【u-boot】u-boot是怎么在H7开发板上运行起来的,

记录一下代码启动的过程：
研究具体代码之前，先了解其中的几个概念
概念一，gd结构体 global_data
为什么要有gd结构体及其作用。
概念二，u-boot代码的分段
主要是分为两段，前段和后段，这个对具体的代码行为会有不同的影响。

对于stm32来说，程序镜像都会存放在起始地址为0x08000000的flash中，其中地址0x08000000存放的是sp指针指向的内存值，0x08000004和之后的一段内存存放的中断向量表，对于Cortex-M7内核来说，中断一共支持256个。
在源码 arch\arm\lib\vectors_m.S中有相关的程序设置。

  .section  .vectors
ENTRY(_start)
	.long	CONFIG_SYS_INIT_SP_ADDR	@ 0 - Reset stack pointer
	.long	reset				    @ 1 - Reset
	.long	__invalid_entry			@ 2 - NMI
	.long	__hard_fault_entry		@ 3 - HardFault
	.long	__mm_fault_entry		@ 4 - MemManage
	.long	__bus_fault_entry		@ 5 - BusFault
	.long	__usage_fault_entry		@ 6 - UsageFault
	.long	__invalid_entry			@ 7 - Reserved
	.long	__invalid_entry			@ 8 - Reserved
	.long	__invalid_entry			@ 9 - Reserved
	.long	__invalid_entry			@ 10 - Reserved
	.long	__invalid_entry			@ 11 - SVCall
	.long	__invalid_entry			@ 12 - Debug Monitor
	.long	__invalid_entry			@ 13 - Reserved
	.long	__invalid_entry			@ 14 - PendSV
	.long	__invalid_entry			@ 15 - SysTick
	.rept	255 - 16
	.long	__invalid_entry			@ 16..255 - External Interrupts
	.endr

代码开头的 .section .vectors表明了这一段程序位于 .vectors段，在链接脚本中，该段被放在程序镜像的初始位置，上电复位的时候会先初始化sp，产生复位中断，执行复位中断函数reset，函数代码在文件 arm\cpu\armv7m\start.S中

.globl	reset
.type reset, %function
reset:
	W(b)	_main

连接脚本中 start.S 中的数据段在向量段之后，可以看出复位中断函数比较简单，直接跳转到了_main 函数， _main 函数做了什么事情呢？
_main 函数在文件 arch\arm\lib\crt0.S中定义，这里做了整个u-boot的初始化工作，下面重点研究该部分。

#if 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
	/ * 前面代码对sp指针做8字节对齐操作* /
	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
	bl	board_init_f

#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	r0, [r9, #GD_START_ADDR_SP]	/* sp = gd->start_addr_sp */
	bic	r0, r0, #7	/* 8-byte alignment for ABI compliance */
	mov	sp, r0
	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_code
here:
/*
 * 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 CONFIG_IS_ENABLED(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. */
#endif

ENDPROC(_main)

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;
}

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;
	/* zero the area */
	memset(gd_ptr, '\0', sizeof(*gd));
	/* set GD unless architecture did it already */
#if !defined(CONFIG_ARM)
	arch_setup_gd(gd_ptr);
#endif
	/* next alloc will be higher by one GD plus 16-byte alignment */
	base += roundup(sizeof(struct global_data), 16);

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

#if CONFIG_VAL(SYS_MALLOC_F_LEN)
	/* go down one 'early malloc arena' */
	gd->malloc_base = base;
	/* next alloc will be higher by one 'early malloc arena' size */
	base += CONFIG_VAL(SYS_MALLOC_F_LEN);
#endif
}

经过上面的函数执行之后，堆栈内存分配如下

初始化之后，r9中存放的是gd的地址，gd->malloc_base中存放的是内存区的基地址，该内存主要用于u-boot前段设备的资源申请。

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) && \
		!defined(CONFIG_EFI_APP) && !CONFIG_IS_ENABLED(X86_64)
	/* NOTREACHED - jump_to_copy() does not return */
	hang();
#endif
}

该函数比较简单，主要的作用就是分别执行一个函数数组中的每个函数，重点分析该函数数组中的函数成员。
该数组定义在文件 common\board_f.c 中

static const init_fnc_t init_sequence_f[] = {
	setup_mon_len,//获取镜像总大小，包括bss段
#ifdef CONFIG_OF_CONTROL
	fdtdec_setup,//获取设备树的存放地址
#endif
#ifdef CONFIG_TRACE
	trace_early_init,
#endif
	initf_malloc,//初始化前段内存相关函数
	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,//配置MPU，设置sdram区可执行代码
	mach_cpu_init,
	initf_dm,//初始化u-boot的设备模型
	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 */
	init_baud_rate,		/* initialze baudrate settings */
	serial_init,		/* serial communications setup */
	console_init_f,		/* stage 1 init of console */
	display_options,	/* say that we are here */
	display_text_info,	/* show debugging info if required */
#if defined(CONFIG_PPC) || defined(CONFIG_M68K) || 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_HARD_SPI)
	init_func_spi,
#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,
#if defined(CONFIG_LOGBUFFER)
	reserve_logbuffer,
#endif
#ifdef CONFIG_PRAM
	reserve_pram,
#endif
	reserve_round_4k,
#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,
};