lowlevel_init 函數
函數 lowlevel_init 在文件 arch/arm/cpu/armv7/lowlevel_init.S 中定義，
/*
* A lowlevel_init function that sets up the stack to call a C function to
* perform further init.
*
* (C) Copyright 2010
* Texas Instruments, <www.ti.com>
*
* Author :
* Aneesh V <[email protected]>
*
* SPDX-License-Identifier: GPL-2.0+
*/
#include <asm-offsets.h>
#include <config.h>
#include <linux/linkage.h>
ENTRY(lowlevel_init)
/*
* Setup a temporary stack. Global data is not available yet.
*/
ldr sp, =CONFIG_SYS_INIT_SP_ADDR
bic sp, sp, #7 /* 8-byte alignment for ABI compliance */
#ifdef CONFIG_SPL_DM
mov r9, #0
#else
/*
* Set up global data for boards that still need it. This will be
* removed soon.
*/
#ifdef CONFIG_SPL_BUILD
ldr r9, =gdata
#else
sub sp, sp, #GD_SIZE
bic sp, sp, #7
mov r9, sp
#endif
#endif
/*
* Save the old lr(passed in ip) and the current lr to stack
*/
push {ip, lr}
/*
* Call the very early init function. This should do only the
* absolute bare minimum to get started. It should not:
*
* - set up DRAM
* - use global_data
* - clear BSS
* - try to start a console
*
* For boards with SPL this should be empty since SPL can do all of
* this init in the SPL board_init_f() function which is called
* immediately after this.
*/
bl s_init
pop {ip, pc}
ENDPROC(lowlevel_init)
第 22 行設置 sp 指向 CONFIG_SYS_INIT_SP_ADDR， CONFIG_SYS_INIT_SP_ADDR 在include/configs/mx6ullevk.h 文件中
IRAM_BASE_ADDR 和 IRAM_SIZE 在 文 件arch/arm/include/asm/arch-mx6/imx-regs.h 中有定義
如果 408 行的條件成立的話 IRAM_SIZE=0X40000，當定義了 CONFIG_MX6SX、CONFIG_MX6U、 CONFIG_MX6SLL 和 CONFIG_MX6SL 中的任意一個的話條件就不成立，在.config 中定義了 CONFIG_MX6UL，所以條件不成立，因此 IRAM_SIZE=0X20000=128KB。
可以得到如下值：
CONFIG_SYS_INIT_RAM_ADDR = IRAM_BASE_ADDR = 0x00900000。
CONFIG_SYS_INIT_RAM_SIZE = 0x00020000 =128KB。
還需要知道GENERATED_GBL_DATA_SIZE的值，在文件include/generated/generic-asm-offsets.h中有定義
GENERATED_GBL_DATA_SIZE=256， GENERATED_GBL_DATA_SIZE 的含義爲
(sizeof(struct global_data) + 15) & ~15 。
綜上所述， CONFIG_SYS_INIT_SP_ADDR 值如下：
CONFIG_SYS_INIT_SP_OFFSET = 0x00020000 – 256 = 0x1FF00。
CONFIG_SYS_INIT_SP_ADDR = 0x00900000 + 0X1FF00 = 0X0091FF00，
此時 sp 指向 0X91FF00，這屬於 IMX6UL/IMX6ULL 的內部 ram。繼續回到文件 lowlevel_init.S，第 23 行對 sp 指針做 8 字節對齊處理！
第 34 行， sp 指針減去 GD_SIZE， GD_SIZE 同樣在 generic-asm-offsets.h 中定了，大小爲248
第 35 行對 sp 做 8 字節對齊，此時 sp 的地址爲 0X0091FF00-248=0X0091FE08，此時 sp 位置如圖所示：
第 36 行將 sp 地址保存在 r9 寄存器中。
第 42 行將 ip 和 lr 壓棧
第 57 行調用函數 s_init
第 58 行將第 36 行入棧的 ip 和 lr 進行出棧，並將 lr 賦給 pc。
s_init 函數
s_init 函數定義在文件arch/arm/cpu/armv7/mx6/soc.c 中
在第 816 行會判斷當前 CPU 類型，如果 CPU 爲 MX6SX、 MX6UL、 MX6ULL 或 MX6SLL中 的 任 意 一 種 ， 那 麼 就 會 直 接 返 回 ， 相 當 於 s_init 函 數 什 麼 都 沒 做 。 所 以 對 於I.MX6UL/I.MX6ULL 來說， s_init 就是個空函數。從 s_init 函數退出以後進入函數 lowlevel_init，但是 lowlevel_init 函數也執行完成了，返回到了函數 cpu_init_crit，函數 cpu_init_crit 也執行完成了，最終返回到 save_boot_params_ret
接下來要執行的是 save_boot_params_ret 中的_main 函數，接下來分析_main 函數。
_main 函數
_main 函數定義在文件 arch/arm/lib/crt0.S 中
/*
