[A133]uboot启动流程

hongxi.zhu 2024-6-21

1. 第一阶段

lds描述

从u-boot.lds中能找到程序的汇编入口ENTRY(_start)

brandy/brandy-2.0/u-boot-2018/u-boot.lds

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

ENTRY(_start)的实现在start.S中，根据平台架构, 有对应的实现，当前平台是armv8

_start

brandy/brandy-2.0/u-boot-2018/arch/arm/cpu/armv8/start.S

.globl	_start
_start:
#if defined(LINUX_KERNEL_IMAGE_HEADER)
#include <asm/boot0-linux-kernel-header.h>
#elif defined(CONFIG_ENABLE_ARM_SOC_BOOT0_HOOK)
/*
 * Various SoCs need something special and SoC-specific up front in
 * order to boot, allow them to set that in their boot0.h file and then
 * use it here.
 */
#include <asm/arch/boot0.h>
#else
	b	reset  /* 跳转到reset块 */
#endif

reset

brandy/brandy-2.0/u-boot-2018/arch/arm/cpu/armv8/start.S

reset:
	/* Allow the board to save important registers */
	b	save_boot_params  /*空实现，并跳转回save_boot_params_ret*/
.globl	save_boot_params_ret
save_boot_params_ret:

	/*
	 * Could be EL3/EL2/EL1, Initial State:
	 * Little Endian, MMU Disabled, i/dCache Disabled
	 */
	adr	x0, vectors 	/*将异常向量表基地址写到x0*/
	switch_el x1, 3f, 2f, 1f	/*根据异常等级选择el3/el2/el1情况处理*/
3:	msr	vbar_el3, x0  /*el3*/
	mrs	x0, scr_el3
	orr	x0, x0, #0xf			/* SCR_EL3.NS|IRQ|FIQ|EA */
	msr	scr_el3, x0
	msr	cptr_el3, xzr			/* Enable FP/SIMD */
	b	0f	/*设置完上述相关寄存器，跳出*/
2:	msr	vbar_el2, x0	/*el2*/
	mov	x0, #0x33ff
	msr	cptr_el2, x0			/* Enable FP/SIMD */
	b	0f
1:	msr	vbar_el1, x0	/*el1*/
	mov	x0, #3 << 20
	msr	cpacr_el1, x0			/* Enable FP/SIMD */
0:  /* 空执行，相当于跳出 */


	/* Apply ARM core specific erratas */
	bl	apply_core_errata  /*arm核的特殊配置*/

	/*
	 * Cache/BPB/TLB Invalidate
	 * i-cache is invalidated before enabled in icache_enable()
	 * tlb is invalidated before mmu is enabled in dcache_enable()
	 * d-cache is invalidated before enabled in dcache_enable()
	 */

	/* Processor specific initialization */
	bl	lowlevel_init  /* A133看起来没做啥事情 */

master_cpu:
	bl	_main  /*跳转到_main*/

_main

brandy/brandy-2.0/u-boot-2018/arch/arm/lib/crt0_64.S

ENTRY(_main)

/*
 * Set up initial C runtime environment and call board_init_f(0).
 */
 	/*清除x0寄存器的最低四位(#0xf即二进制1111)并赋值给sp栈指针, x0寄存器的值是调用方传递*/
	bic	sp, x0, #0xf	/* 16-byte alignment for ABI compliance */
	mov	x0, sp		/*将栈指针sp的值写入x0寄存器作为下一条bl命令的参数，这个参数就是global区域的顶部地址top*/
	bl	board_init_f_alloc_reserve		/*跳转到C中的board_init_f_alloc_reserve，给全局gd变量分配内存*/
	mov	sp, x0		/*上面的bl命令执行后，它的返回值存放在x0中，将这个新的栈指针值赋值给sp指针*/
	/* set up gd here, outside any C code */
	mov	x18, x0		/*x18寄存器用作全局数据（Global Data, gd）的指针，上面board_init_f_alloc_reserve的返回值就是需要设置的gd的指针地址*/
	bl	board_init_f_init_reserve	/*跳转到C中的board_init_f_init_reserve， 初始化gd变量的内容为0，并确定后续分配的gd变量内容偏移基地址*/

	mov	x0, #0		/*x0清零，下一个bl命令传入参数值为0*/
	bl	board_init_f		/*跳转到C中的board_init_f初始化一些早期硬件，并为重定位准备*/

