UBOOT官网下载地址：http://ftp.denx.de/pub/u-boot/
很详细的UBOOT解释： https://www.crifan.com/files/doc/docbook/uboot_starts_analysis/release/html/uboot_starts_analysis.html
U-BOOT主要作用和执行流程
①一句话描述
U-BOOT对硬件进行前期的初始化并准备堆栈，之后载入内核并向内核传递必要的参数，便于内核启动。
②执行流程概况
u-boot载入芯片后nor flash从0地址开始执行，nand flash会将前4K考入芯片的内存然后从0地址开始执行，程序最还是从_start标签开始（lds文件定义，start.s有具体实现）。
u-boot一般启动分为两阶段：1.硬件相关的用汇编实现，初始化及重定位代码完成后交给第二阶段 2.第二阶段较为通用，使用c代码编写。
1. UBoot内存划分， lds文件
内存划分存在于对应芯片架构中的u-boot.lds文件中，这个文件是用于连接器连接时对程序各个段空间进行划分之用。
OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
OUTPUT_ARCH(arm)
ENTRY(_start) //程序开始标号，既程序从标号为_start的位置开始执行，这个标号在对应CPU（如arm920t）的start.s文件中，接下来程序分析将分析这个文件
SECTIONS
{
. = 0x00000000; //程序最开始的地址为0
. = ALIGN(4); //4字节地址对齐
.text : //文本段开始位置
{
__image_copy_start = .; //拷贝image开始的地方（其实也是0）
CPUDIR/start.o (.text) //保证start.s的text段放在最前面
*(.text) //u-boot中所有程序的文本段都会放到这个位置
}
. = ALIGN(4); //4字节对齐
.rodata : { *(SORT_BY_ALIGNMENT(SORT_BY_NAME(.rodata*))) } //只读数据从这里开始存放
. = ALIGN(4);
.data : {
*(.data) //所有文件的数据段
}
. = ALIGN(4);
. = .; //不i知道，可能重定位一下定位器的位置比较好玩儿吧
__u_boot_cmd_start = .; //同上，定义了一个宏，保存当前位置信息
.u_boot_cmd : { *(.u_boot_cmd) } //存储u_boot_cmd
__u_boot_cmd_end = .;
. = ALIGN(4);
__image_copy_end = .;
.rel.dyn : {
__rel_dyn_start = .;
*(.rel*)
__rel_dyn_end = .;
}
.dynsym : {
__dynsym_start = .;
*(.dynsym)
}
_end = .;
/*
* Deprecated: this MMU section is used by pxa at present but
* should not be used by new boards/CPUs.
*/
. = ALIGN(4096);
.mmutable : {
*(.mmutable)
}
.bss __rel_dyn_start (OVERLAY) : {
__bss_start = .;
*(.bss)
. = ALIGN(4);
__bss_end__ = .;
}
/DISCARD/ : { *(.dynstr*) }
/DISCARD/ : { *(.dynamic*) }
/DISCARD/ : { *(.plt*) }
/DISCARD/ : { *(.interp*) }
/DISCARD/ : { *(.gnu*) }
}
2. 按代码执行流程分析各个函数
2.1 主要功能
- 设置CPU模式
- 关闭看门狗
- 关闭中断
- 设置堆栈sp指针
- 清除bss段
- 异常中断处理
2.2 主要函数
函数入口：
.globl _start //全局声明，在lds中定义为entry
_start: b start_code //主要执行函数，进入代码后直接跳转到start_code
ldr pc, _undefined_instruction //ldr，将地址导入寄存器，pc为程序指针。导入pc后会进行调用跳转
ldr pc, _software_interrupt
ldr pc, _prefetch_abort
ldr pc, _data_abort
ldr pc, _not_used
ldr pc, _irq
ldr pc, _fiq初始化函数：start_code
start_code:
/*
* set the cpu to SVC32 mode
*/
mrs r0, cpsr //cspr值存入寄存器，方便修改
bic r0, r0, #0x1f //将低5位【4：0】清零
orr r0, r0, #0xd3 //或操作，进行置位
msr cpsr, r0 //修改后的cspr放入寄存器
/*以上为设置cspr值的代码段，使系统进入SVC方式*/
#if defined(CONFIG_AT91RM9200DK) || defined(CONFIG_AT91RM9200EK)
/*
* relocate exception table
*/
ldr r0, =_start //标号地址放入r0，重载时确定起始位置
ldr r1, =0x0 //地址0x00放入r1, 重载后为0地址
mov r2, #16 //循环次数计数， 16*4 存前64字节？