* 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 76 行，設置 sp 指針爲 CONFIG_SYS_INIT_SP_ADDR，也就是 sp 指向 0X0091FF00。
第 83 行， sp 做 8 字節對齊。
第 85 行，讀取 sp 到寄存器 r0 裏面，此時 r0=0X0091FF00。
第 86 行，調用函數 board_init_f_alloc_reserve，此函數有一個參數，參數爲 r0 中的值，也就是 0X0091FF00，此函數定義在文件 common/init/board_init.c 中
函數 board_init_f_alloc_reserve 主要是留出早期的 malloc 內存區域和 gd 內存區域，其中CONFIG_SYS_MALLOC_F_LEN=0X400( 在 文 件 include/generated/autoconf.h 中 定 義 ) ，sizeof(struct global_data)=248(GD_SIZE 值)
函數 board_init_f_alloc_reserve 是有返回值的，返回值爲新的 top 值，從圖 32.2.4.1 可知，此時 top=0X0091FA00。
第 87 行，將 r0 寫入到 sp 裏面， r0 保存着函數board_init_f_alloc_reserve 的返回值，所以這一句也就是設置 sp=0X0091FA00。
第 89 行，將 r0 寄存器的值寫到寄存器 r9 裏面，因爲 r9 寄存器存放着全局變量 gd 的地址，在文件 arch/arm/include/asm/global_data.h 中有如圖所示宏定義
uboot 中定義了一個指向 gd_t 的指針 gd， gd 存放在寄存器 r9 裏面的，因此 gd 是個全局變量。 gd_t 是個結構體，在 include/asm-generic/global_data.h 裏面有定義，
*
* Copyright (c) 2012 The Chromium OS Authors.
* (C) Copyright 2002-2010
* Wolfgang Denk, DENX Software Engineering, wd@denx.de.
*
* SPDX-License-Identifier: GPL-2.0+
*/
#ifndef __ASM_GENERIC_GBL_DATA_H
#define __ASM_GENERIC_GBL_DATA_H
/*
* The following data structure is placed in some memory which is
* available very early after boot (like DPRAM on MPC8xx/MPC82xx, or
* some locked parts of the data cache) to allow for a minimum set of
* global variables during system initialization (until we have set
* up the memory controller so that we can use RAM).
*
* Keep it *SMALL* and remember to set GENERATED_GBL_DATA_SIZE > sizeof(gd_t)
*
* Each architecture has its own private fields. For now all are private
*/
#ifndef __ASSEMBLY__
#include <membuff.h>
#include <linux/list.h>
typedef struct global_data {
bd_t *bd;
unsigned long flags;
unsigned int baudrate;
unsigned long cpu_clk; /* CPU clock in Hz! */
unsigned long bus_clk;
/* We cannot bracket this with CONFIG_PCI due to mpc5xxx */
unsigned long pci_clk;
unsigned long mem_clk;
#if defined(CONFIG_LCD) || defined(CONFIG_VIDEO)
unsigned long fb_base; /* Base address of framebuffer mem */
#endif
#if defined(CONFIG_POST) || defined(CONFIG_LOGBUFFER)
unsigned long post_log_word; /* Record POST activities */
unsigned long post_log_res; /* success of POST test */
unsigned long post_init_f_time; /* When post_init_f started */
#endif
#ifdef CONFIG_BOARD_TYPES
unsigned long board_type;
#endif
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 */
unsigned long env_valid; /* Checksum of Environment valid? */
unsigned long ram_top; /* Top address of RAM used by U-Boot */
unsigned long relocaddr; /* Start address of U-Boot in RAM */
phys_size_t ram_size; /* RAM size */
#ifdef CONFIG_SYS_MEM_RESERVE_SECURE
#define MEM_RESERVE_SECURE_SECURED 0x1
#define MEM_RESERVE_SECURE_MAINTAINED 0x2
#define MEM_RESERVE_SECURE_ADDR_MASK (~0x3)
/*
* Secure memory addr
* This variable needs maintenance if the RAM base is not zero,
* or if RAM splits into non-consecutive banks. It also has a
* flag indicating the secure memory is marked as secure by MMU.