#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	x0, [x18, #GD_START_ADDR_SP]	/* x0 <- gd->start_addr_sp */
	bic	sp, x0, #0xf	/* 16-byte alignment for ABI compliance */
	ldr	x18, [x18, #GD_NEW_GD]		/* x18 <- gd->new_gd */

	adr	lr, relocation_return

	/* Add in link-vs-relocation offset */
	ldr	x9, [x18, #GD_RELOC_OFF]	/* x9 <- gd->reloc_off */
	add	lr, lr, x9	/* new return address after relocation */
	ldr	x0, [x18, #GD_RELOCADDR]	/* x0 <- gd->relocaddr */
	b	relocate_code

relocation_return:

/*
 * Set up final (full) environment
 */
	bl	c_runtime_cpu_setup		/* still call old routine */
#endif /* !CONFIG_SPL_BUILD */

/*
 * Clear BSS section
 */
	ldr	x0, =__bss_start		/* this is auto-relocated! */
	ldr	x1, =__bss_end			/* this is auto-relocated! */
clear_loop:
	str	xzr, [x0], #8
	cmp	x0, x1
	b.lo	clear_loop

	/* call board_init_r(gd_t *id, ulong dest_addr) */
	mov	x0, x18				/* gd_t */
	ldr	x1, [x18, #GD_RELOCADDR]	/* dest_addr */
	b	board_init_r			/* PC relative jump */

	/* NOTREACHED - board_init_r() does not return */

ENDPROC(_main)

board_init_f_alloc_reserve

brandy/brandy-2.0/u-boot-2018/common/init/board_init.c

ulong board_init_f_alloc_reserve(ulong top)
{
	...
	/* LAST : reserve GD (rounded up to a multiple of 16 bytes) */
 	// 从global区域分配一块大小为struct global_data大小的16字节对齐的内存存放全局的global_data变量
    // top就是x0寄存器传入的值(具体值需要调试才知道)
	top = rounddown(top-sizeof(struct global_data), 16);

	return top;
}

board_init_f_init_reserve

brandy/brandy-2.0/u-boot-2018/common/init/board_init.c

void board_init_f_init_reserve(ulong base)
{
	struct global_data *gd_ptr;

	/*
	 * clear GD entirely and set it up.
	 * Use gd_ptr, as gd may not be properly set yet.
	 */

	gd_ptr = (struct global_data *)base;  // 拿到寄存器x18里存的gd变量内存地址
	/* zero the area */
	memset(gd_ptr, '\0', sizeof(*gd));  //将这块内存数据，初始化为0
	/* set GD unless architecture did it already */
	...
	/* next alloc will be higher by one GD plus 16-byte alignment */
	base += roundup(sizeof(struct global_data), 16);  //内存向上对齐16字节，这个地址就是gd变量后续分配内容的偏移基地址

	/*
	 * record early malloc arena start.
	 * Use gd as it is now properly set for all architectures.
	 */

	...
}

board_init_f

brandy/brandy-2.0/u-boot-2018/common/board_f.c

void board_init_f(ulong boot_flags)
{
	gd->flags = boot_flags; // 这里汇编传入的x0是0，所以boot_flags = 0
	gd->have_console = 0;

