u-boot移植总结（一）start.S分析

本次移植u-boot-2010.09是基于S3C2440的FL440板子，板子自带NANDFLASH而没有NORFLASH，所以在U-BOOT启动的过程中必须实现从NANDFLASH到SDRAM的重定向。

其中最重要的就是在U-BOOT开始的start.S汇编代码,这段代码要完成工作：

1，异常中断向量表，复位后异常向量处理

2， 跳转到代码实际执行处start_code

3，关闭看门狗WATCHDOG

3，关闭所有中断INTERRUPT

4，设置时钟分频,主要设置寄存器CLKDVN,MPLLCON,UPLLCON

5，关闭MMU和CACHE，并调用lowlevel_init.S完成SDRAM和NANDFLASH的初始化，为代码的重定向做准备

6，设置堆栈，并且跳入第二阶段的C代码

7，异常向量处理代码

以下为start.S的分析：

1，异常中断向量表，复位后异常向量处理

//声明一个全局标量，在cpu/arm920t/u-boot.lds中有定义，即代码的入口地址，也是编译地址_start: b   start_code   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  //.word 定义一个32位的地址标识_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   .balignl 16,0xdeadbeef//将地址偏移为16的整数倍，空余的内容填上0xdeadbeef，这个数即“Magic Number”,可用判断当前u-boot执行位置_TEXT_BASE:   .word   TEXT_BASE    //在config.mk中有定义，即u-boot自启时flash从定向到sdram的地址.globl _armboot_start   //声明一个全局变量,之后要调用_armboot_start:   .word _start/* * These are defined in the board-specific linker script. */.globl _bss_start                   //连接脚本u-boot.lds中有定义_bss_start:   .word __bss_start.globl _bss_end//连接脚本u-boot.lds中有定义</span>_bss_end:  .word _end#ifdef CONFIG_USE_IRQ           //堆栈设置/* IRQ stack memory (calculated at run-time) */.globl IRQ_STACK_STARTIRQ_STACK_START:  .word   0x0badc0de/* IRQ stack memory (calculated at run-time) */.globl FIQ_STACK_STARTFIQ_STACK_START:   .word 0x0badc0de#endif

