uboot组织架构正在朝着linux架构方向发展，不同版本稍有不同，一下以U-Boot 2016.03为例。
分析入口：以u-boot.lds（其决定了各个段的排布方式）开始：
1、u-boot.lds:
//设置输出文件大小端格式
OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
//设置文件以 ARM 可执行文件格式输出
OUTPUT_ARCH(arm)
//指令以_start标号开始
ENTRY(_start)
SECTIONS
{
. = 0x00000000;
. = ALIGN(4);
.text :
{
*(.__image_copy_start)
*(.vectors)
//text 段以start.o开始
arch/arm/cpu/armv7/start.o (.text*)
//其他text段
*(.text*)
}
. = ALIGN(4);
//只读数据段，首先将所有文件的只读数据段以名字（即 asc 码排序），然后以对齐方式排序存放到 text 段后面。
.rodata : { *(SORT_BY_ALIGNMENT(SORT_BY_NAME(.rodata*))) }
. = ALIGN(4);
//所有的数据段
.data : {
*(.data*)
}
. = ALIGN(4);
. = .;
. = ALIGN(4);
.u_boot_list : {
KEEP(*(SORT(.u_boot_list*)));
}
. = ALIGN(4);
.image_copy_end :
{
*(.__image_copy_end)
}
.rel_dyn_start :
{
*(.__rel_dyn_start)
}
.rel.dyn : {
*(.rel*)
}
.rel_dyn_end :
{
*(.__rel_dyn_end)
}
.end :
{
*(.__end)
}
_image_binary_end = .;
. = ALIGN(4096);
.mmutable : {
*(.mmutable)
}
.bss_start __rel_dyn_start (OVERLAY) : {
KEEP(*(.__bss_start));
__bss_base = .;
}
.bss __bss_base (OVERLAY) : {
*(.bss*)
. = ALIGN(4);
__bss_limit = .;
}
.bss_end __bss_limit (OVERLAY) : {
KEEP(*(.__bss_end));
}
.dynsym _image_binary_end : { *(.dynsym) }
.dynbss : { *(.dynbss) }
.dynstr : { *(.dynstr*) }
.dynamic : { *(.dynamic*) }
.plt : { *(.plt*) }
.interp : { *(.interp*) }
.gnu.hash : { *(.gnu.hash) }
.gnu : { *(.gnu*) }
.ARM.exidx : { *(.ARM.exidx*) }
.gnu.linkonce.armexidx : { *(.gnu.linkonce.armexidx.*) }
}
通过u-boot.lds我们知道，第一个运行的地方是_start，它在arch/arm/lib/vector.S中被定义。
2、vector.S：_start
文件路径：arch/arm/lib/vector.S
_start:
#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
通过_start我们知道，其之后跳转到了reset，其在arch/arm/cpu/armv7/start.S中被定义。
3、start.S：reset
文件位置：arch/arm/cpu/armv7/start.S
#include <asm-offsets.h>
#include <config.h>
#include <asm/system.h>
#include <linux/linkage.h>
/*************************************************************************
* Startup Code (reset vector)
* Do important init only if we don't start from memory!
* Setup memory and board specific bits prior to relocation.
* Relocate armboot to ram. Setup stack.