    //执行init_sequence_f数组中的每一个函数指针,会依次初始化cpu/dm/外设总线/串口/optee/等
	if (initcall_run_list(init_sequence_f))
		hang();
}

init_sequence_f数组中特别注意的是下面的函数，将接下来的链接重定位息息相关：

static const init_fnc_t init_sequence_f[] = {
	setup_mon_len, /*设置内存区域长度 gd->mon_len = __bss_end - __image_copy_start;*/
#ifdef CONFIG_OF_CONTROL
	fdtdec_setup, /*设置dtb的地址gd->fdt_blob*/
#endif
#ifdef CONFIG_TRACE_EARLY
	trace_early_init,
#endif
	initf_malloc,  //看起来这里并没有实现f_malloc的区域，也许别的地方实现了
	log_init,
	initf_bootstage,	/* uses its own timer, so does not need DM */
	initf_console_record,
#if defined(CONFIG_HAVE_FSP)
	arch_fsp_init,
#endif
	arch_cpu_init,		/* basic arch cpu dependent setup */
	mach_cpu_init,		/* SoC/machine dependent CPU setup */
	initf_dm,
	arch_cpu_init_dm,
#if defined(CONFIG_BOARD_EARLY_INIT_F)
	board_early_init_f,
#endif
#if defined(CONFIG_PPC) || defined(CONFIG_SYS_FSL_CLK) || defined(CONFIG_M68K)
	/* get CPU and bus clocks according to the environment variable */
	get_clocks,		/* get CPU and bus clocks (etc.) */
#endif
#if !defined(CONFIG_M68K)
	timer_init,		/* initialize timer */
#endif
#if defined(CONFIG_BOARD_POSTCLK_INIT)
	board_postclk_init,
#endif
	env_init,		/* initialize environment */  //初始化env驱动
	init_baud_rate,		/* initialze baudrate settings */  //从env读取出设置的串口波特率
	serial_init,		/* serial communications setup */  //初始化串口
	console_init_f,		/* stage 1 init of console */  //从uboot的设备树中获取调试等级debug_mode等
	display_options,	/* say that we are here */  //"打印 U-Boot 2018.07 (xxxx) Allwinner Technology"
	display_text_info,	/* show debugging info if required *///打印bss段的内存起止地址(未初始化全局变量数据段)、text段内存起始地址(代码段)
#if defined(CONFIG_PPC) || defined(CONFIG_SH) || defined(CONFIG_X86)
	checkcpu,
#endif
#if defined(CONFIG_DISPLAY_CPUINFO)
	print_cpuinfo,		/* display cpu info (and speed) */
#endif
#if defined(CONFIG_DTB_RESELECT)
	embedded_dtb_select,
#endif
#if defined(CONFIG_DISPLAY_BOARDINFO)
	show_board_info,
#endif
	INIT_FUNC_WATCHDOG_INIT
#if defined(CONFIG_MISC_INIT_F)
	misc_init_f,
#endif
	INIT_FUNC_WATCHDOG_RESET
#if defined(CONFIG_SYS_I2C)
	init_func_i2c,
#endif

#if defined(CONFIG_VID) && !defined(CONFIG_SPL)
	init_func_vid,
#endif
#if defined(CONFIG_HARD_SPI)
	init_func_spi,
#endif
#if defined(CONFIG_OPTEE25)
	smc_init,
#endif

	announce_dram_init,
	dram_init,		/* configure available RAM banks */
#ifdef CONFIG_POST
	post_init_f,
#endif
	INIT_FUNC_WATCHDOG_RESET
#if defined(CONFIG_SYS_DRAM_TEST)
	testdram,
#endif /* CONFIG_SYS_DRAM_TEST */
	INIT_FUNC_WATCHDOG_RESET