* Flags used: 0x1 secured
* 0x2 maintained
*/
phys_addr_t secure_ram;
#endif
unsigned long mon_len; /* monitor len */
unsigned long irq_sp; /* irq stack pointer */
unsigned long start_addr_sp; /* start_addr_stackpointer */
unsigned long reloc_off;
struct global_data *new_gd; /* relocated global data */
#ifdef CONFIG_DM
struct udevice *dm_root; /* Root instance for Driver Model */
struct udevice *dm_root_f; /* Pre-relocation root instance */
struct list_head uclass_root; /* Head of core tree */
#endif
#ifdef CONFIG_TIMER
struct udevice *timer; /* Timer instance for Driver Model */
#endif
const void *fdt_blob; /* Our device tree, NULL if none */
void *new_fdt; /* Relocated FDT */
unsigned long fdt_size; /* Space reserved for relocated FDT */
struct jt_funcs *jt; /* jump table */
char env_buf[32]; /* buffer for getenv() before reloc. */
#ifdef CONFIG_TRACE
void *trace_buff; /* The trace buffer */
#endif
#if defined(CONFIG_SYS_I2C)
int cur_i2c_bus; /* current used i2c bus */
#endif
#ifdef CONFIG_SYS_I2C_MXC
void *srdata[10];
#endif
unsigned long timebase_h;
unsigned long timebase_l;
#ifdef CONFIG_SYS_MALLOC_F_LEN
unsigned long malloc_base; /* base address of early malloc() */
unsigned long malloc_limit; /* limit address */
unsigned long malloc_ptr; /* current address */
#endif
#ifdef CONFIG_PCI
struct pci_controller *hose; /* PCI hose for early use */
phys_addr_t pci_ram_top; /* top of region accessible to PCI */
#endif
#ifdef CONFIG_PCI_BOOTDELAY
int pcidelay_done;
#endif
struct udevice *cur_serial_dev; /* current serial device */
struct arch_global_data arch; /* architecture-specific data */
#ifdef CONFIG_CONSOLE_RECORD
struct membuff console_out; /* console output */
struct membuff console_in; /* console input */
#endif
#ifdef CONFIG_DM_VIDEO
ulong video_top; /* Top of video frame buffer area */
ulong video_bottom; /* Bottom of video frame buffer area */
#endif
} gd_t;
#endif
/*
* Global Data Flags - the top 16 bits are reserved for arch-specific flags
*/
#define GD_FLG_RELOC 0x00001 /* Code was relocated to RAM */
#define GD_FLG_DEVINIT 0x00002 /* Devices have been initialized */
#define GD_FLG_SILENT 0x00004 /* Silent mode */
#define GD_FLG_POSTFAIL 0x00008 /* Critical POST test failed */
#define GD_FLG_POSTSTOP 0x00010 /* POST seqeunce aborted */
#define GD_FLG_LOGINIT 0x00020 /* Log Buffer has been initialized */
#define GD_FLG_DISABLE_CONSOLE 0x00040 /* Disable console (in & out) */
#define GD_FLG_ENV_READY 0x00080 /* Env. imported into hash table */
#define GD_FLG_SERIAL_READY 0x00100 /* Pre-reloc serial console ready */
#define GD_FLG_FULL_MALLOC_INIT 0x00200 /* Full malloc() is ready */
#define GD_FLG_SPL_INIT 0x00400 /* spl_init() has been called */
#define GD_FLG_SKIP_RELOC 0x00800 /* Don't relocate */
#define GD_FLG_RECORD 0x01000 /* Record console */
#endif /* __ASM_GENERIC_GBL_DATA_H */
這一行代碼就是設置 gd 所指向的位置，也就是 gd 指向 0X0091FA00。
第 90 行調用函數 board_init_f_init_reserve，此函數在文件common/init/board_init.c 中有定義
/*
* Code shared between SPL and U-Boot proper
*
* Copyright (c) 2015 Google, Inc
* Written by Simon Glass <[email protected]>
*
* SPDX-License-Identifier: GPL-2.0+
*/
#include <common.h>
DECLARE_GLOBAL_DATA_PTR;
/*
* It isn't trivial to figure out whether memcpy() exists. The arch-specific
* memcpy() is not normally available in SPL due to code size.
