[TI-Sitara]启动流程

前段时间在准备AM437x启动相关的一些事情，对MLO、SPL等事情也是有些糊涂，于是分享下面这篇文章

转自：http://blog.csdn.net/psvoldemort/article/details/41861959

参考文件：

1，AM335x ARM Cortex-A8 Microprocessors (MPUs) Technical Reference Manual.pdf；

2，am3359.pdf；

目录

• 1.am335x的cpu上电后，会跳到哪个地址去执行？
• 1.1.第一级 bootloader：
• 1.2.第二级 bootloader：
• 1.3.第三级 bootloader：
• 2.第二级 bootloader：MLO（SPL）做了哪些事情？
• 2.1.SPL内存重映射
• 2.2.保存启动参数 bl    save_boot_params
• 2.3.设置 CPU 为 SVC32 模式
• 2.4.CPU的初始化
• 2.5.执行 s_init() 函数，实现 CPU 的初始化
• 2.6.使能第二级缓冲区
• 2.7.关闭看门狗（WDT）
• 2.8.给外设设置好 PLL 和 时钟频率等
• 2.9.使能 32-KHz 频率的实时时钟
• 2.10.使能UART0
• 2.11.初始化 定时器
• 2.12.初始化控制台，通过UART可以查看相关信息
• 2.13.配置 DDR
• 2.14.设置 internal RAM 内存空间的栈指针，调用 board_init_f()函数
• 2.15.代码重定向
• 2.16.清空 bss 段
• 2.17.调用函数 board_init_r，用以完成 MLO（SPI）阶段的所有初始化，并跳转到 uboot.img 阶段
• 3.第三级 bootloader：uboot.img 做了哪些事情？

1.am335x的cpu上电后，会跳到哪个地址去执行？

答：芯片到uboot启动流程 ：Internal-ROM → MLO(SPL)→ uboot.img

AM335x 中bootloader被分成了 3 个部分：

1.1.第一级 bootloader：

引导加载程序，板子上电后会自动执行这些代码，如选择哪种方式启动（NAND，SDcard，UART。。。），然后跳转转到第二级 bootloader。这些代码应该是存放在 176KB 的 ROM 中。

1.2.第二级 bootloader：

MLO（SPL），用以硬件初始化：关闭看门狗，关闭中断，设置 CPU 时钟频率、速度等操作。然后会跳转到第三级bootloader。MLO文件应该会被映射到 64 KB的 Internal SRAM 中。

1.3.第三级 bootloader：

uboot.img，C代码的入口。

其中第一级 bootloader 是板子固化的，第二级和第三级是通过编译 uboot 所得的。

2.第二级 bootloader：MLO（SPL）做了哪些事情？

MLO(SPL)内存分布如下：

2.1.SPL内存重映射

< PATH : /arch/arm/cpu/armv7/omap-common/u-boot-spl.lds >
MEMORY { .sram : ORIGIN = CONFIG_SPL_TEXT_BASE,
        LENGTH = CONFIG_SPL_MAX_SIZE }
MEMORY { .sdram : ORIGIN = CONFIG_SPL_BSS_START_ADDR, 
        LENGTH = CONFIG_SPL_BSS_MAX_SIZE }
 
OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
OUTPUT_ARCH(arm)
ENTRY(_start)
SECTIONS
{
    .text      :
    {
    __start = .;
      arch/arm/cpu/armv7/start.o    (.text)
      *(.text*)
    } >.sram
 
    . = ALIGN(4);
    .rodata : { *(SORT_BY_ALIGNMENT(.rodata*)) } >.sram
 
    . = ALIGN(4);
    .data : { *(SORT_BY_ALIGNMENT(.data*)) } >.sram
    . = ALIGN(4);
    __image_copy_end = .;
    _end = .;
 
    .bss :
    {
        . = ALIGN(4);
        __bss_start = .;
        *(.bss*)
        . = ALIGN(4);
        __bss_end__ = .;
    } >.sdram
}

<PATH : /include/configs/am335x_evm.h>
#define CONFIG_SPL_TEXT_BASE        0x402F0400
#define CONFIG_SPL_MAX_SIZE     (46 * 1024)
#define CONFIG_SPL_STACK        LOW_LEVEL_SRAM_STACK
 