*************************************************************************/
.globl reset
.globl save_boot_params_ret
reset:
/* Allow the board to save important registers */
b save_boot_params
save_boot_params_ret:
/*
* disable interrupts (FIQ and IRQ), also set the cpu to SVC32 mode,
* except if in HYP mode already
*/
//设置CPSR寄存器里设置CPU为SVC模式，禁止中断
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
/*
* 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
//调用cpu_init_cp15，初始化协处理器CP15,从而禁用MMU和TLB
bl cpu_init_cp15
//调用cpu_init_crit，进行一些关键的初始化动作，也就是平台级和板级的初始化
bl cpu_init_crit
#endif
//跳转到主函数，也就是板级初始化函数
bl _main
通过start.S我们知道，其之后分别执行了cpu_init_cp15（本文件中定义）和cpu_init_crit（在arch\arm\cpu\armv7\ lowlevel_init.S中定义）。
4、lowlevel_init.S：cpu_init_crit
文件位置：arch\arm\cpu\armv7\ lowlevel_init.S
其中跳转执行s_init，在arch\arm\cpu\armv7\mx6\ soc.c中定义。
之后结束了cpu_init_crit的执行，会返回到start.S中执行bl _main，此处也就是第二阶段_main在arch\arm\lib\crt0.S中定义。
5、crt0.S：_main
文件位置：arch\arm\lib\crt0.S/*
* crt0 - C-runtime startup Code for ARM U-Boot
*
* Copyright (c) 2012 Albert ARIBAUD <[email protected]>
*
* SPDX-License-Identifier: GPL-2.0+
*/
#include <config.h>
#include <asm-offsets.h>
#include <linux/linkage.h>
#ifdef CONFIG_CPU_V7M
#include <asm/armv7m.h>
#endif
/*
* This file handles the target-independent stages of the U-Boot
* start-up where a C runtime environment is needed. Its entry point
* is _main and is branched into from the target's start.S file.
*
* _main execution sequence is:
*
* 1. Set up initial environment for calling board_init_f().
* This environment only provides a stack and a place to store
* the GD ('global data') structure, both located in some readily
* available RAM (SRAM, locked cache...). In this context, VARIABLE
* global data, initialized or not (BSS), are UNAVAILABLE; only
* CONSTANT initialized data are available. GD should be zeroed
* before board_init_f() is called.
*
* 2. Call board_init_f(). This function prepares the hardware for
* execution from system RAM (DRAM, DDR...) As system RAM may not
* be available yet, , board_init_f() must use the current GD to
* store any data which must be passed on to later stages. These
* data include the relocation destination, the future stack, and
* the future GD location.
*
* 3. Set up intermediate environment where the stack and GD are the
* ones allocated by board_init_f() in system RAM, but BSS and
* initialized non-const data are still not available.
*
* 4a.For U-Boot proper (not SPL), call relocate_code(). This function
* relocates U-Boot from its current location into the relocation
* destination computed by board_init_f().
*
* 4b.For SPL, board_init_f() just returns (to crt0). There is no
* code relocation in SPL.
*
* 5. Set up final environment for calling board_init_r(). This
* environment has BSS (initialized to 0), initialized non-const
* data (initialized to their intended value), and stack in system
* RAM (for SPL moving the stack and GD into RAM is optional - see
* CONFIG_SPL_STACK_R). GD has retained values set by board_init_f().
*
* 6. For U-Boot proper (not SPL), some CPUs have some work left to do
* at this point regarding memory, so call c_runtime_cpu_setup.
*
* 7. Branch to board_init_r().
*
* For more information see 'Board Initialisation Flow in README.
*/
/*
* entry point of crt0 sequence
*/
ENTRY(_main)
/*
* Set up initial C runtime environment and call board_init_f(0).
*/
#if defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_STACK)
ldr sp, =(CONFIG_SPL_STACK)
#else
ldr sp, =(CONFIG_SYS_INIT_SP_ADDR)
#endif
#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
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
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 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_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 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. */
#endif
ENDPROC(_main)
第二阶段主要函数有设置C，加载C环境、board_init_f(common\Board_f.c）、重定位代码relocate_code 、执行board_init_r。程序再此处不应该返回，而是陷入死循环。
6、Board_f.c：board_init_f
文件位置：common\Board_f.c
void board_init_f(ulong boot_flags)
{
#ifdef CONFIG_SYS_GENERIC_GLOBAL_DATA
/*
* For some archtectures, global data is initialized and used before
* calling this function. The data should be preserved. For others,
* CONFIG_SYS_GENERIC_GLOBAL_DATA should be defined and use the stack
* here to host global data until relocation.
*/
gd_t data;
gd = &data;
/*
* Clear global data before it is accessed at debug print
* in initcall_run_list. Otherwise the debug print probably
* get the wrong vaule of gd->have_console.