copyex:
subs r2, r2, #1 //减数做循环
ldr r3, [r0], #4 //r0地址存入r3，并且r9加4
str r3, [r1], #4 //r3地址数据（start开始的地方），放入r1的地址（0地址），r1加4
bne copyex //r2不为0则循环写数据，因为r2做了减法，cspr状态值改变只会因为r2的操作产生
#endif
#ifdef CONFIG_S3C24X0
/* turn off the watchdog */
# if defined(CONFIG_S3C2400)
# define pWTCON 0x15300000
# define INTMSK 0x14400008 /* Interrupt-Controller base addresses */
# define CLKDIVN 0x14800014 /* clock divisor register */
#else
# define pWTCON 0x53000000
# define INTMSK 0x4A000008 /* Interrupt-Controller base addresses */
# define INTSUBMSK 0x4A00001C
# define CLKDIVN 0x4C000014 /* clock divisor register */
# endif
ldr r0, =pWTCON //看门狗地址存入r0，查datasheet
mov r1, #0x0 //0存入r1
str r1, [r0] //看门狗地址存入0，既关看门狗
/*
* mask all IRQs by setting all bits in the INTMR - default
*/
mov r1, #0xffffffff
ldr r0, =INTMSK
str r1, [r0] //中断地址存入全1，禁止中断
# if defined(CONFIG_S3C2410)
ldr r1, =0x3ff
ldr r0, =INTSUBMSK
str r1, [r0]
# endif
/* FCLK:HCLK:PCLK = 1:2:4 */
/* default FCLK is 120 MHz ! */
ldr r0, =CLKDIVN
mov r1, #3
str r1, [r0] //修改时钟参数
#endif /* CONFIG_S3C24X0 */
/*
* we do sys-critical inits only at reboot,
* not when booting from ram!
*/
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
bl cpu_init_crit //非从ram启动时会调用这个函数
#endif
/* Set stackpointer in internal RAM to call board_init_f */
call_board_init_f: //设置了lowlevel初始化从这里初始化板子
ldr sp, =(CONFIG_SYS_INIT_SP_ADDR) //设置栈顶指针，宏在config.mk
bic sp, sp, #7 /* 8-byte alignment for ABI compliance */ //清除后四位，8字节对齐
ldr r0,=0x00000000
bl board_init_f //调用函数，bl相对于b调用区别：会填充R4（lr）寄存器，调用结束会返回，函数在board.c中。返回后重新执行下面的函数，正好对应c中的relocate_code
//cpu_init_crit最终也会调用这个函数
非从RAM启动的情况会执行这个函数：cpu_init_critcpu_init_crit:
/*
* flush v4 I/D caches
*/
mov r0, #0mcr p15, 0, r0, c7, c7, 0 //关闭ICaches(指令缓存,关闭是为了降低MMU查表带来的开销)和DCaches(数据缓存,DCaches使用的是虚拟地址，开启MMU之前必须关闭)
mcr p15, 0, r0, c8, c7, 0 //使无效整个数据TLB和指令TLB(TLB就是负责将虚拟内存地址翻译成实际的物理内存地址/*
* disable MMU stuff and caches
*/mrc p15, 0, r0, c1, c0, 0
bic r0, r0, #0x00002300 @ clear bits 13, 9:8 (--V- --RS) //bit8:系统不保护,bit9:ROM不保护,bit13:设置中断向量表的位置为0x0~0x1c,即异常模式基地址为0X0
bic r0, r0, #0x00000087 @ clear bits 7, 2:0 (B--- -CAM) //bit0~2:禁止MMU，禁止地址对齐检查,禁止数据Cache.bit7:设为小端模式
orr r0, r0, #0x00000002 @ set bit 2 (A) Align //bit2:开启数据Cache
orr r0, r0, #0x00001000 @ set bit 12 (I) I-Cache //bit12:开启指令Cache
mcr p15, 0, r0, c1, c0, 0/*
* before relocating, we have to setup RAM timing
* because memory timing is board-dependend, you will
* find a lowlevel_init.S in your board directory.