*/
#if !defined(CONFIG_SPL_BUILD) || \
(defined(CONFIG_SPL_LIBGENERIC_SUPPORT) && \
!defined(CONFIG_USE_ARCH_MEMSET))
#define _USE_MEMCPY
#endif
/* Unfortunately x86 or ARM can't compile this code as gd cannot be assigned */
#if !defined(CONFIG_X86) && !defined(CONFIG_ARM)
__weak void arch_setup_gd(struct global_data *gd_ptr)
{
gd = gd_ptr;
}
#endif /* !CONFIG_X86 && !CONFIG_ARM */
/*
* Allocate reserved space for use as 'globals' from 'top' address and
* return 'bottom' address of allocated space
*
* Notes:
*
* Actual reservation cannot be done from within this function as
* it requires altering the C stack pointer, so this will be done by
* the caller upon return from this function.
*
* IMPORTANT:
*
* Alignment constraints may differ for each 'chunk' allocated. For now:
*
* - GD is aligned down on a 16-byte boundary
*
* - the early malloc arena is not aligned, therefore it follows the stack
* alignment constraint of the architecture for which we are bulding.
*
* - GD is allocated last, so that the return value of this functions is
* both the bottom of the reserved area and the address of GD, should
* the calling context need it.
*/
ulong board_init_f_alloc_reserve(ulong top)
{
/* Reserve early malloc arena */
#if defined(CONFIG_SYS_MALLOC_F)
top -= CONFIG_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;
}
/*
* Initialize reserved space (which has been safely allocated on the C
* stack from the C runtime environment handling code).
*
* Notes:
*
* Actual reservation was done by the caller; the locations from base
* to base+size-1 (where 'size' is the value returned by the allocation
* function above) can be accessed freely without risk of corrupting the
* C runtime environment.
*
* IMPORTANT:
*
* Upon return from the allocation function above, on some architectures
* the caller will set gd to the lowest reserved location. Therefore, in
* this initialization function, the global data MUST be placed at base.
*
* ALSO IMPORTANT:
*
* On some architectures, gd will already be good when entering this
* function. On others, it will only be good once arch_setup_gd() returns.
* Therefore, global data accesses must be done:
*
* - through gd_ptr if before the call to arch_setup_gd();
*
* - through gd once arch_setup_gd() has been called.
*
* Do not use 'gd->' until arch_setup_gd() has been called!
*
* IMPORTANT TOO:
*
* Initialization for each "chunk" (GD, early malloc arena...) ends with
* an incrementation line of the form 'base += <some size>'. The last of
* these incrementations seems useless, as base will not be used any
* more after this incrementation; but if/when a new "chunk" is appended,
* this increment will be essential as it will give base right value for
* this new chunk (which will have to end with its own incrementation
* statement). Besides, the compiler's optimizer will silently detect
* and remove the last base incrementation, therefore leaving that last
* (seemingly useless) incrementation causes no code increase.