*/
zero_global_data();
#endif
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)
/* NOTREACHED - jump_to_copy() does not return */
hang();
#endif
}init_sequence_f：
static init_fnc_t init_sequence_f[] = {
#ifdef CONFIG_SANDBOX
setup_ram_buf,
#endif
setup_mon_len,
#ifdef CONFIG_OF_CONTROL
fdtdec_setup,
#endif
#ifdef CONFIG_TRACE
trace_early_init,
#endif
initf_malloc,
initf_console_record,
#if defined(CONFIG_MPC85xx) || defined(CONFIG_MPC86xx)
/* TODO: can this go into arch_cpu_init()? */
probecpu,
#endif
#if defined(CONFIG_X86) && defined(CONFIG_HAVE_FSP)
x86_fsp_init,
#endif
arch_cpu_init, /* basic arch cpu dependent setup */
initf_dm,
arch_cpu_init_dm,
mark_bootstage, /* need timer, go after init dm */
#if defined(CONFIG_BOARD_EARLY_INIT_F)
board_early_init_f,
#endif
/* TODO: can any of this go into arch_cpu_init()? */
#if defined(CONFIG_PPC) && !defined(CONFIG_8xx_CPUCLK_DEFAULT)
get_clocks, /* get CPU and bus clocks (etc.) */
#if defined(CONFIG_TQM8xxL) && !defined(CONFIG_TQM866M) \
&& !defined(CONFIG_TQM885D)
adjust_sdram_tbs_8xx,
#endif
/* TODO: can we rename this to timer_init()? */
init_timebase,
#endif
#if defined(CONFIG_ARM) || defined(CONFIG_MIPS) || \
defined(CONFIG_BLACKFIN) || defined(CONFIG_NDS32) || \
defined(CONFIG_SPARC)
timer_init, /* initialize timer */
#endif
#ifdef CONFIG_SYS_ALLOC_DPRAM
#if !defined(CONFIG_CPM2)
dpram_init,
#endif
#endif
#if defined(CONFIG_BOARD_POSTCLK_INIT)
board_postclk_init,
#endif
#if defined(CONFIG_SYS_FSL_CLK) || defined(CONFIG_M68K)
get_clocks,
#endif
env_init, /* initialize environment */
#if defined(CONFIG_8xx_CPUCLK_DEFAULT)
/* get CPU and bus clocks according to the environment variable */
get_clocks_866,
/* adjust sdram refresh rate according to the new clock */
sdram_adjust_866,
init_timebase,
#endif
init_baud_rate, /* initialze baudrate settings */
serial_init, /* serial communications setup */
console_init_f, /* stage 1 init of console */
#ifdef CONFIG_SANDBOX
sandbox_early_getopt_check,
#endif
#ifdef CONFIG_OF_CONTROL
fdtdec_prepare_fdt,
#endif
display_options, /* say that we are here */
display_text_info, /* show debugging info if required */
#if defined(CONFIG_MPC8260)
prt_8260_rsr,
prt_8260_clks,
#endif /* CONFIG_MPC8260 */
#if defined(CONFIG_MPC83xx)
prt_83xx_rsr,
#endif
#if defined(CONFIG_PPC) || defined(CONFIG_M68K)
checkcpu,
#endif
print_cpuinfo, /* display cpu info (and speed) */
#if defined(CONFIG_MPC5xxx)
prt_mpc5xxx_clks,
#endif /* CONFIG_MPC5xxx */
#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_HARD_I2C) || defined(CONFIG_SYS_I2C)
init_func_i2c,
#endif
#if defined(CONFIG_HARD_SPI)
init_func_spi,
#endif
announce_dram_init,
/* TODO: unify all these dram functions? */
#if defined(CONFIG_ARM) || defined(CONFIG_X86) || defined(CONFIG_NDS32) || \
defined(CONFIG_MICROBLAZE) || defined(CONFIG_AVR32)
dram_init, /* configure available RAM banks */
#endif
#if defined(CONFIG_MIPS) || defined(CONFIG_PPC) || defined(CONFIG_M68K)
init_func_ram,
#endif
#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_BLACKFIN)