*/
mov ip, lr //保存lr的值，用于返回调用cpu_init_crit的函数处，用于调用board_init_fbl lowlevel_init //函数在对应的board文件夹中，第二阶段启动代码，主要作用是初始化两个重要数据结构，对SDRAM的内存分配设置，对各种需要用到的外设进行初始化，最后循环跳入main_loop()函数
mov lr, ip
mov pc, lrboard_init_f函数
涉及到两个重要的数据结构：1）bd_t结构体，关于开发板信息（波特率，ip, 平台号，启动参数）。2）gd_t结构体成员主要是一些全局的系统初始化参数。需要用到时，用宏定义DECLARD_GLOBAL_DATA_PTT,指定占用寄存器r8，具体定义如下：
typedef struct bd_info {
int bi_baudrate; /* serial console baudrate串口波特率 */
unsigned long bi_ip_addr; /* IP Address IP 地址*/
ulong bi_arch_number; /* unique id for this board 板子的id */
ulong bi_boot_params; /* where this board expects params 启动参数*/
struct /* RAM configuration RAM 配置*/
{
ulong start;
ulong size;
} bi_dram[CONFIG_NR_DRAM_BANKS];
} bd_t;
Gd_t结构体定义，如下：
typedef struct global_data {
bd_t *bd;
unsigned long flags; //指示标志，如设备已经初始化标志等
unsigned long baudrate; //串行口通信速率
unsigned long have_console; /* serial_init() was called */
#ifdef CONFIG_PRE_CONSOLE_BUFFER
unsigned long precon_buf_idx; /* Pre-Console buffer index */
#endif
unsigned long env_addr; /* Address of Environment struct 环境参数地址 */
unsigned long env_valid; /* Checksum of Environment valid? 环境参数CRC检验有效标志*/
unsigned long fb_base; /* base address of frame buffer 帧缓冲区基地址*/
……
} gd_t;
c函数，计算重定位后地址分配，填充bd和gd数据：board_init_fvoid board_init_f(ulong bootflag)
{
bd_t *bd;
init_fnc_t **init_fnc_ptr;
gd_t *id;
ulong addr, addr_sp;
#ifdef CONFIG_PRAM
ulong reg;
#endif
bootstage_mark_name(BOOTSTAGE_ID_START_UBOOT_F, "board_init_f");
/* Pointer is writable since we allocated a register for it */
gd = (gd_t *) ((CONFIG_SYS_INIT_SP_ADDR) & ~0x07);
/* compiler optimization barrier needed for GCC >= 3.4 */
__asm__ __volatile__("": : :"memory"); /*内存屏障，防止编译器优化赋值顺序*/
memset((void *)gd, 0, sizeof(gd_t));
gd->mon_len = _bss_end_ofs; /*_bss_end_ofs的定义在start.S中： .globl _bss_end_ofs，这里计算出来了U-BOOT的大小，用于后面做减法计算重定位的起始地址*/
#ifdef CONFIG_OF_EMBED
/* Get a pointer to the FDT */
gd->fdt_blob = _binary_dt_dtb_start;
#elif defined CONFIG_OF_SEPARATE
/* FDT is at end of image */
gd->fdt_blob = (void *)(_end_ofs + _TEXT_BASE);
#endif
/* Allow the early environment to override the fdt address */
gd->fdt_blob = (void *)getenv_ulong("fdtcontroladdr", 16,
(uintptr_t)gd->fdt_blob);
for (init_fnc_ptr = init_sequence; *init_fnc_ptr; ++init_fnc_ptr) { /*初始化函数的数组，会逐个调用数组中的初始化函数*/
if ((*init_fnc_ptr)() != 0) {
hang ();
}
}
#ifdef CONFIG_OF_CONTROL
/* For now, put this check after the console is ready */
if (fdtdec_prepare_fdt()) {
panic("** CONFIG_OF_CONTROL defined but no FDT - please see "
"doc/README.fdt-control");
}
#endif
debug("monitor len: %08lX\n", gd->mon_len);
/*
* Ram is setup, size stored in gd !!
*/
debug("ramsize: %08lX\n", gd->ram_size);
#if defined(CONFIG_SYS_MEM_TOP_HIDE)
/*
* Subtract specified amount of memory to hide so that it won't
* get "touched" at all by U-Boot. By fixing up gd->ram_size
* the Linux kernel should now get passed the now "corrected"
* memory size and won't touch it either. This should work
* for arch/ppc and arch/powerpc. Only Linux board ports in
* arch/powerpc with bootwrapper support, that recalculate the
* memory size from the SDRAM controller setup will have to
* get fixed.
