uboot移植---start.s分析

具体见注释

#include <config.h>
#include <version.h>
#ifdef CONFIG_ENABLE_MMU
#include <asm/proc/domain.h>
#endif
#include <asm/arch/s3c6410.h>

#if !defined(CONFIG_ENABLE_MMU) && !defined(CONFIG_SYS_PHY_UBOOT_BASE)
#define CONFIG_SYS_PHY_UBOOT_BASE CONFIG_SYS_UBOOT_BASE
#endif

/*
*************************************************************************
*
* Jump vector table as in table 3.1 in [1]
*
*************************************************************************
*/

.globl _start
_start: b reset
#ifndef CONFIG_NAND_SPL
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

_undefined_instruction:
.word undefined_instruction
_software_interrupt:
.word software_interrupt
_prefetch_abort:
.word prefetch_abort
_data_abort:
.word data_abort
_not_used:
.word not_used
_irq:
.word irq
_fiq:
.word fiq
_pad:
.word 0x12345678 /* now 16*4=64 */
#else
. = _start + 64
#endif

.global _end_vect
_end_vect:
.balignl 16,0xdeadbeef
/*
*************************************************************************
*
* 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
*
*************************************************************************
*/

_TEXT_BASE:
.word TEXT_BASE

/*
* Below variable is very important because we use MMU in U-Boot.
* Without it, we cannot run code correctly before MMU is ON.
* by scsuh.
*/
_TEXT_PHY_BASE:
.word CONFIG_SYS_PHY_UBOOT_BASE

.globl _armboot_start
_armboot_start:
.word _start

/*
* These are defined in the board-specific linker script.
*/
.globl _bss_start
_bss_start:
.word __bss_start

.globl _bss_end
_bss_end:
.word _end

/*
* the actual reset code
*/
/* 从reset开始看，前面就不分析了 */
reset:
/*
* 设置SVC超级用户模式,就是这么规定，可参考
*《ARM ArchitectureReference Manual》 的 Program status registers 一节
*/
mrs r0, cpsr
bic r0, r0, #0x3f
orr r0, r0, #0xd3
msr cpsr, r0

/*
*************************************************************************
*
* CPU_init_critical registers
* setup important registers
* setup memory timing
*
*************************************************************************
*/
/*
* we do sys-critical inits only at reboot,
* not when booting from ram!
*/
cpu_init_crit:
/*
* When booting from NAND - it has definitely been a reset, so, no need
* to flush caches and disable the MMU
*/
#ifndef CONFIG_NAND_SPL
/*
* flush v4 I/D caches 清除数据缓存和指令缓存
*/
mov r0, #0
mcr p15, 0, r0, c7, c7, 0 /* flush v3/v4 cache */
mcr p15, 0, r0, c8, c7, 0 /* flush v4 TLB */

/*
* disable MMU stuff and caches
* 关闭MMU和数据缓存，开启指令缓存
* 设置参照arm1176jzfs文档3.2.7节
*/
mrc p15, 0, r0, c1, c0, 0
bic r0, r0, #0x00002300 @ clear bits 13, 9:8 (--V- --RS)
bic r0, r0, #0x00000087 @ clear bits 7, 2:0 (B--- -CAM)
orr r0, r0, #0x00000002 @ set bit 2 (A) Align
orr r0, r0, #0x00001000 @ set bit 12 (I) I-Cache
/*
* 下面这行原先是在第193行，关闭了MMU，如果不注释掉的话当执行到 MOV PC,R2后就应该是
* 转跳到了内存中间运行，而此时我们的DDR和其他外设没初始化，而且代码也没有重定位
*/
mcr p15, 0, r0, c1, c0, 0
/* Prepare to disable the MMU */
#if 0
adr r1, mmu_disable_phys
/* We presume we're within the first 1024 bytes */
and r1, r1, #0x3fc
ldr r2, _TEXT_PHY_BASE
ldr r3, =0xfff00000
and r2, r2, r3
orr r2, r2, r1
b mmu_disable