#define CONFIG_SPL_BSS_START_ADDR   0x80000000
#define CONFIG_SPL_BSS_MAX_SIZE     0x80000     /* 512 KB */ <span style="font-size:16px;color:#003399;"><strong></strong></span>

2.2.保存启动参数 bl    save_boot_params

<PATH : /arch/arm/cpu/armv7/start.S>
/*
 * the actual reset code
 */
 
reset:
    bl  save_boot_params

<PATH : /arch/arm/cpu/armv7/omap-common/lowlevel_init.S>
.global save_boot_params
save_boot_params:
    /*
     * See if the rom code passed pointer is valid:
     * It is not valid if it is not in non-secure SRAM
     * This may happen if you are booting with the help of
     * debugger
     */
    ldr     r2, =NON_SECURE_SRAM_START
    cmp r2, r0
    bgt 1f
    ldr r2, =NON_SECURE_SRAM_END
    cmp r2, r0
    blt 1f
 
    /*
     * store the boot params passed from rom code or saved
     * and passed by SPL
     */
    cmp r0, #0
    beq 1f
    ldr r1, =boot_params
    str r0, [r1]

/*《PATH: /arch/arm/include/asm/arch-ti81xx/omap.h》
 * Non-secure SRAM Addresses
 * Non-secure RAM starts at 0x40300000 for GP devices. But we keep SRAM_BASE
 * at 0x40304000(EMU base) so that our code works for both EMU and GP
 */
#define NON_SECURE_SRAM_START   0x40304000
#define NON_SECURE_SRAM_END 0x4030E000
#define LOW_LEVEL_SRAM_STACK    0x4030B7FC

问题：这些参数是保存在哪里的？大概有哪些参数？

答：这些参数保存的内存地址为 64 KB 的 OCM RAM 中：

注：Dowloaded Image 区域：是用来保存 MLO（SPL） 文件的，其最大可达到 109 KB

2.3.设置 CPU 为 SVC32 模式

    <PATH :/arch/arm/cpu/armv7/start.S>       
        /*
     * set the cpu to SVC32 mode
     */
    mrs r0, cpsr
    bic r0, r0, #0x1f
    orr r0, r0, #0xd3
    msr cpsr,r0

• CPSR:程序状态寄存器(current program status register)(当前程序状态寄存器)，在任何处理器模式下被访问。它包含了条件标志位、中断禁止位、当前处理器模式标志以及其他的一些控制和状态位。

CPSR在用户级编程时用于存储条件码。

• SPSR:程序状态保存寄存器（saved program statusregister）,每一种处理器模式下都有一个状态寄存器SPSR,SPSR用于保存CPSR的状态，以便异常返回后恢复异常发生时的工作状态。当特定的异常中断发生时，这个寄存器用于存放当前程序状态寄存器的内容。在异常中断退出时，可以用SPSR来恢复CPSR。由于用户模式和系统模式不是异常中断模式，所以他没有SPSR。当用户在用户模式或系统模式访问SPSR，将产生不可预知的后果。

详解：http://blog.chinaunix.net/uid-28458801-id-3487199.html

2.4.CPU的初始化

《PATH : /arch/arm/cpu/armv7/start.S》
    /* the mask ROM code should have PLL and others stable */
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
    bl  cpu_init_crit
#endif

<PATH : /arch/arm/cpu/armv7/omap-common/lowlevel_init.S>

.globl lowlevel_init
lowlevel_init:
    /*
     * Setup a temporary stack
     */
    ldr sp, =LOW_LEVEL_SRAM_STACK
 
    /*
     * Save the old lr(passed in ip) and the current lr to stack
     */
    push    {ip, lr}
 
    /*
     * go setup pll, mux, memory
     */
    bl  s_init
    pop {ip, pc}

问题：CPU的初始化有哪些内容？

答：@b1@ 首先要设置堆栈区，因为将会调用 C函数来实现CPU的初始化

问题：这个堆栈在什么位置，其内存大小是多少？

答: 

《PATH ：/arch/arm/include/asm/arch-ti81xx/omap.h》
#define LOW_LEVEL_SRAM_STACK    0x4030B7FC<strong></strong>