*/
gd->ram_size -= CONFIG_SYS_MEM_TOP_HIDE;
#endif
addr = CONFIG_SYS_SDRAM_BASE + gd->ram_size; /*这样addr是内存的高地址，0x30000000+256M， 关于gd->ram_size,在初始化函数dram_init中，已经给过值了*/
#ifdef CONFIG_LOGBUFFER
#ifndef CONFIG_ALT_LB_ADDR
/* reserve kernel log buffer */
addr -= (LOGBUFF_RESERVE);
debug("Reserving %dk for kernel logbuffer at %08lx\n", LOGBUFF_LEN,
addr);
#endif
#endif
#ifdef CONFIG_PRAM
/*
* reserve protected RAM
*/
reg = getenv_ulong("pram", 10, CONFIG_PRAM); /*从环境变量获取代码地址，pram*/
addr -= (reg << 10); /* size is in kB */
debug("Reserving %ldk for protected RAM at %08lx\n", reg, addr);
#endif /* CONFIG_PRAM */
#if !(defined(CONFIG_SYS_ICACHE_OFF) && defined(CONFIG_SYS_DCACHE_OFF))
/* reserve TLB table */
addr -= (4096 * 4);
/* round down to next 64 kB limit */
addr &= ~(0x10000 - 1); //64K对齐
gd->tlb_addr = addr;
debug("TLB table at: %08lx\n", addr);
#endif
/* round down to next 4 kB limit */
addr &= ~(4096 - 1); // K对齐，此处前面已经64K对齐了，就不需改动
debug("Top of RAM usable for U-Boot at: %08lx\n", addr);
#ifdef CONFIG_LCD
#ifdef CONFIG_FB_ADDR
gd->fb_base = CONFIG_FB_ADDR;
#else
/* reserve memory for LCD display (always full pages) */
addr = lcd_setmem(addr);
gd->fb_base = addr;
#endif /* CONFIG_FB_ADDR */
#endif /* CONFIG_LCD */
/*
* reserve memory for U-Boot code, data & bss
* round down to next 4 kB limit
*/
addr -= gd->mon_len; /*重定位地址再减去U-BOOT的大小，此时就是重定位时代码要拷贝到的目的位置*/
addr &= ~(4096 - 1);
debug("Reserving %ldk for U-Boot at: %08lx\n", gd->mon_len >> 10, addr);
#ifndef CONFIG_SPL_BUILD
/*
* reserve memory for malloc() arena
*/
addr_sp = addr - TOTAL_MALLOC_LEN; /*当前地址减去预留给堆的空间后，作为栈的初始地址*/
debug("Reserving %dk for malloc() at: %08lx\n",
TOTAL_MALLOC_LEN >> 10, addr_sp);
/*
* (permanently) allocate a Board Info struct
* and a permanent copy of the "global" data
*/
addr_sp -= sizeof (bd_t); /*栈中用于放bd_t的数据，预留出足够的空间*/
bd = (bd_t *) addr_sp;
gd->bd = bd; /*bd指向新分配的地址，代码重定位把数据copy过去后可以直接使用地址指针*/
debug("Reserving %zu Bytes for Board Info at: %08lx\n",
sizeof (bd_t), addr_sp);
#ifdef CONFIG_MACH_TYPE
gd->bd->bi_arch_number = CONFIG_MACH_TYPE; /* board id for Linux */
#endif
addr_sp -= sizeof (gd_t); /*栈顶指针减法，预留出gd_t的空间，参考bd_t*/
id = (gd_t *) addr_sp;
debug("Reserving %zu Bytes for Global Data at: %08lx\n",
sizeof (gd_t), addr_sp);
/* setup stackpointer for exeptions */
gd->irq_sp = addr_sp; /*此时栈顶的地址放入gd结构体，作为ird中断使用的栈顶地址*/
#ifdef CONFIG_USE_IRQ
addr_sp -= (CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ);
debug("Reserving %zu Bytes for IRQ stack at: %08lx\n",
CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ, addr_sp);
#endif
/* leave 3 words for abort-stack */
addr_sp -= 12; /*为中断预留的栈空间*/
/* 8-byte alignment for ABI compliance */
addr_sp &= ~0x07;
#else
addr_sp += 128; /* leave 32 words for abort-stack */
gd->irq_sp = addr_sp;
#endif
debug("New Stack Pointer is: %08lx\n", addr_sp);
#ifdef CONFIG_POST