/* Blackfin u-boot monitor should be on top of the ram */
reserve_uboot,
#endif
#if defined(CONFIG_SPARC)
reserve_prom,
#endif
#if defined(CONFIG_LOGBUFFER) && !defined(CONFIG_ALT_LB_ADDR)
reserve_logbuffer,
#endif
#ifdef CONFIG_PRAM
reserve_pram,
#endif
reserve_round_4k,
#if !(defined(CONFIG_SYS_ICACHE_OFF) && defined(CONFIG_SYS_DCACHE_OFF)) && \
defined(CONFIG_ARM)
reserve_mmu,
#endif
#ifdef CONFIG_DM_VIDEO
reserve_video,
#else
# ifdef CONFIG_LCD
reserve_lcd,
# endif
/* TODO: Why the dependency on CONFIG_8xx? */
# if defined(CONFIG_VIDEO) && (!defined(CONFIG_PPC) || defined(CONFIG_8xx)) && \
!defined(CONFIG_ARM) && !defined(CONFIG_X86) && \
!defined(CONFIG_BLACKFIN) && !defined(CONFIG_M68K)
reserve_legacy_video,
# endif
#endif /* CONFIG_DM_VIDEO */
reserve_trace,
#if !defined(CONFIG_BLACKFIN)
reserve_uboot,
#endif
#ifndef CONFIG_SPL_BUILD
reserve_malloc,
reserve_board,
#endif
setup_machine,
reserve_global_data,
reserve_fdt,
reserve_arch,
reserve_stacks,
setup_dram_config,
show_dram_config,
#if defined(CONFIG_PPC) || defined(CONFIG_M68K) || defined(CONFIG_MIPS)
setup_board_part1,
#endif
#if defined(CONFIG_PPC) || defined(CONFIG_M68K)
INIT_FUNC_WATCHDOG_RESET
setup_board_part2,
#endif
display_new_sp,
#ifdef CONFIG_SYS_EXTBDINFO
setup_board_extra,
#endif
INIT_FUNC_WATCHDOG_RESET
reloc_fdt,
setup_reloc,
#if defined(CONFIG_X86) || defined(CONFIG_ARC)
copy_uboot_to_ram,
clear_bss,
do_elf_reloc_fixups,
#endif
#if !defined(CONFIG_ARM) && !defined(CONFIG_SANDBOX)
jump_to_copy,
#endif
NULL,
};之后返回crt0.S，继续执行后续函数，如board_init_r在common/Board_r.c中定义
7、Board_r.c：board_init_r
文件位置：common/Board_r.c
void board_init_r(gd_t *new_gd, ulong dest_addr)
{
#ifdef CONFIG_NEEDS_MANUAL_RELOC
int i;
#endif
#ifdef CONFIG_AVR32
mmu_init_r(dest_addr);
#endif
#if !defined(CONFIG_X86) && !defined(CONFIG_ARM) && !defined(CONFIG_ARM64)
gd = new_gd;
#endif
#ifdef CONFIG_NEEDS_MANUAL_RELOC
for (i = 0; i < ARRAY_SIZE(init_sequence_r); i++)
init_sequence_r[i] += gd->reloc_off;
#endif
if (initcall_run_list(init_sequence_r))
hang();
/* NOTREACHED - run_main_loop() does not return */
hang();
}
之后调用了init_sequence_r中函数在common/Board_r.c同文件中定义。
init_fnc_t init_sequence_r[] = {
initr_trace,
initr_reloc,
/* TODO: could x86/PPC have this also perhaps? */
#ifdef CONFIG_ARM
initr_caches,
/* 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 availble
* until board_init() is called below to remap IFC to high
* region.
*/
#endif
initr_reloc_global_data,
#if defined(CONFIG_SYS_INIT_RAM_LOCK) && defined(CONFIG_E500)
initr_unlock_ram_in_cache,
#endif
initr_barrier,
initr_malloc,
initr_console_record,
#ifdef CONFIG_SYS_NONCACHED_MEMORY
initr_noncached,
#endif
bootstage_relocate,
#ifdef CONFIG_DM
initr_dm,
#endif
initr_bootstage,
#if defined(CONFIG_ARM) || defined(CONFIG_NDS32)
board_init, /* Setup chipselects */
#endif
/*
* TODO: printing of the clock inforamtion of the board is now
* implemented as part of bdinfo command. Currently only support for
* davinci SOC's is added. Remove this check once all the board
* implement this.
