本文僅探討s3c6410從nand Flash啟動u-boot時的代碼重定位過程

參考：

1）《USER'S MANUAL-S3C6410X》第二章 MEMORY MAP 第八章 NAND FLASH CONTROLLER

2）u-boot源碼：

u-boot-x.x.x/board/samsumg/smdk6410/lowlevel_init.S

u-boot-x.x.x/cpu/s3c64xx/start.S

u-boot-x.x.x/cpu/s3c64xx/nand_cp.c

代碼重定位過程簡述

由于在nand flash中無法運行代碼，所以當開發板從nand flash啟動時，我們需要將存儲在外設nand flash中的u-boot代碼搬運到sdram中運行，如何完成這個搬運工作呢？這需要借助一個跳板，即“stepping stone”，它是s3c6410的一塊內置sram，開發板上電時，nand flash控制器自動將nand flash的前8K的內容拷貝到sram中并執行，這一小段啟動代碼除了初始化硬件外，最重要的一個工作就是將nand flash中的所有u-boot代碼拷貝（即重定位）到sdram的指定地址上去，然后跳轉到sdram中執行。

重定位代碼解析：

1）nand接口初始化

u-boot啟動時，首先執行相應硬件平臺的start.S，start.S中調用lowlevel_init對時鐘，uart，nand，mmu等底層硬件作初始化。

start.S:

...bl    lowlevel_init    /* go setup pll,mux,memory */...

lowlevel_init.S:

.../* * Nand Interface Init for SMDK6400 */nand_asm_init:    ldr    r0, =ELFIN_NAND_BASE    ldr    r1, [r0, #NFCONF_OFFSET]    orr    r1, r1, #0x70    orr    r1, r1, #0x7700    str     r1, [r0, #NFCONF_OFFSET]    ldr    r1, [r0, #NFCONT_OFFSET]    orr    r1, r1, #0x03    str     r1, [r0, #NFCONT_OFFSET]    mov    pc, lr...

2）代碼重定位

從nand flash啟動時，重定位代碼如下：

start.S:

/* when we already run in ram, we don't need to relocate U-Boot.     * and actually, memory controller must be configured before U-Boot     * is running in ram.     */    ldr    r0, =0xff000fff    bic    r1, pc, r0        /* r0 <- current base addr of code */    ldr    r2, _TEXT_BASE        /* r1 <- original base addr in ram */    bic    r2, r2, r0        /* r0 <- current base addr of code */    cmp     r1, r2                  /* compare r0, r1                  */    beq     after_copy        /* r0 == r1 then skip flash copy   */#ifdef CONFIG_BOOT_NAND    mov    r0, #0x1000    bl    copy_from_nand#endif

r1存放當前代碼運行的起始地址，r2存放u-boot即將在sdram中運行的地址，如果兩個地址相等，說明此時u-boot已經在sdram中運行了，無需再執行從nand拷貝數據到sdram的動作；否則，此時u-boot還在它的臨時住所sram中執行，此地不可久留，需要執行copy_from_nand將u-boot代碼完完整整地拷貝到sdram中去，然后跳轉到sdram中去執行剩下的代碼。

/* * copy U-Boot to SDRAM and jump to ram (from NAND or OneNAND) * r0: size to be compared * Load 1'st 2blocks to RAM because U-boot's size is larger than 1block(128k) size */    .globl copy_from_nandcopy_from_nand:    mov    r10, lr        /* save return address */    mov    r9, r0    /* get ready to call C functions */    ldr    sp, _TEXT_PHY_BASE    /* setup temp stack pointer */    sub    sp, sp, #12    mov    fp, #0            /* no PRevious frame, so fp=0 */    mov    r9, #0x1000    bl    copy_uboot_to_ram3:    tst     r0, #0x0    bne    copy_failed    ldr    r0, =0x0c000000    ldr    r1, _TEXT_PHY_BASE1:    ldr    r3, [r0], #4    ldr    r4, [r1], #4    teq    r3, r4    bne    compare_failed    /* not matched */    subs    r9, r9, #4    bne    1b4:    mov    lr, r10        /* all is OK */    mov    pc, lrcopy_failed:    nop            /* copy from nand failed */    b    copy_failedcompare_failed:    nop            /* compare failed */    b    compare_failed

真正執行拷貝動作的是copy_uboot_to_ram函數，它定義在u-boot-x.x.x/cpu/s3c64xx/nand_cp.c中，

int copy_uboot_to_ram (void){    int large_block = 0;    int i;    vu_char id;            NAND_ENABLE_CE();        NFCMD_REG = NAND_CMD_READID;        NFADDR_REG =  0x00;    /* wait for a while */        for (i=0; i<200; i++);    id = NFDATA8_REG;    id = NFDATA8_REG;    if (id > 0x80)        large_block = 1;    /* read NAND Block.     * 128KB ->240KB because of U-Boot size increase. by scsuh     * So, read 0x3c000 bytes not 0x20000(128KB).     */    return nandll_read_blocks(CFG_PHY_UBOOT_BASE, 0x3c000, large_block);}

nand flash支持兩種頁大小，512B和2KB，large_block = 0時，頁大小為512字節，large_block = 1時，頁大小為2K字節。nandll_read_blocks拷貝nand flash從第0頁開始的0x3c00（240K）大小的數據到sdram的CFG_PHY_UBOOT_BASE地址處。

/* * Read data from NAND. */static int nandll_read_blocks (ulong dst_addr, ulong size, int large_block){        uchar *buf = (uchar *)dst_addr;        int i;    uint page_shift = 9;    if (large_block)        page_shift = 11;        /* Read pages */        for (i = 0; i < (0x3c000>>page_shift); i++, buf+=(1<<page_shift)) {                nandll_read_page(buf, i, large_block);        }        return 0;}

首先根據large_block判斷nand flash一個頁的大小，從而計算需要拷貝的頁的數量，即需要拷貝(0x3c000>>page_shift)個頁，nandll_read_page每次只拷貝一個頁的數據。

/* * address format *              17 16         9 8            0 * -------------------------------------------- * | block(12bit) | page(5bit) | offset(9bit) | * -------------------------------------------- */static int nandll_read_page (uchar *buf, ulong addr, int large_block){        int i;    int page_size = 512;    if (large_block)        page_size = 2048;        NAND_ENABLE_CE();        NFCMD_REG = NAND_CMD_READ0;        /* Write Address */        NFADDR_REG = 0;    if (large_block)            NFADDR_REG = 0;    NFADDR_REG = (addr) & 0xff;    NFADDR_REG = (addr >> 8) & 0xff;    NFADDR_REG = (addr >> 16) & 0xff;    if (large_block)        NFCMD_REG = NAND_CMD_READSTART;        NF_TRANSRnB();    /* for compatibility(2460). u32 cannot be used. by scsuh */    for(i=0; i < page_size; i++) {                *buf++ = NFDATA8_REG;        }        NAND_DISABLE_CE();        return 0;}

從nand flash中讀取數據的流程為片選（NAND_ENABLE_CE）->發讀命令（NFCMD_REG）->發地址（NFADDR_REG）->發讀命令（NFCMD_REG）->等待數據可讀（NF_TRANSRnB）->讀數據（NFDATA8_REG）。由于每次從NFDATA8_REG中只可讀取1個字節的數據，所以拷貝一頁需要讀取512或2048次。

當執行完copy_uboot_to_ram返回到start.S時，nand flash中的代碼重定位便完成了，此后程序跳轉到sdram中執行，stepping stone的職責就此結束。