post_bootmode_init();
post_run(NULL, POST_ROM | post_bootmode_get(0));
#endif
gd->bd->bi_baudrate = gd->baudrate;
/* Ram ist board specific, so move it to board code ... */
dram_init_banksize();
display_dram_config(); /* and display it */
gd->relocaddr = addr; /*u-boot重新搬运后的起始地址。*/
gd->start_addr_sp = addr_sp;
gd->reloc_off = addr - _TEXT_BASE;
debug("relocation Offset is: %08lx\n", gd->reloc_off);
memcpy(id, (void *)gd, sizeof(gd_t));
relocate_code(addr_sp, id, addr); /*relocate_code(addr_sp, id, addr)；在start.S中定义，C又回到了汇编*/
/* NOTREACHED - relocate_code() does not return */
}
被c函数调用，用于重定位代码，存在于start.s中：relocate_code.globl relocate_code
relocate_code:
mov r4, r0 /* save addr_sp */
mov r5, r1 /* save addr of gd */
mov r6, r2 /* save addr of destination */
/*以上为保存传入的3个参数*/
/* Set up the stack */
stack_setup:
mov sp, r4
adr r0, _start
cmp r0, r6
beq clear_bss /* skip relocation */
mov r1, r6 /* r1 <- scratch for copy_loop */
ldr r3, _bss_start_ofs
add r2, r0, r3 /* r2 <- source end address */
copy_loop:
ldmia r0!, {r9-r10} /* copy from source address [r0] */
stmia r1!, {r9-r10} /* copy to target address [r1] */
cmp r0, r2 /* until source end address [r2] */
blo copy_loop
/*上述代码将U-BOOT拷贝到了新的重定位后的位置，即上面片段计算出来的addr的地址*/
#ifndef CONFIG_SPL_BUILD
/*
* fix .rel.dyn relocations
*/
/*使用 .rel.dyn字段进行重定位*/
ldr r0, _TEXT_BASE /* r0 <- Text base */
sub r9, r6, r0 /* r9 <- relocation offset */
ldr r10, _dynsym_start_ofs /* r10 <- sym table ofs */
add r10, r10, r0 /* r10 <- sym table in FLASH */
ldr r2, _rel_dyn_start_ofs /* r2 <- rel dyn start ofs */
add r2, r2, r0 /* r2 <- rel dyn start in FLASH */
ldr r3, _rel_dyn_end_ofs /* r3 <- rel dyn end ofs */
add r3, r3, r0 /* r3 <- rel dyn end in FLASH */
fixloop:
ldr r0, [r2] /* r0 <- location to fix up, IN FLASH! */
add r0, r0, r9 /* r0 <- location to fix up in RAM */
ldr r1, [r2, #4]
and r7, r1, #0xff
cmp r7, #23 /* relative fixup? */
beq fixrel
cmp r7, #2 /* absolute fixup? */
beq fixabs
/* ignore unknown type of fixup */
b fixnext
fixabs:
/* absolute fix: set location to (offset) symbol value */
mov r1, r1, LSR #4 /* r1 <- symbol index in .dynsym */
add r1, r10, r1 /* r1 <- address of symbol in table */
ldr r1, [r1, #4] /* r1 <- symbol value */
add r1, r1, r9 /* r1 <- relocated sym addr */
b fixnext
fixrel:
/* relative fix: increase location by offset */
ldr r1, [r0]
add r1, r1, r9
fixnext:
str r1, [r0]
add r2, r2, #8 /* each rel.dyn entry is 8 bytes */
cmp r2, r3
blo fixloop
#endif
clear_bss:
#ifndef CONFIG_SPL_BUILD
ldr r0, _bss_start_ofs
ldr r1, _bss_end_ofs
mov r4, r6 /* reloc addr */
add r0, r0, r4
add r1, r1, r4
mov r2, #0x00000000 /* clear */
clbss_l:str r2, [r0] /* clear loop... */
add r0, r0, #4
cmp r0, r1
bne clbss_l
bl coloured_LED_init
bl red_led_on
#endif
/*
* We are done. Do not return, instead branch to second part of board
* initialization, now running from RAM.