2.5.执行 s_init() 函数，实现 CPU 的初始化

<PATH : /board/ti/am335x/evm.c>
/*
 * early system init of muxing and clocks.
 */
void s_init(void)
{
    /* Can be removed as A8 comes up with L2 enabled */
    l2_cache_enable();
 
    /* WDT1 is already running when the bootloader gets control
     * Disable it to avoid "random" resets
     */
    __raw_writel(0xAAAA, WDT_WSPR);
    while(__raw_readl(WDT_WWPS) != 0x0);
    __raw_writel(0x5555, WDT_WSPR);
    while(__raw_readl(WDT_WWPS) != 0x0);
 
#ifdef CONFIG_SPL_BUILD
    /* Setup the PLLs and the clocks for the peripherals */
    pll_init();
 
    /* Enable RTC32K clock */
    rtc32k_enable();
 
    /* UART softreset */
    u32 regVal;
    u32 uart_base = DEFAULT_UART_BASE;
 
    enable_uart0_pin_mux();
    /* IA Motor Control Board has default console on UART3*/
    /* XXX: This is before we've probed / set board_id */
    if (board_id == IA_BOARD) {
        uart_base = UART3_BASE;
    }
 
    regVal = __raw_readl(uart_base + UART_SYSCFG_OFFSET);
    regVal |= UART_RESET;
    __raw_writel(regVal, (uart_base + UART_SYSCFG_OFFSET) );
    while ((__raw_readl(uart_base + UART_SYSSTS_OFFSET) &
            UART_CLK_RUNNING_MASK) != UART_CLK_RUNNING_MASK);
 
    /* Disable smart idle */
    regVal = __raw_readl((uart_base + UART_SYSCFG_OFFSET));
    regVal |= UART_SMART_IDLE_EN;
    __raw_writel(regVal, (uart_base + UART_SYSCFG_OFFSET));
 
    /* Initialize the Timer */
    init_timer();
 
    preloader_console_init();
 
    printf("
location /board/ti/am335x
");        //@@
/*@@*/
//  led();
/*@@*/
     
    config_am335x_ddr();
 
#endif
}

2.6.使能第二级缓冲区

/* Can be removed as A8 comes up with L2 enabled */
l2_cache_enable();
 
<PATH : /arch/arm/cpu/armv7/ti81xx/cache.S>
l2_cache_enable:
    push    {r0, r1, r2, lr}
    mrc 15, 0, r3, cr1, cr0, 1
    orr r3, r3, #2
    mcr 15, 0, r3, cr1, cr0, 1
    pop {r1, r2, r3, pc}

 

2.7.关闭看门狗（WDT）

/* WDT1 is already running when the bootloader gets control
 * Disable it to avoid "random" resets
 */
__raw_writel(0xAAAA, WDT_WSPR);
while(__raw_readl(WDT_WWPS) != 0x0);
__raw_writel(0x5555, WDT_WSPR);
while(__raw_readl(WDT_WWPS) != 0x0);

<PATH : /arch/arm/include/asm/arch-ti81xx/cpu.h>
#define WDT_WSPR    (WDT_BASE + 0x048)
 
 
<PATH : /arch/arm/include/asm/arch-ti81xx/hardware.h>
/* Watchdog Timer */
#ifdef CONFIG_AM335X
#define WDT_BASE            0x44E35000
#else
#define WDT_BASE            0x480C2000
#endif

2.8.给外设设置好 PLL 和 时钟频率等

/* Setup the PLLs and the clocks for the peripherals */
pll_init();
 
 
<PATH : /board/ti/am335x/pll.c>
/*
 * Configure the PLL/PRCM for necessary peripherals
 */
void pll_init()
{
    mpu_pll_config(MPUPLL_M_500);
    core_pll_config();
    per_pll_config();
    ddr_pll_config();
    /* Enable the required interconnect clocks */
    interface_clocks_enable();
    /* Enable power domain transition */
    power_domain_transition_enable();
    /* Enable the required peripherals */
    per_clocks_enable();
}

  

2.9.使能 32-KHz 频率的实时时钟

/* Enable RTC32K clock */
rtc32k_enable();
 