*/
void board_init_f_init_reserve(ulong base)
{
struct global_data *gd_ptr;
#ifndef _USE_MEMCPY
int *ptr;
#endif
/*
* 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 */
#ifdef _USE_MEMCPY
memset(gd_ptr, '\0', sizeof(*gd));
#else
for (ptr = (int *)gd_ptr; ptr < (int *)(gd_ptr + 1); )
*ptr++ = 0;
#endif
/* 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 defined(CONFIG_SYS_MALLOC_F)
/* go down one 'early malloc arena' */
gd->malloc_base = base;
/* next alloc will be higher by one 'early malloc arena' size */
base += CONFIG_SYS_MALLOC_F_LEN;
#endif
}
可以看出，此函數用於初始化 gd，其實就是清零處理。 另外，此函數還設置了gd->malloc_base 爲 gd 基地址+gd 大小=0X0091FA00+248=0X0091FAF8，在做 16 字節對齊，最終 gd->malloc_base=0X0091FB00，這個也就是 early malloc 的起始地址。
繼續回到代碼main 中，第 92 行設置 R0 爲 0。
第 93 行，調用 board_init_f 函數，此函數定義在文件 common/board_f.c 中！主要用來初始化 DDR，定時器，完成代碼拷貝等等，此函數我們後面在詳細的分析。
第 103 行，重新設置環境(sp 和 gd)、獲取 gd->start_addr_sp 的值賦給 sp，在函數 board_init_f中會初始化 gd 的所有成員變量，其中 gd->start_addr_sp=0X9EF44E90， 所以這裏相當於設置sp=gd->start_addr_sp=0X9EF44E90。 0X9EF44E90 是 DDR 中的地址，說明新的 sp 和 gd 將會存放到 DDR 中，而不是內部的 RAM 了。 GD_START_ADDR_SP=64，參考示例代碼 32.2.2.4。
第 109 行， sp 做 8 字節對齊。
第 111 行，獲取 gd->bd 的地址賦給 r9，此時 r9 存放的是老的 gd，這裏通過獲取 gd->bd 的地址來計算出新的 gd 的位置。 GD_BD=0，參考示例代碼 32.2.2.4。
第 112 行，新的 gd 在 bd 下面，所以 r9 減去 gd 的大小就是新的 gd 的位置，獲取到新的 gd的位置以後賦值給 r9。
第 114 行，設置 lr 寄存器爲 here，這樣後面執行其他函數返回的時候就返回到了第 122 行的 here 位置處。
第 115，讀取 gd->reloc_off 的值複製給 r0 寄存器， GD_RELOC_OFF=68
第 116 行， lr 寄存器的值加上 r0 寄存器的值，重新賦值給 lr 寄存器。因爲接下來要重定位代碼，也就是把代碼拷貝到新的地方去(現在的 uboot 存放的起始地址爲 0X87800000，下面要將 uboot 拷貝到 DDR 最後面的地址空間出，將 0X87800000 開始的內存空出來)，其中就包括here，因此 lr 中的 here 要使用重定位後的位置。
第 120 行，讀取 gd->relocaddr 的值賦給 r0 寄存器，此時 r0 寄存器就保存着 uboot 要拷貝的目的地址，爲 0X9FF47000。GD_RELOCADDR=48，參考示例代碼 32.2.2.4。
第 121 行，調用函數 relocate_code，也就是代碼重定位函數，此函數負責將 uboot 拷貝到新的地方去，此函數定義在文件 arch/arm/lib/relocate.S 中稍後會詳細分析此函數。
第 127 行，調用函數 relocate_vectors，對中斷向量表做重定位，此函數定義在文件arch/arm/lib/relocate.S 中，稍後會詳細分析此函數。
第 131 行，調用函數 c_runtime_cpu_setup，此函數定義在文件 arch/arm/cpu/armv7/start.S 中，
函數內容如下：
79 * If I-cache is enabled invalidate it
80 */
81 #ifndef CONFIG_SYS_ICACHE_OFF
82 mcr p15, 0, r0, c7, c5, 0 @ invalidate icache
83 mcr p15, 0, r0, c7, c10, 4 @ DSB
84 mcr p15, 0, r0, c7, c5, 4 @ ISB
85 #endif
86
87 bx lr
88
89 ENDPROC(c_runtime_cpu_setup)
第 141~159 行，清除 BSS 段。
第 167 行，設置函數 board_init_r 的兩個參數，函數 board_init_r 聲明如下：board_init_r(gd_t *id, ulong dest_addr) 第一個參數是 gd，因此讀取 r9 保存到 r0 裏面。
第 168 行，設置函數 board_init_r 的第二個參數是目的地址，因此 r1= gd->relocaddr。
第 174 行、調用函數 board_init_r，此函數定義在文件 common/board_r.c 中，稍後會詳細的分析此函數。
這個就是_main 函數的運行流程，在_main 函數裏面調用了 board_init_f、 relocate_code、relocate_vectors 和 board_init_r 這 4 個函數