*/
#ifdef CONFIG_NAND_SPL
ldr r0, _nand_boot_ofs
mov pc, r0
_nand_boot_ofs:
.word nand_boot //nand启动
#else
ldr r0, _board_init_r_ofs
adr r1, _start
add lr, r0, r1
add lr, lr, r9
/* setup parameters for board_init_r */
mov r0, r5 /* gd_t */
mov r1, r6 /* dest_addr */
/* jump to it ... */
mov pc, lr //这里lr为board_init_r地址
_board_init_r_ofs:
.word board_init_r - _start
#endif
_rel_dyn_start_ofs:
.word __rel_dyn_start - _start
_rel_dyn_end_ofs:
.word __rel_dyn_end - _start
_dynsym_start_ofs:
.word __dynsym_start - _start
第二阶段：board_init_r最终调用main_loop解析参数启动内核void board_init_r(gd_t *id, ulong dest_addr)
{
ulong malloc_start;
#if !defined(CONFIG_SYS_NO_FLASH)
ulong flash_size;
#endif
gd = id;
gd->flags |= GD_FLG_RELOC; /* tell others: relocation done */
bootstage_mark_name(BOOTSTAGE_ID_START_UBOOT_R, "board_init_r");
monitor_flash_len = _end_ofs;
/* Enable caches */
enable_caches();
debug("monitor flash len: %08lX\n", monitor_flash_len);
board_init(); /* Setup chipselects */ /*设置板级的初始化*/
/*
* 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
#ifdef CONFIG_SERIAL_MULTI
serial_initialize();
#endif
debug("Now running in RAM - U-Boot at: %08lx\n", dest_addr);
#ifdef CONFIG_LOGBUFFER
logbuff_init_ptrs();
#endif
#ifdef CONFIG_POST
post_output_backlog();
#endif
/* The Malloc area is immediately below the monitor copy in DRAM */
malloc_start = dest_addr - TOTAL_MALLOC_LEN; /*剩余出堆内存的空间*/
mem_malloc_init (malloc_start, TOTAL_MALLOC_LEN);
#if !defined(CONFIG_SYS_NO_FLASH)
puts("Flash: ");
flash_size = flash_init();
if (flash_size > 0) {
# ifdef CONFIG_SYS_FLASH_CHECKSUM
char *s = getenv("flashchecksum");
print_size(flash_size, "");
/*
* Compute and print flash CRC if flashchecksum is set to 'y'
*
* NOTE: Maybe we should add some WATCHDOG_RESET()? XXX
*/
if (s && (*s == 'y')) {
printf(" CRC: %08X", crc32(0,
(const unsigned char *) CONFIG_SYS_FLASH_BASE,
flash_size));
}
putc('\n');
# else /* !CONFIG_SYS_FLASH_CHECKSUM */
print_size(flash_size, "\n");
# endif /* CONFIG_SYS_FLASH_CHECKSUM */
} else {
puts(failed);
hang();
}
#endif
#if defined(CONFIG_CMD_NAND)
puts("NAND: ");
nand_init(); /* go init the NAND */
#endif
#if defined(CONFIG_CMD_ONENAND)
onenand_init();
#endif
#ifdef CONFIG_GENERIC_MMC
puts("MMC: ");
mmc_initialize(gd->bd);
#endif
#ifdef CONFIG_HAS_DATAFLASH
AT91F_DataflashInit();
dataflash_print_info();
#endif
/* initialize environment */
env_relocate();
#if defined(CONFIG_CMD_PCI) || defined(CONFIG_PCI)
arm_pci_init();
#endif
/* IP Address */
gd->bd->bi_ip_addr = getenv_IPaddr("ipaddr");
stdio_init(); /* get the devices list going. */
jumptable_init();
#if defined(CONFIG_API)
/* Initialize API */
api_init();
#endif
console_init_r(); /* fully init console as a device */
#if defined(CONFIG_ARCH_MISC_INIT)
/* miscellaneous arch dependent initialisations */
arch_misc_init();
#endif
#if defined(CONFIG_MISC_INIT_R)
/* miscellaneous platform dependent initialisations */
misc_init_r();
#endif
/* set up exceptions */
interrupt_init();