2， 跳转到代码实际执行处（设置管理模式=>关闭看门狗=>关闭所有中断=>设置时钟分频）

/* * the actual start code */start_code:    /*     * set the cpu to SVC32 mode       */                              //设置管理模式  31  30  29  28      7   6   4    3    2    1    0      mrs r0, cpsr              //    CPSR      N   Z   C   V       I   F   M4   M3   M2   M1   M0    bic r0, r0, #0x1f         //                                          1    0    0    1    1    orr r0, r0, #0xd3      msr cpsr, r0           // CPSR为状态寄存器，用于设置系统运行状态，只能用MSR MRS指令#if defined(CONFIG_AT91RM9200DK) || defined(CONFIG_AT91RM9200EK)   //系统中断重定向于RAM中，以便快速响应中断,搬运的代码为4*16bytes/*  relocate exception table */    ldr r0, =_start    ldr r1, =0x0    mov r2, #16copyex:     subs r2, r2, #1    ldr r3, [r0], #4     str r3, [r1], #4     bne copyex#endif//关闭看门狗#if defined(CONFIG_S3C2400) || defined(CONFIG_S3C2410) || defined(CONFIG_S3C2440)    /* turn off the watchdog */#if defined(CONFIG_S3C2400)    #define pWTCON    0x15300000#define INTMSK    0x14400008  /* Interupt-Controller base addresses */#define CLKDIVN   0x14800014  /* clock divisor register */#else//查阅s3c2440的datesheet中指出寄存器地址#define pWTCON    0x53000000 #define INTMSK    0x4A000008  /* Interupt-Controller base addresses */#define INTSUBMSK 0x4A00001C#define CLKDIVN   0x4C000014  /* clock divisor register */#endif#define CLK_CTL_BASE 0x4C000000    //添加时钟分频寄存器地址，用于时钟分频设置#define MDIV_405 0x7f<<12#define PSDIV_405 0x21#define MDIV_200 0xa1<<12#define PSDIV_200 0x31 </span>    ldr r0, =pWTCON    mov r1, #0x0    str r1, [r0] /*  mask all IRQs by setting all bits in the INTMR - default */    mov r1, #0xffffffff                                //ARM920T有32个中断源，禁止所有中断，32位中断屏蔽寄存器置位     ldr r0, =INTMSK     str r1, [r0]#if defined(CONFIG_S3C2440)||defined(CONFIG_S3C2410) /* add by zhou */     ldr r1, =0x7ff<span style="white-space:pre">//屏蔽所有的中断源，S3C2440中寄存器只有前15位有效，故0x7ff置位INTSUNMSK     ldr r0, =INTSUBMSK     str r1, [r0]#endif//设置时钟频率#if defined(CONFIG_S3C2440)    mov  r1, #5    str  r1, [r0]    mrc  p15, 0, r1, c1, c0, 0     orr  r1, r1, #0xc0000000    mcr  p15, 0, r1, c1, c0, 0     mov  r1, #CLK_CTL_BASE//S3C2440系统主频为405MHZ，USB为48MHZ，要求MPLLCON = (0x7f<<12) | (0x02<<4) | (0x01) = 0x7f021    mov  r2, #MDIV_405    add  r2, r2, #PSDIV_405    str  r2, [r1, #0x04]   #else    /* FCLK:HCLK:PCLK = 1:2:4 */    /* default FCLK is 120 MHz ! */    ldr r0, =CLKDIVN    mov r1, #3    str r1, [r0]    mrc p15, 0, r1, c1, c0, 0    orr r1, r1, #0xc0000000    mcr p15, 0, r1, c1, c0, 0    mov r1, #CLK_CTL_BASE    mov r2, #MDIV_200    add r2, r2,#PSDIV_200    str r2, [r1,#0x04]#endif#endif /* (CONFIG_S3C2400) || (CONFIG_S3C2410) || (CONFIG_S3C2440) */

3，关闭MMU和CACHE，并调用lowlevel_init.S完成SDRAM和NANDFLASH的初始化，为代码的重定向做准备

   /*******************************************     * we do sys-critical inits only at reboot,     * not when booting from ram!     ******************************************/#ifndef CONFIG_SKIP_LOWLEVEL_INIT    bl  cpu_init_crit  //关闭MMU和CACHE，并调用lowlevel_init.S完成SDRAM和NANDFLASH的初始化#endif................................./* ************************************************************************* * * CPU_init_critical registers * * setup important registers * setup memory timing * ************************************************************************* */#ifndef CONFIG_SKIP_LOWLEVEL_INITcpu_init_crit:    /*     * flush v4 I/D caches     */    mov r0, #0      //具体设置看下图,详细参考CP15指令：http://blog.csdn.net/gooogleman/article/details/3635238    mcr p15, 0, r0, c7, c7, 0 //向c7写入0将使ICache与DCache 无效flush v3/v4 cache     mcr p15, 0, r0, c8, c7, 0  //向c8写入0将使TLB失效 flush v4 TLB     /*     * disable MMU stuff and caches  <span style="color:#ff0000;">   //协处理器CP15的C1处理器可以设置MMU和caches,具体参考下图     */    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    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       //由于有两层调用，需要把lr保存到ip，以防止破坏    bl  lowlevel_init//调用c函数，初始化FLASH和SDRAM，为代码重定向做准备    mov lr, ip     mov pc, lr      //返回#endif /* CONFIG_SKIP_LOWLEVEL_INIT */

代码重定向基本思路:

1.内存运行与否，是则设置堆栈，跳入c函数阶段

2.若不在内存运行，判断是在norflash还在nandflash运行

//代码重定向部分/***************CHECK_CODE_POSITION******************************/    adr r0, _start      /* r0 <- current position of code   */   //检查代码是否在已经SDRAM中运行，是则设置堆栈，并跳入c代码部分    ldr r1, _TEXT_BASE      /* test if we run from flash or RAM */  //_start为u-boot的真正运行地址，_TEXT_BASE为FLASH加载到SDRAM的地址，在config.mk中定义为0x33f80000    cmp r0, r1          /* don't reloc during debug         */      //若相等，说明已经在SDRAM中运行，设置堆栈，并且调转到第二阶段的C函数    beq stack_setup//若不相等，则要判断是从NORFLASF或NANDFLASH启动/***************CHECK_CODE_POSITION******************************//***************CHECK_BOOT_FLASH********************************/    ldr r1, =((4<<28)|(3<<4)|(3<<2))  /*address in 4000003c*/    mov r0, #0     //NANDFLASH的启动原理，启动时4K SRAM，即Stepping Stone,会映射到nGCS0,0x0000 0000地址，同时它还是会被映射到0x4000 0000地址     str r0,[r1]   //而NORFLASH支持片上运行，并会被一直挂载到nGCS0,0x0000 0000，具体可以参照NANDFLASH启动原理    mov r1, #0x3c   /*address in 0x3c*/ //NANDFLASH启动时，因为地址为16倍数对齐，此时0x0000 003c 和 0x4000 003c都为唯一确定的0xdeadbeef，即"Magic Mumber"    ldr r0, [r1]    //当0x4000 003c清零，若0x0000 003c读出也是零，则u-boot代码从NANDFLASH启动，否则从NORFLASH    cmp r0, #0    bne relocate    /*recover 0x4000003c */    ldr r0, =(0xdeadbeef)     //若在NANDFLASH启动，必须保证代码和前4K拷贝到SRAM一致，否则会进入死循环    ldr r1, =((4<<28)|(3<<4)|(3<<2))    str r0, [r1]/***************CHECK_BOOT_FLASH********************************//***************NAND_BOOT********************************/#ifdef  CONFIG_S3C2440//支持S3C2440的NANDFLASH#define LENGTH_UBOOT  0x60000#define NAND_CTL_BASE 0x4E000000/* Offset */#define oNFCONF 0x00#define oNFCONT 0x04#define oNFCMD  0x08#define oNFSTAT 0x20    mov  r1, #NAND_CTL_BASE //NAND Flash配置寄存器设置    ldr  r2, =( (7<<12)|(7<<8)|(7<<4)|(0<<0) )    str  r2, [r1, #oNFCONF]    ldr  r2, [r1, #oNFCONF]    ldr  r2, =( (1<<4)|(0<<1)|(1<<0) ) /* @ Active low CE Control */    str  r2, [r1, #oNFCONT]     //NAND Flash控制寄存器设置    ldr  r2, [r1, #oNFCONT]    ldr  r2, =(0x6)     /*  @ RnB Clear *///NAND Flash状态寄存器设置    str  r2, [r1, #oNFSTAT]    ldr  r2, [r1, #oNFSTAT]    mov  r2, #0xff//NAND Flash命令寄存器设置    strb r2, [r1, #oNFCMD]    mov  r3, #0     nand1:    add  r3, r3, #0x1    cmp  r3, #0xa    blt  nand1nand2:    ldr r2, [r1, #oNFSTAT]    tst r2, #0x4    beq nand2     ldr r2, [r1, #oNFCONT]    orr r2, r2, #0x2    str r2, [r1, #oNFCONT] /* get read to call C functions (for nand_read()) */     ldr sp, DW_STACK_START     mov fp, #0 /* copy U-Boot to RAM */     ldr r0, =TEXT_BASE //汇编调用c函数nand_read_ll，第一个参数存于r0搬运到内存地址     mov r1, #0x0     //第二个参数存于r1，NANDFLASH中u-boot地址     mov r2, #LENGTH_UBOOT /第三个参数存于r2，u-boot的总大小     bl nand_read_ll //在nand_read.c定义，支持不同NANDFLASH芯片代码拷贝     tst r0, #0x0 //r0是存放函数返回的参数，返回值为0则正确拷贝，否则进入死循环      beq ok_nand_readbad_nand_read:loop2:    b loop2ok_nand_read: //前4K代码比较，即判定Stepping Stone中4k代码和c函数搬运的代码是否一致     mov r0, #0     ldr r1, =TEXT_BASE     mov r2, #0x400 /* 4 bytes * 1024 = 4Kbytes */go_next:     ldr r3, [r0], #4 //r3存放NANDFLASH上u-boot的代码    ldr r4, [r1], #4 //r4为在内存中的u-boot代码     teq r3, r4    bne notmatch //不一致则进入死循环    subs r2, r2, #4    beq stack_setup    bne go_nextnotmatch:loop3:    b loop3#endif#ifdef CONFIG_S3C2410/* Offset */#define oNFCONF 0x00#define oNFCMD  0x04#define oNFSTAT 0x10    mov  r1, #NAND_CTL_BASE    ldr  r2, =0xf830    str  r2, [r1, #oNFCONF]    ldr  r2, [r1, #oNFCONF]    bic  r2,  r2, #0x800     /* enable chip  */    str  r2, [r1, #oNFCONF]    mov  r2, #0xff        /*  @ RESET command  +  strb  r2, [r1, #oNFCMD] */    strb r2, [r1, #oNFCMD]    mov  r3, #0 nand1:    add  r3, r3, #0x1    cmp  r3, #0xa    blt  nand1nand2:    ldr  r2, [r1, #oNFSTAT] /* @ wait ready */    tst  r2, #0x1    beq  nand2    ldr  r2, [r1, #oNFCONF]    orr  r2,  r2, #0x800   /*@ disable chip */    str  r2, [r1, #oNFCONF]    /*    @ get read to call C functions (for nand_read()) */    ldr  sp, DW_STACK_START    /* @ setup stack pointer */    mov  fp, #0                /* @ no previous frame, so fp=0 */    /*  @ copy U-Boot to RAM */    ldr r0, =TEXT_BASE    mov r1, #0x0    mov r2, #LENGTH_UBOOT    bl nand_read_ll    tst r0, #0x0    beq ok_nand_readbad_nand_read:loop2:    b  loop2  /*@ infinite loop */ok_nand_read:/*     @ verify */    mov r0, #0     ldr r1, =TEXT_BASE    mov r2, #0x400 /* @ 4 bytes * 1024 = 4K-bytes */go_next:    ldr  r3, [r0], #4    ldr  r4, [r1], #4    teq  r3, r4    bne  notmatch    subs r2, r2, #4    beq  stack_setup    bne  go_nextnotmatch:loop3:    b    loop3  /*@ infinite loop */#endif/***************** NAND_BOOT ************************************************/</span>