#ifdef CONFIG_POST
	init_post,
#endif
	INIT_FUNC_WATCHDOG_RESET
	/*
	 * Now that we have DRAM mapped and working, we can
	 * relocate the code and continue running from DRAM.
	 *
	 * Reserve memory at end of RAM for (top down in that order):
	 *  - area that won't get touched by U-Boot and Linux (optional)
	 *  - kernel log buffer
	 *  - protected RAM
	 *  - LCD framebuffer
	 *  - monitor code
	 *  - board info struct
	 */
	setup_dest_addr,  //获取内存区域的top地址
#ifdef CONFIG_PRAM
	reserve_pram,
#endif
	reserve_round_4k,  //内存4K对齐
#ifdef CONFIG_ARM
	reserve_mmu,
#endif
	reserve_video,
	reserve_trace,
	reserve_uboot,
	reserve_malloc,
	reserve_board,
	setup_machine,
	reserve_global_data,
	reserve_fdt,
	reserve_bootstage,
	reserve_arch,
	reserve_stacks,
	dram_init_banksize,
	show_dram_config,
#if defined(CONFIG_M68K) || defined(CONFIG_MIPS) || defined(CONFIG_PPC) || \
	defined(CONFIG_SH)
	setup_board_part1,
#endif
#if defined(CONFIG_PPC) || defined(CONFIG_M68K)
	INIT_FUNC_WATCHDOG_RESET
	setup_board_part2,
#endif
	display_new_sp,
#ifdef CONFIG_OF_BOARD_FIXUP
	fix_fdt,
#endif
	INIT_FUNC_WATCHDOG_RESET
	reloc_fdt,
	reloc_bootstage,
	setup_reloc,
#if defined(CONFIG_X86) || defined(CONFIG_ARC)
	copy_uboot_to_ram,
	do_elf_reloc_fixups,
	clear_bss,
#endif
#if defined(CONFIG_XTENSA)
	clear_bss,
#endif
#if !defined(CONFIG_ARM) && !defined(CONFIG_SANDBOX) && \
		!CONFIG_IS_ENABLED(X86_64)
	jump_to_copy,
#endif
	NULL,
};

init_sequence_f数组中特别注意的是下面的函数，将接下来的链接重定位内存结构息息相关

	setup_dest_addr,  //获取内存区域的top地址
	reserve_round_4k,  //内存4K对齐
	reserve_mmu,  //在内存中为MMU TLB页表分配16KB内存
	reserve_video,  //fb相关，这里空实现
	reserve_uboot,  //在内存中为U-Boot text, data & bss段分配790KB内存
	reserve_malloc,  //在内存中为malloc区域预留500MB内存
	reserve_board,  //在内存中为board info预留96 Byte内存
	reserve_global_data,    // 在内存中为global_data预留336 Byte内存
	reserve_fdt,  // 在内存中为fdt 设备树预留149KB内存
	reserve_stacks,  // 为栈空间预留空间(IRQ stack和stack)。sp指针指向的base地址
	dram_init_banksize,  //空实现

相关打印

Ram size: 40000000  // 1GB
Ram top: 80000000  //高地址
TLB table from 7fff0000 to 7fff4000
Reserving 790k for U-Boot at: 7ff2a000
Reserving 512128k for malloc() at: 60b0a000
Reserving 96 Bytes for Board Info at: 60b09fa0
Reserving 336 Bytes for Global Data at: 60b09e50
Reserving 153216 Bytes for FDT at: 60ae47d0
Reserving 8192 Bytes for IRQ stack at: 60ae27c0
New Stack Pointer is: 60ae27b0
[01.588]Relocation Offset is: 35f2a000
Relocating to 7ff2a000, new gd at 60b09e50, sp at 60ae27b0