/* enable exceptions */
enable_interrupts();
/* Perform network card initialisation if necessary */
#if defined(CONFIG_DRIVER_SMC91111) || defined (CONFIG_DRIVER_LAN91C96)
/* XXX: this needs to be moved to board init */
if (getenv("ethaddr")) {
uchar enetaddr[6];
eth_getenv_enetaddr("ethaddr", enetaddr);
smc_set_mac_addr(enetaddr);
}
#endif /* CONFIG_DRIVER_SMC91111 || CONFIG_DRIVER_LAN91C96 */
/* Initialize from environment */
load_addr = getenv_ulong("loadaddr", 16, load_addr);
#if defined(CONFIG_CMD_NET)
{
char *s = getenv("bootfile");
if (s != NULL)
copy_filename(BootFile, s, sizeof(BootFile));
}
#endif
#ifdef CONFIG_BOARD_LATE_INIT
board_late_init();
#endif
#ifdef CONFIG_BITBANGMII
bb_miiphy_init();
#endif
#if defined(CONFIG_CMD_NET)
puts("Net: ");
eth_initialize(gd->bd);
#if defined(CONFIG_RESET_PHY_R)
debug("Reset Ethernet PHY\n");
reset_phy();
#endif
#endif
#ifdef CONFIG_POST
post_run(NULL, POST_RAM | post_bootmode_get(0));
#endif
#if defined(CONFIG_PRAM) || defined(CONFIG_LOGBUFFER)
/*
* Export available size of memory for Linux,
* taking into account the protected RAM at top of memory
*/
{
ulong pram = 0;
uchar memsz[32];
#ifdef CONFIG_PRAM
pram = getenv_ulong("pram", 10, CONFIG_PRAM);
#endif
#ifdef CONFIG_LOGBUFFER
#ifndef CONFIG_ALT_LB_ADDR
/* Also take the logbuffer into account (pram is in kB) */
pram += (LOGBUFF_LEN + LOGBUFF_OVERHEAD) / 1024;
#endif
#endif
sprintf((char *)memsz, "%ldk", (gd->ram_size / 1024) - pram);
setenv("mem", (char *)memsz);
}
#endif
/* main_loop() can return to retry autoboot, if so just run it again. */
for (;;) {
main_loop();
}
/* NOTREACHED - no way out of command loop except booting */
}main.c中的main_loop函数
执行到启动内核的调用流程：parse_file_outer->parse_stream_outer->run_list->run_list_real->run_pipe_real->cmd_process->(通过注册到u_boot_cmd结构的的do_bootm函数后向下继续调用)->do_bootm->do_bootm_linux->【boot_prep_linux(填充a_tag)，boot_jump_linux（控制权移交内核，内核入口从image的laodaddr获取）】，到此u_boot寿终正寝，权限交给内核。
void main_loop (void)
{
#ifndef CONFIG_SYS_HUSH_PARSER
static char lastcommand[CONFIG_SYS_CBSIZE] = { 0, };
int len;
int rc = 1;
int flag;
#endif
#if defined(CONFIG_BOOTDELAY) && (CONFIG_BOOTDELAY >= 0)
char *s;
int bootdelay;
#endif
#ifdef CONFIG_PREBOOT
char *p;
#endif
#ifdef CONFIG_BOOTCOUNT_LIMIT
unsigned long bootcount = 0;
unsigned long bootlimit = 0;
char *bcs;
char bcs_set[16];
#endif /* CONFIG_BOOTCOUNT_LIMIT */
#ifdef CONFIG_BOOTCOUNT_LIMIT
bootcount = bootcount_load();
bootcount++;
bootcount_store (bootcount);
sprintf (bcs_set, "%lu", bootcount);
setenv ("bootcount", bcs_set);
bcs = getenv ("bootlimit");
bootlimit = bcs ? simple_strtoul (bcs, NULL, 10) : 0;
#endif /* CONFIG_BOOTCOUNT_LIMIT */
#ifdef CONFIG_MODEM_SUPPORT
debug ("DEBUG: main_loop: do_mdm_init=%d\n", do_mdm_init);
if (do_mdm_init) {
char *str = strdup(getenv("mdm_cmd"));
setenv ("preboot", str); /* set or delete definition */
if (str != NULL)
free (str);
mdm_init(); /* wait for modem connection */
}
#endif /* CONFIG_MODEM_SUPPORT */
