u-boot第二阶段初始化内容的入口函数是_main,_main位于arch/arm/lib/crt0.S文件中：
_main函数中先为调用board_init_f准备初始化环境（设置栈指针sp和并给gd_t结构分配空间）：
.global _main _main: /* * Set up initial C runtime environment and call board_init_f(0). */ #if defined(CONFIG_NAND_SPL) /* deprecated, use instead CONFIG_SPL_BUILD */ ldr sp, =(CONFIG_SYS_INIT_SP_ADDR) #elif defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_STACK) ldr sp, =(CONFIG_SPL_STACK) #else ldr sp, =(CONFIG_SYS_INIT_SP_ADDR) #endif bic sp, sp, #7 /* 8-byte alignment for ABI compliance */ sub sp, #GD_SIZE /* allocate one GD above SP */ bic sp, sp, #7 /* 8-byte alignment for ABI compliance */ mov r8, sp /* GD is above SP */
然后调用board_init_f（0），这是因为r0可以作为输入参数：
mov r0, #0 bl board_init_f
board_init_f(0)代码如下：
init_fnc_t *init_sequence[] = {
arch_cpu_init, /* basic arch cpu dependent setup */
mark_bootstage,
#ifdef CONFIG_OF_CONTROL
fdtdec_check_fdt,
#endif
#if defined(CONFIG_BOARD_EARLY_INIT_F)
board_early_init_f,
#endif
timer_init, /* initialize timer */
#ifdef CONFIG_BOARD_POSTCLK_INIT
board_postclk_init,
#endif
#ifdef CONFIG_FSL_ESDHC
get_clocks,
#endif
env_init, /* initialize environment */
init_baudrate, /* initialze baudrate settings */
serial_init, /* serial communications setup */
console_init_f, /* stage 1 init of console */
display_banner, /* say that we are here */
#if defined(CONFIG_DISPLAY_CPUINFO)
print_cpuinfo, /* display cpu info (and speed) */
#endif
#if defined(CONFIG_DISPLAY_BOARDINFO)
checkboard, /* display board info */
#endif
#if defined(CONFIG_HARD_I2C) || defined(CONFIG_SOFT_I2C)
init_func_i2c,
#endif
dram_init, /* configure available RAM banks */
NULL,
};
void 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 void *new_fdt = NULL; size_t fdt_size = 0; memset((void *)gd, 0, sizeof(gd_t)); gd->mon_len = _bss_end_ofs; #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) { 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 ", gd->mon_len); /* * Ram is setup, size stored in gd !! */ debug("ramsize: %08lX ", 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; #ifdef CONFIG_LOGBUFFER #ifndef CONFIG_ALT_LB_ADDR /* reserve kernel log buffer */ addr -= (LOGBUFF_RESERVE); debug("Reserving %dk for kernel logbuffer at %08lx ", LOGBUFF_LEN, addr); #endif #endif #ifdef CONFIG_PRAM /* * reserve protected RAM */ reg = getenv_ulong("pram", 10, CONFIG_PRAM); addr -= (reg << 10); /* size is in kB */ debug("Reserving %ldk for protected RAM at %08lx ", reg, addr); #endif /* CONFIG_PRAM */ #if !(defined(CONFIG_SYS_ICACHE_OFF) && defined(CONFIG_SYS_DCACHE_OFF)) /* reserve TLB table */ gd->tlb_size = 4096 * 4; addr -= gd->tlb_size; /* round down to next 64 kB limit */ addr &= ~(0x10000 - 1); gd->tlb_addr = addr; debug("TLB table from %08lx to %08lx ", addr, addr + gd->tlb_size); #endif /* round down to next 4 kB limit */ addr &= ~(4096 - 1); debug("Top of RAM usable for U-Boot at: %08lx ", 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; addr &= ~(4096 - 1); debug("Reserving %ldk for U-Boot at: %08lx ", gd->mon_len >> 10, addr); #ifndef CONFIG_SPL_BUILD /* * reserve memory for malloc() arena */ addr_sp = addr - TOTAL_MALLOC_LEN; debug("Reserving %dk for malloc() at: %08lx ", 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); bd = (bd_t *) addr_sp; gd->bd = bd; debug("Reserving %zu Bytes for Board Info at: %08lx ", 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); id = (gd_t *) addr_sp; debug("Reserving %zu Bytes for Global Data at: %08lx ", sizeof (gd_t), addr_sp); #if defined(CONFIG_OF_SEPARATE) && defined(CONFIG_OF_CONTROL) /* * If the device tree is sitting immediate above our image then we * must relocate it. If it is embedded in the data section, then it * will be relocated with other data. */ if (gd->fdt_blob) { fdt_size = ALIGN(fdt_totalsize(gd->fdt_blob) + 0x1000, 32); addr_sp -= fdt_size; new_fdt = (void *)addr_sp; debug("Reserving %zu Bytes for FDT at: %08lx ", fdt_size, addr_sp); } #endif /* setup stackpointer for exeptions */ gd->irq_sp = addr_sp; #ifdef CONFIG_USE_IRQ addr_sp -= (CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ); debug("Reserving %zu Bytes for IRQ stack at: %08lx ", CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ, addr_sp); #endif /* leave 3 words for abort-stack */ addr_sp -= 12; /* 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 ", 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; gd->start_addr_sp = addr_sp; gd->reloc_off = addr - _TEXT_BASE; debug("relocation Offset is: %08lx ", gd->reloc_off); if (new_fdt) { memcpy(new_fdt, gd->fdt_blob, fdt_size); gd->fdt_blob = new_fdt; } memcpy(id, (void *)gd, sizeof(gd_t)); }