为即将重定位做的内存分配，此时结构如下

reloc_fdt

重定位设备树文件

brandy/brandy-2.0/u-boot-2018/common/board_f.c

static int reloc_fdt(void)
{
#ifndef CONFIG_OF_EMBED
	,..
	if (gd->new_fdt) {  //前面的reserved fdt就分配了内存区域
		memcpy(gd->new_fdt, gd->fdt_blob, gd->fdt_size);  //将fdt文件拷贝到内存的新地址中
		gd->fdt_blob = gd->new_fdt;  //修改gd中fdt地址指向
	}
#endif

	return 0;
}

setup_reloc

重定位uboot text&bss&data段到内存中的新地址

brandy/brandy-2.0/u-boot-2018/common/board_f.c

static int setup_reloc(void)
{
	//gd->relocaddr就是前面内存模型中给uboot分配的空间的基地址
    //__image_copy_start是lds文件中描述拷贝uboot的起始地址
	gd->reloc_off = gd->relocaddr - (unsigned long)__image_copy_start;  //重新设置gd中uboot重定位地址偏移

	memcpy(gd->new_gd, (char *)gd, sizeof(gd_t));  //将gd从flash中拷贝内存中的global data区域, 后续都使用new_gd

	tick_printf("Relocation Offset is: %08lx\n", gd->reloc_off);
	debug("Relocating to %08lx, new gd at %08lx, sp at %08lx\n",
	      gd->relocaddr, (ulong)map_to_sysmem(gd->new_gd),
	      gd->start_addr_sp);

	return 0;
}

到这里重定位的准备工作都做完了，board_init_f函数执行结束并返回到_main汇编标签往下执行

/*
 * 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	x0, [x18, #GD_START_ADDR_SP]	/* x0 <- gd->start_addr_sp */
	bic	sp, x0, #0xf	/* 16-byte alignment for ABI compliance */
	ldr	x18, [x18, #GD_NEW_GD]		/* x18 <- gd->new_gd */

	adr	lr, relocation_return

	/* Add in link-vs-relocation offset */
	ldr	x9, [x18, #GD_RELOC_OFF]	/* x9 <- gd->reloc_off */
	add	lr, lr, x9	/* new return address after relocation */
	ldr	x0, [x18, #GD_RELOCADDR]	/* x0 <- gd->relocaddr */
	b	relocate_code

relocation_return:

逐行解析：

ldr	x0, [x18, #GD_START_ADDR_SP]	/* x0 <- gd->start_addr_sp */

使用ARM64的ldr指令从寄存器x18指向的结构体中加载GD_START_ADDR_SP偏移量处的值到寄存器x0。实际上x18是gd的指针。

GD_START_ADDR_SP从下面的汇编到C的map可知是gd->start_addr_sp

a133_linux/brandy/brandy-2.0/u-boot-2018/lib/asm-offsets.c

DEFINE(GD_START_ADDR_SP, offsetof(struct global_data, start_addr_sp));

