// 
ENDPROC(cpu_init_cp15)

cpu_init_crit呢？与具体开发板相关

/*************************************************************************
 *
 * CPU_init_critical registers
 *
 * setup important registers
 * setup memory timing
 *
 *************************************************************************/
ENTRY(cpu_init_crit)
	/*
	 * Jump to board specific initialization...
	 * The Mask ROM will have already initialized
	 * basic memory. Go here to bump up clock rate and handle
	 * wake up conditions.
	 */
	b	lowlevel_init		@ go setup pll,mux,memory
ENDPROC(cpu_init_crit)

环境准备：_main

主要作用是用于初始化C语言环境

// arch/arm/lib/crt0.S

/*
 * entry point of crt0 sequence
 */

ENTRY(_main)

先来看前半段

/*
 * Set up initial C runtime environment and call board_init_f(0).
 */

#if defined(CONFIG_TPL_BUILD) && defined(CONFIG_TPL_NEEDS_SEPARATE_STACK)
	ldr	r0, =(CONFIG_TPL_STACK)
#elif defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_STACK)
	ldr	r0, =(CONFIG_SPL_STACK)
#else
	ldr	r0, =(CONFIG_SYS_INIT_SP_ADDR)
#endif
	bic	r0, r0, #7	/* 8-byte alignment for ABI compliance */
	mov	sp, r0
	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

#if defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_EARLY_BSS)
	CLEAR_BSS
#endif

	mov	r0, #0
	bl	board_init_f

设置sp寄存器指向CONFIG_SYS_INIT_SP_ADDR && 0x1000（8字节对齐）

board_init_f_alloc_reserve用于为早期malloc和GD全局数据区留出空间，如下，并且返回修改后的sp寄存器，并且在board_init_f_init_reserve初始化GD的值

GD保存了uboot中用到的所有全局变量

ulong board_init_f_alloc_reserve(ulong top)
{
   
	/* Reserve early malloc arena */
#if CONFIG_VAL(SYS_MALLOC_F_LEN)
	top -= CONFIG_VAL(SYS_MALLOC_F_LEN);
#endif
	/* LAST : reserve GD (rounded up to a multiple of 16 bytes) */
	top = rounddown(top-sizeof(struct global_data), 16);

	return top;
}

board_init_f用于调用init_sequence_f中的函数，如下，其作用是初始化uboot的一些功能以及填充GD表

void board_init_f(ulong boot_flags)
{
   
	gd->flags = boot_flags;
	gd->have_console = 0;

	if (initcall_run_list(init_sequence_f))
		hang();

#if !defined(CONFIG_ARM) && !defined(CONFIG_SANDBOX)