#ifdef CONFIG_VERSION_VARIABLE
{
setenv ("ver", version_string); /* set version variable */
}
#endif /* CONFIG_VERSION_VARIABLE */
#ifdef CONFIG_SYS_HUSH_PARSER
u_boot_hush_start ();
#endif
#if defined(CONFIG_HUSH_INIT_VAR)
hush_init_var ();
#endif
#ifdef CONFIG_PREBOOT
if ((p = getenv ("preboot")) != NULL) {
# ifdef CONFIG_AUTOBOOT_KEYED
int prev = disable_ctrlc(1); /* disable Control C checking */
# endif
run_command(p, 0);
# ifdef CONFIG_AUTOBOOT_KEYED
disable_ctrlc(prev); /* restore Control C checking */
# endif
}
#endif /* CONFIG_PREBOOT */
#if defined(CONFIG_UPDATE_TFTP)
update_tftp (0UL);
#endif /* CONFIG_UPDATE_TFTP */
#if defined(CONFIG_BOOTDELAY) && (CONFIG_BOOTDELAY >= 0)
s = getenv ("bootdelay");
bootdelay = s ? (int)simple_strtol(s, NULL, 10) : CONFIG_BOOTDELAY;
debug ("### main_loop entered: bootdelay=%d\n\n", bootdelay);
#if defined(CONFIG_MENU_SHOW)
bootdelay = menu_show(bootdelay);
#endif
# ifdef CONFIG_BOOT_RETRY_TIME
init_cmd_timeout ();
# endif /* CONFIG_BOOT_RETRY_TIME */
#ifdef CONFIG_POST
if (gd->flags & GD_FLG_POSTFAIL) {
s = getenv("failbootcmd");
}
else
#endif /* CONFIG_POST */
#ifdef CONFIG_BOOTCOUNT_LIMIT
if (bootlimit && (bootcount > bootlimit)) {
printf ("Warning: Bootlimit (%u) exceeded. Using altbootcmd.\n",
(unsigned)bootlimit);
s = getenv ("altbootcmd");
}
else
#endif /* CONFIG_BOOTCOUNT_LIMIT */
s = getenv ("bootcmd");
debug ("### main_loop: bootcmd=\"%s\"\n", s ? s : "<UNDEFINED>");
if (bootdelay >= 0 && s && !abortboot (bootdelay)) {
# ifdef CONFIG_AUTOBOOT_KEYED
int prev = disable_ctrlc(1); /* disable Control C checking */
# endif
run_command(s, 0);
# ifdef CONFIG_AUTOBOOT_KEYED
disable_ctrlc(prev); /* restore Control C checking */
# endif
}
# ifdef CONFIG_MENUKEY
if (menukey == CONFIG_MENUKEY) {
s = getenv("menucmd");
if (s)
run_command(s, 0);
}
#endif /* CONFIG_MENUKEY */
#endif /* CONFIG_BOOTDELAY */
/*
* Main Loop for Monitor Command Processing
*/
#ifdef CONFIG_SYS_HUSH_PARSER
parse_file_outer(); /*2410走这里，最终通过cmd_process根据命令行的入参调用内核启动，启动函数定义在U_BOOT_CMD结构体定义的结构中*/
/* This point is never reached */
for (;;);
#else
for (;;) {
#ifdef CONFIG_BOOT_RETRY_TIME
if (rc >= 0) {
/* Saw enough of a valid command to
* restart the timeout.
*/
reset_cmd_timeout();
}
#endif
len = readline (CONFIG_SYS_PROMPT);
flag = 0; /* assume no special flags for now */
if (len > 0)
strcpy (lastcommand, console_buffer);
else if (len == 0)
flag |= CMD_FLAG_REPEAT;
#ifdef CONFIG_BOOT_RETRY_TIME
else if (len == -2) {
/* -2 means timed out, retry autoboot
*/
puts ("\nTimed out waiting for command\n");
# ifdef CONFIG_RESET_TO_RETRY
/* Reinit board to run initialization code again */
do_reset (NULL, 0, 0, NULL);
# else
return; /* retry autoboot */
# endif
}
#endif
if (len == -1)
puts ("<INTERRUPT>\n");
else
rc = run_command(lastcommand, flag);
if (rc <= 0) {
/* invalid command or not repeatable, forget it */
lastcommand[0] = 0;
}
}
#endif /*CONFIG_SYS_HUSH_PARSER*/
}
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