bic	sp, x0, #0xf	/* 16-byte alignment for ABI compliance */

bic（位清除）指令将x0（即gd->start_addr_sp）的低4位清零，以确保栈指针（sp）是16字节对齐的。这是为了满足ABI的要求

ldr	x18, [x18, #GD_NEW_GD]		/* x18 <- gd->new_gd */

更新x18寄存器，使其指向gd->new_gd。代码重定位后，全局描述符的gd变量指向需要改变

adr	lr, relocation_return

使用adr（地址寄存器）指令获取relocation_return标签的当前地址，并将其存储在链接寄存器（lr）中。这将是代码重定位后的返回地址。

ldr	x9, [x18, #GD_RELOC_OFF]	/* x9 <- gd->reloc_off */

从gd(new_gd)结构体中加载GD_RELOC_OFF（可能是重定位偏移量）到x9

add	lr, lr, x9	/* new return address after relocation */

将x9（重定位偏移量）加到lr寄存器中，以得到重定位后的最终返回地址

ldr	x0, [x18, #GD_RELOCADDR]	/* x0 <- gd->relocaddr */

从gd(new_gd)结构体中加载GD_RELOCADDR到x0

b	relocate_code

跳转到relocate_code标签执行，传入的参数为GD_RELOCADDR

relocate_code

/*
 * void relocate_code (addr_moni)
 *
 * This function relocates the monitor code.
 * x0 holds the destination address.
 */
ENTRY(relocate_code)
	stp	x29, x30, [sp, #-32]!	/* create a stack frame */
	mov	x29, sp
	str	x0, [sp, #16]
	/*
	 * Copy u-boot from flash to RAM
	 */
	adr	x1, __image_copy_start	/* x1 <- Run &__image_copy_start */
	subs	x9, x0, x1		/* x8 <- Run to copy offset */
	b.eq	relocate_done		/* skip relocation */
	/*
	 * Don't ldr x1, __image_copy_start here, since if the code is already
	 * running at an address other than it was linked to, that instruction
	 * will load the relocated value of __image_copy_start. To
	 * correctly apply relocations, we need to know the linked value.
	 *
	 * Linked &__image_copy_start, which we know was at
	 * CONFIG_SYS_TEXT_BASE, which is stored in _TEXT_BASE, as a non-
	 * relocated value, since it isn't a symbol reference.
	 */
	ldr	x1, _TEXT_BASE		/* x1 <- Linked &__image_copy_start */
	subs	x9, x0, x1		/* x9 <- Link to copy offset */

	adr	x1, __image_copy_start	/* x1 <- Run &__image_copy_start */
	adr	x2, __image_copy_end	/* x2 <- Run &__image_copy_end */
copy_loop:
	ldp	x10, x11, [x1], #16	/* copy from source address [x1] */
	stp	x10, x11, [x0], #16	/* copy to   target address [x0] */
	cmp	x1, x2			/* until source end address [x2] */
	b.lo	copy_loop
	str	x0, [sp, #24]

	/*
	 * Fix .rela.dyn relocations
	 */
	adr	x2, __rel_dyn_start	/* x2 <- Run &__rel_dyn_start */
	adr	x3, __rel_dyn_end	/* x3 <- Run &__rel_dyn_end */
fixloop:
	ldp	x0, x1, [x2], #16	/* (x0,x1) <- (SRC location, fixup) */
	ldr	x4, [x2], #8		/* x4 <- addend */
	and	x1, x1, #0xffffffff
	cmp	x1, #R_AARCH64_RELATIVE
	bne	fixnext

	/* relative fix: store addend plus offset at dest location */
	add	x0, x0, x9
	add	x4, x4, x9
	str	x4, [x0]
fixnext:
	cmp	x2, x3
	b.lo	fixloop

relocate_done:
	switch_el x1, 3f, 2f, 1f
	bl	hang
3:	mrs	x0, sctlr_el3
	b	0f
2:	mrs	x0, sctlr_el2
	b	0f
1:	mrs	x0, sctlr_el1
0:	tbz	w0, #2, 5f	/* skip flushing cache if disabled */
	tbz	w0, #12, 4f	/* skip invalidating i-cache if disabled */
	ic	iallu		/* i-cache invalidate all */
	isb	sy
4:	ldp	x0, x1, [sp, #16]
	bl	__asm_flush_dcache_range
5:	ldp	x29, x30, [sp],#32
	ret
ENDPROC(relocate_code)

这个的内容比较繁杂涉及复杂的栈操作和链接操作，主要做了两件事：

• 拷贝flash上面的uboot代码段到内存中
• 重置链接中的.rela.dyn内容(与重定位相关)

完成重定位后，返回到_main中，进入uboot第二阶段

2. 第二阶段

relocation_return:

/*
 * Set up final (full) environment
 */
	bl	c_runtime_cpu_setup		/* still call old routine */  /*重定向向量表基址寄存器*/
#endif /* !CONFIG_SPL_BUILD */

/*
 * Clear BSS section  重定向后需要清除bss段内容，为接下来运行重定向的uboot做准备
 */
	ldr	x0, =__bss_start		/* this is auto-relocated! */
	ldr	x1, =__bss_end			/* this is auto-relocated! */
clear_loop:
	str	xzr, [x0], #8
	cmp	x0, x1
	b.lo	clear_loop