从代码中可以看出，board_init_f对gd内容进行初始化并将其拷贝到id变量中。
此外，代码中还对init_sequence中的所有函数按顺序依次执行，
从而完成了对MPLL、UPLL、GPIO、Timer、波特率、串口配置以及显示u-boot版本号、显示CPU型号、显示DRAM信息等功能
接着设置新sp和gd，然后跳转到relocate_code进行重定位，relocate_code的代码如下：
.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 */ adr r0, _start cmp r0, r6 moveq r9, #0 /* no relocation. relocation offset(r9) = 0 */ beq relocate_done /* 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 #ifndef CONFIG_SPL_BUILD /* * fix .rel.dyn relocations */ 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 relocate_done: mov pc, lr
relocate_code刚开始执行前先将here的值保存到lr寄存器中，然后执行重定位，执行完毕后跳转到lr寄存器(即here)执行。
然后对BSS段进行初始化：
here: /* Set up final (full) environment */ bl c_runtime_cpu_setup /* we still call old routine here */ ldr r0, =__bss_start /* this is auto-relocated! */ 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 */ strlo r2, [r0] /* clear 32-bit BSS word */ addlo r0, r0, #4 /* move to next */ blo clbss_l
接着调用board_init_r()函数：
bl coloured_LED_init bl red_led_on #if defined(CONFIG_NAND_SPL) /* call _nand_boot() */ ldr pc, =nand_boot #else /* call board_init_r(gd_t *id, ulong dest_addr) */ mov r0, r8 /* gd_t */ ldr r1, [r8, #GD_RELOCADDR] /* dest_addr */ /* call board_init_r */ ldr pc, =board_init_r /* this is auto-relocated! */ #endif
board_init_r函数代码如下：
void board_init_r(gd_t *id, ulong dest_addr) { ulong malloc_start; #if !defined(CONFIG_SYS_NO_FLASH) ulong flash_size; #endif 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 ", monitor_flash_len); board_init(); /* Setup chipselects */ /* * 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 serial_initialize(); debug("Now running in RAM - U-Boot at: %08lx ", 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; mem_malloc_init (malloc_start, TOTAL_MALLOC_LEN); #ifdef CONFIG_ARCH_EARLY_INIT_R arch_early_init_r(); #endif power_init_board(); #if !defined(CONFIG_SYS_NO_FLASH) puts("Flash: "); flash_size = flash_init(); if (flash_size > 0) { # ifdef CONFIG_SYS_FLASH_CHECKSUM 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 (getenv_yesno("flashchecksum") == 1) { printf(" CRC: %08X", crc32(0, (const unsigned char *) CONFIG_SYS_FLASH_BASE, flash_size)); } putc(' '); # else /* !CONFIG_SYS_FLASH_CHECKSUM */ print_size(flash_size, " "); # 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 */ if (should_load_env()) env_relocate(); else set_default_env(NULL); #if defined(CONFIG_CMD_PCI) || defined(CONFIG_PCI) arm_pci_init(); #endif 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 */ #ifdef CONFIG_DISPLAY_BOARDINFO_LATE # ifdef CONFIG_OF_CONTROL /* Put this here so it appears on the LCD, now it is ready */ display_fdt_model(gd->fdt_blob); # else checkboard(); # endif #endif #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(); /* Initialize from environment */ load_addr = getenv_ulong("loadaddr", 16, load_addr); #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 "); 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 */ }
在board_init_r函数中依次执行使能数据Cache和指令Cache，三个串口初始化，内存分配初始化，flash初始化，nand初始化，mmc设备初始化，stdio初始化，stdout和stderr初始化，开中断,网卡初始化等操作，
然后反复执行main_loop函数。
u-boot第二个阶段的执行内容如下：
（1）为board_init_f准备sp和gd
（2）执行board_init_f,对MPLL、UPLL、GPIO、Timer、波特率、串口配置，显示u-boot版本号、CPU型号和DRAM信息，并初始化gd内容。
（3）调用relocate_code进行代码重定位
（4）初始化BSS
（5）调用board_init_r，使能数据Cache和指令Cache，三个串口初始化，内存分配初始化，flash初始化，nand初始化，
mmc设备初始化，stdio初始化，stdout和stderr初始化，开中断，网卡初始化，然后执行main_loop等待用户输入命令或者超时后进入linux内核。