.align 5
/* Run in a single cache-line */
mmu_disable:
mcr p15, 0, r0, c1, c0, 0
nop
nop
mov pc, r2
#endif
#endif
/* 设置外设基地址 */
mmu_disable_phys:
/* Peri port setup */
ldr r0, =0x70000000
orr r0, r0, #0x13
mcr p15,0,r0,c15,c2,4 @ 256M (0x70000000 - 0x7fffffff)

/*
* 跳到 lowlevel_init 执行底层初始化 ,
* 至于怎么初始化先不管，暂且认为他初始化好了DDR，UART.....等等外设
*/
bl lowlevel_init /* go setup pll,mux,memory */

/* CONFIG_ENABLE_MMU没有定义，下面这段代码不用看 */
after_copy:
#if 0
#ifdef CONFIG_ENABLE_MMU
enable_mmu:
/* enable domain access */
ldr r5, =0x0000ffff
mcr p15, 0, r5, c3, c0, 0 /* load domain access register */

/* Set the TTB register */
ldr r0, _mmu_table_base
ldr r1, =CONFIG_SYS_PHY_UBOOT_BASE
ldr r2, =0xfff00000
bic r0, r0, r2
orr r1, r0, r1
mcr p15, 0, r1, c2, c0, 0

/* Enable the MMU */
mrc p15, 0, r0, c1, c0, 0
orr r0, r0, #1 /* Set CR_M to enable MMU */

/* Prepare to enable the MMU */
adr r1, skip_hw_init
and r1, r1, #0x3fc
ldr r2, _TEXT_BASE
ldr r3, =0xfff00000
and r2, r2, r3
orr r2, r2, r1
b mmu_enable

.align 5
/* Run in a single cache-line */
mmu_enable:

mcr p15, 0, r0, c1, c0, 0
nop
nop
mov pc, r2
#endif
#endif
/* 下一步是设置栈地址，其内存分布图如下:
*
* +---------------------------------------+ <----- r0 = CONFIG_SYS_UBOOT_BASE = 0x57e00000
* | |
* | CONFIG_SYS_MALLOC_LEN |
* | |
* +---------------------------------------+ <----- r0 = r0 - CONFIG_SYS_MALLOC_LEN
* | |
* | CONFIG_SYS_GBL_DATA_SIZE |
* | |
* +---------------------------------------+ <----- r0 = r0 - CONFIG_SYS_GBL_DATA_SIZE
* | |
* | 12 BYTE |
* +---------------------------------------+ <----- sp = r0 - 12
*/
skip_hw_init:
/* Set up the stack */
stack_setup:
ldr r0, =CONFIG_SYS_UBOOT_BASE /* base of copy in DRAM */
sub r0, r0, #CONFIG_SYS_MALLOC_LEN /* malloc area */
sub r0, r0, #CONFIG_SYS_GBL_DATA_SIZE /* bdinfo */
sub sp, r0, #12 /* leave 3 words for abort-stack */

/* 清除BSS段 */
clear_bss:
ldr r0, _bss_start /* find start of bss segment */
ldr r1, _bss_end /* stop here */
mov r2, #0 /* clear */
clbss_l:
str r2, [r0] /* clear loop... */
add r0, r0, #4
cmp r0, r1
ble clbss_l

#ifndef CONFIG_NAND_SPL
/* 转跳到start_armboot去执行，正常来说跳转过去后是不会回来了的，所以后面的代码不用看了 */
ldr pc, _start_armboot

_start_armboot:
.word start_armboot
#else
b nand_boot
/* .word nand_boot*/
#endif

<span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">我们发现start.s并未对代码进行重定位,而且lowlevel_init和start_armboot也没有对代码重定位，所以代码重定位的工作就需要我们自己做了</span>

下篇准备分许lowlevel_init，那重定位就下下篇说吧

实际上 配置CONFIG_NAND_SPL应该是会进行代码重定位，不过还没时间去试试

等有时间试试看