	/* call board_init_r(gd_t *id, ulong dest_addr) */
	mov	x0, x18				/* gd_t */  /*传递新的gd的地址*/
	ldr	x1, [x18, #GD_RELOCADDR]	/* dest_addr */  /*gd->relocaddr就是重定向后，uboot在内存上运行的入口地址*/
	b	board_init_r			/* PC relative jump */  /*入口函数就是board_init_r函数，且不再返回*/

	/* NOTREACHED - board_init_r() does not return */

ENDPROC(_main)

board_init_r

brandy/brandy-2.0/u-boot-2018/common/board_r.c

void board_init_r(gd_t *new_gd, ulong dest_addr)
{
	gd->flags &= ~GD_FLG_LOG_READY;

	if (initcall_run_list(init_sequence_r))
		hang();

	/* NOTREACHED - run_main_loop() does not return */
	hang();
}

static init_fnc_t init_sequence_r[] = {
	initr_trace,
	initr_reloc,  // 更新fdt的地址到env fdtaddr字段中
	/* TODO: could x86/PPC have this also perhaps? */
#ifdef CONFIG_ARM
	initr_caches,  //配置cpu smp和使能icache和dcache
	/* Note: For Freescale LS2 SoCs, new MMU table is created in DDR.
	 *	 A temporary mapping of IFC high region is since removed,
	 *	 so environmental variables in NOR flash is not available
	 *	 until board_init() is called below to remap IFC to high
	 *	 region.
	 */
#endif
	initr_reloc_global_data,  //更新gd中一些地址偏移，A133这里基本空实现
	initr_barrier,  //空实现
	initr_malloc,  //初始化malloc区域
	log_init,
	initr_bootstage,	/* Needs malloc() but has its own timer */
	initr_console_record,  //空实现
	bootstage_relocate,
#ifdef CONFIG_DM
	initr_dm,  //初始化dm驱动框架
#endif
#if defined(CONFIG_ARM) || defined(CONFIG_NDS32) || defined(CONFIG_RISCV)
	board_init,	/* Setup chipselects */  //全志平台相关的初始化，如CPU/axp Power/rtc/dma等
#endif

	stdio_init_tables,  //stdio相关的配置
	initr_serial,  //串口驱动初始化
	initr_announce,  //打印说明当前uboot是运行在ram中
#ifdef CONFIG_SUNXI_LEDC
	initr_ledc,  // led demo
#endif
	power_init_board,  //空实现
#ifdef CONFIG_MMC
	initr_mmc, //初始化mmc设备驱动，空实现
#endif
	initr_secondary_cpu,  //初始化第二个cpu，一般uboot都是只运行在cpu0上，这里空实现
	stdio_add_devices,  //注册标准输入输出的各种函数，如puts/gets/tstc
	initr_jumptable,  //初始化函数跳转表，例如上面的stdio的这些函数
	console_init_r,		/* fully init console as a device */  //初始化终端输出，可以是串口或者lcd，并映射stdout/stdin/stderr
	interrupt_init,  //空实现
#ifdef CONFIG_ARM
	initr_enable_interrupts,  //空实现
#endif
#ifdef CONFIG_SUNXI_FAST_BURN_KEY
	sunxi_fast_burn_key,  //快速烧key相关功能
#endif
#ifdef CONFIG_ARCH_SUNXI
	  initr_sunxi_plat, //初始化flash相关
#endif
	board_env_late_init,  //获取pmu bootreason和keybox初始化
#ifdef CONFIG_ARCH_SUNXI
	sunxi_burn_key,  //烧key功能
#endif

	run_main_loop,  //进入主循环，引导内核或者进入cli
};

因为uboot相当于重新运行了，所以init_sequence_r中首先需要做各种初始化，最后进入引导linux内核启动流程或者进入cli_loop循坏中。