*/
#ifdef CONFIG_CLOCKS
set_cpu_clk_info, /* Setup clock information */
#endif
stdio_init_tables,
initr_serial,
initr_announce,
INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_NEEDS_MANUAL_RELOC
initr_manual_reloc_cmdtable,
#endif
#if defined(CONFIG_PPC) || defined(CONFIG_M68K)
initr_trap,
#endif
#ifdef CONFIG_ADDR_MAP
initr_addr_map,
#endif
#if defined(CONFIG_BOARD_EARLY_INIT_R)
board_early_init_r,
#endif
INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_LOGBUFFER
initr_logbuffer,
#endif
#ifdef CONFIG_POST
initr_post_backlog,
#endif
INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_SYS_DELAYED_ICACHE
initr_icache_enable,
#endif
#if defined(CONFIG_PCI) && defined(CONFIG_SYS_EARLY_PCI_INIT)
/*
* Do early PCI configuration _before_ the flash gets initialised,
* because PCU ressources are crucial for flash access on some boards.
*/
initr_pci,
#endif
#ifdef CONFIG_WINBOND_83C553
initr_w83c553f,
#endif
#ifdef CONFIG_ARCH_EARLY_INIT_R
arch_early_init_r,
#endif
power_init_board,
#ifndef CONFIG_SYS_NO_FLASH
initr_flash,
#endif
INIT_FUNC_WATCHDOG_RESET
#if defined(CONFIG_PPC) || defined(CONFIG_M68K) || defined(CONFIG_X86) || \
defined(CONFIG_SPARC)
/* initialize higher level parts of CPU like time base and timers */
cpu_init_r,
#endif
#ifdef CONFIG_PPC
initr_spi,
#endif
#ifdef CONFIG_CMD_NAND
initr_nand,
#endif
#ifdef CONFIG_CMD_ONENAND
initr_onenand,
#endif
#ifdef CONFIG_GENERIC_MMC
initr_mmc,
#endif
#ifdef CONFIG_HAS_DATAFLASH
initr_dataflash,
#endif
initr_env,
#ifdef CONFIG_SYS_BOOTPARAMS_LEN
initr_malloc_bootparams,
#endif
INIT_FUNC_WATCHDOG_RESET
initr_secondary_cpu,
#if defined(CONFIG_ID_EEPROM) || defined(CONFIG_SYS_I2C_MAC_OFFSET)
mac_read_from_eeprom,
#endif
INIT_FUNC_WATCHDOG_RESET
#if defined(CONFIG_PCI) && !defined(CONFIG_SYS_EARLY_PCI_INIT)
/*
* Do pci configuration
*/
initr_pci,
#endif
stdio_add_devices,
initr_jumptable,
#ifdef CONFIG_API
initr_api,
#endif
console_init_r, /* fully init console as a device */
#ifdef CONFIG_DISPLAY_BOARDINFO_LATE
show_board_info,
#endif
#ifdef CONFIG_ARCH_MISC_INIT
arch_misc_init, /* miscellaneous arch-dependent init */
#endif
#ifdef CONFIG_MISC_INIT_R
misc_init_r, /* miscellaneous platform-dependent init */
#endif
INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_CMD_KGDB
initr_kgdb,
#endif
interrupt_init,
#if defined(CONFIG_ARM) || defined(CONFIG_AVR32)
initr_enable_interrupts,
#endif
#if defined(CONFIG_MICROBLAZE) || defined(CONFIG_AVR32) || defined(CONFIG_M68K)
timer_init, /* initialize timer */
#endif
#if defined(CONFIG_STATUS_LED)
initr_status_led,
#endif
/* PPC has a udelay(20) here dating from 2002. Why? */
#ifdef CONFIG_CMD_NET
initr_ethaddr,
#endif