3，设置堆栈，并且跳入第二阶段的C代码

 /* Set up the stack */stack_setup:    ldr r0, _TEXT_BASE      /* upper 128 KiB: relocated uboot   */    sub r0, r0, #CONFIG_SYS_MALLOC_LEN  /* malloc area              */    sub r0, r0, #CONFIG_SYS_GBL_DATA_SIZE /* bdinfo                 */#ifdef CONFIG_USE_IRQ    sub r0, r0, #(CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ)#endif    sub sp, r0, #12     /* leave 3 words for abort-stack    */clear_bss://将未初始化.bss段初始化0    ldr r0, _bss_start      /* find start of bss segment        */    ldr r1, _bss_end        /* stop here                        */    mov r2, #0x00000000     /* clear                            */clbss_l:    str r2, [r0]        /* clear loop...                    */    add r0, r0, #4    cmp r0, r1    ble clbss_l    ldr pc, _start_armboot  //跳到第二阶段c函数，进一步初始化板子硬件_start_armboot: .word start_armboot#define STACK_BASE 0x33f00000    //为nand_read_ll函数调用设置堆栈#define STACK_SIZE 0x10000  .align 2DW_STACK_START: .word  STACK_BASE+STACK_SIZE-4 </span>

4，异常向量处理代码

/************************************************************************* * * Interrupt handling * ************************************************************************* */......

详细可参考http://www.crifan.com/files/doc/docbook/uboot_starts_analysis/release/html/uboot_starts_analysis.html