#ifdef CONFIG_BOARD_LATE_INIT
board_late_init,
#endif
#ifdef CONFIG_FSL_FASTBOOT
initr_fastboot_setup,
#endif
#if defined(CONFIG_CMD_AMBAPP)
ambapp_init_reloc,
#if defined(CONFIG_SYS_AMBAPP_PRINT_ON_STARTUP)
initr_ambapp_print,
#endif
#endif
#ifdef CONFIG_CMD_SCSI
INIT_FUNC_WATCHDOG_RESET
initr_scsi,
#endif
#ifdef CONFIG_CMD_DOC
INIT_FUNC_WATCHDOG_RESET
initr_doc,
#endif
#ifdef CONFIG_BITBANGMII
initr_bbmii,
#endif
#ifdef CONFIG_CMD_NET
INIT_FUNC_WATCHDOG_RESET
initr_net,
#endif
#ifdef CONFIG_POST
initr_post,
#endif
#if defined(CONFIG_CMD_PCMCIA) && !defined(CONFIG_CMD_IDE)
initr_pcmcia,
#endif
#if defined(CONFIG_CMD_IDE)
initr_ide,
#endif
#ifdef CONFIG_LAST_STAGE_INIT
INIT_FUNC_WATCHDOG_RESET
/*
* Some parts can be only initialized if all others (like
* Interrupts) are up and running (i.e. the PC-style ISA
* keyboard).
*/
last_stage_init,
#endif
#ifdef CONFIG_CMD_BEDBUG
INIT_FUNC_WATCHDOG_RESET
initr_bedbug,
#endif
#if defined(CONFIG_PRAM) || defined(CONFIG_LOGBUFFER)
initr_mem,
#endif
#ifdef CONFIG_PS2KBD
initr_kbd,
#endif
#if defined(CONFIG_SPARC)
prom_init,
#endif
#ifdef CONFIG_FSL_FASTBOOT
initr_check_fastboot,
#endif
run_main_loop,
};static int run_main_loop(void)
{
#ifdef CONFIG_SANDBOX
sandbox_main_loop_init();
#endif
/* main_loop() can return to retry autoboot, if so just run it again */
for (;;)
main_loop();
return 0;
}main_loop在common/ main.c中定义
8、main.c： main_loop
common/ main.c
void main_loop(void)
{
const char *s;
bootstage_mark_name(BOOTSTAGE_ID_MAIN_LOOP, "main_loop");
#ifndef CONFIG_SYS_GENERIC_BOARD
puts("Warning: Your board does not use generic board. Please read\n");
puts("doc/README.generic-board and take action. Boards not\n");
puts("upgraded by the late 2014 may break or be removed.\n");
#endif
#ifdef CONFIG_VERSION_VARIABLE
setenv("ver", version_string); /* set version variable */
#endif /* CONFIG_VERSION_VARIABLE */
cli_init();
run_preboot_environment_command();
#if defined(CONFIG_UPDATE_TFTP)
update_tftp(0UL, NULL, NULL);
#endif /* CONFIG_UPDATE_TFTP */
s = bootdelay_process();
if (cli_process_fdt(&s))
cli_secure_boot_cmd(s);
autoboot_command(s);
cli_loop();
}
main_loop中执行延时函数，并决定是否启动内核，或者解析命令。欢迎继续阅读：
点击：
imx6ul-U-Boot 2016.03u-boot分析之main_loop函数-ARM Cortex-A7
小结：
uboot的执行流程：
（1）_start（arch/arm/lib/vector.S）
b reset
（2）reset（arch/arm/cpu/armv7/start.S）
bl cpu_init_cp15
bl cpu_init_crit
bl _main
（3）_main（arch\arm\lib\crt0.S）
board_init_f (common\Board_f.c）
b relocate_code
ldr lr, =board_init_r（common/Board_r.c）
run_main_loop（）
main_loop()
（4）main_loop（）例如，uboot中LCD的初始化流程如下所示：
LCD的初始化流程：ENTRY(_main)-》board_init_f-》init_sequence_f-》board_init_r-》init_sequence_r-》stdio_add_devices-》
board_video_skip-》drv_video_init-》