 《PATH : /board/ti/am335x/evm.c》
static void rtc32k_enable(void)
{
    /* Unlock the rtc's registers */
    __raw_writel(0x83e70b13, (AM335X_RTC_BASE + RTC_KICK0_REG));
    __raw_writel(0x95a4f1e0, (AM335X_RTC_BASE + RTC_KICK1_REG));
 
    /* Enable the RTC 32K OSC */
    __raw_writel(0x48, (AM335X_RTC_BASE + RTC_OSC_REG));
}
 
<PATH : /arch/arm/include/asm/arch-ti81xx/hardware.h>
/* RTC base address */
#define AM335X_RTC_BASE            0x44E3E000
 
<PATH : /board/ti/am335x/evm.c>
#define RTC_KICK0_REG        0x6c
#define RTC_KICK1_REG        0x70
#define RTC_OSC_REG        0x54

2.10.使能UART0

  /* UART softreset */
    u32 regVal;
    u32 uart_base = DEFAULT_UART_BASE;
 
    enable_uart0_pin_mux();
    /* IA Motor Control Board has default console on UART3*/
    /* XXX: This is before we've probed / set board_id */
    if (board_id == IA_BOARD) {
        uart_base = UART3_BASE;
    }
 
    regVal = __raw_readl(uart_base + UART_SYSCFG_OFFSET);
    regVal |= UART_RESET;
    __raw_writel(regVal, (uart_base + UART_SYSCFG_OFFSET) );
    while ((__raw_readl(uart_base + UART_SYSSTS_OFFSET) &
            UART_CLK_RUNNING_MASK) != UART_CLK_RUNNING_MASK);
 
    /* Disable smart idle */
    regVal = __raw_readl((uart_base + UART_SYSCFG_OFFSET));
    regVal |= UART_SMART_IDLE_EN;
    __raw_writel(regVal, (uart_base + UART_SYSCFG_OFFSET));
 
 
<PATH : /arch/arm/include/asm/arch-ti81xx/cpu.h>
#ifdef CONFIG_AM335X
#define DEFAULT_UART_BASE       UART0_BASE
#endif
 
<PATH : /arch/arm/include/asm/arch-ti81xx/hardware.h>
#ifdef CONFIG_AM335X
#define UART0_BASE          0x44E09000
#else
#define UART0_BASE          0x48020000
#endif

2.11.初始化 定时器

/* Initialize the Timer */
init_timer();
  
<PATH : /board/ti/am335x/evm.c>
static void init_timer(void)
{
    /* Reset the Timer */
    __raw_writel(0x2, (DM_TIMER2_BASE + TSICR_REG));
 
    /* Wait until the reset is done */
    while (__raw_readl(DM_TIMER2_BASE + TIOCP_CFG_REG) & 1);
 
    /* Start the Timer */
    __raw_writel(0x1, (DM_TIMER2_BASE + TCLR_REG));
}
 
<PATH : /arch/arm/include/asm/arch-ti81xx/hardware.h>
/* DM Timer base addresses */
#define DM_TIMER0_BASE          0x4802C000
#define DM_TIMER1_BASE          0x4802E000
#define DM_TIMER2_BASE          0x48040000
#define DM_TIMER3_BASE          0x48042000
#define DM_TIMER4_BASE          0x48044000
#define DM_TIMER5_BASE          0x48046000
#define DM_TIMER6_BASE          0x48048000
#define DM_TIMER7_BASE          0x4804A000

2.12.初始化控制台，通过UART可以查看相关信息

preloader_console_init();
 
《PATH : /arch/arm/cpu/armv7/omap-common/spl.c》
/* This requires UART clocks to be enabled */
void preloader_console_init(void)
{
    const char *u_boot_rev = U_BOOT_VERSION;
    char rev_string_buffer[50];
 
    gd = &gdata;
    gd->bd = &bdata;
    gd->flags |= GD_FLG_RELOC;
    gd->baudrate = CONFIG_BAUDRATE;
 
    serial_init();      /* serial communications setup */
 
    /* Avoid a second "U-Boot" coming from this string */
    u_boot_rev = &u_boot_rev[7];
 
    printf("
U-Boot SPL %s (%s - %s)
", u_boot_rev, U_BOOT_DATE,
        U_BOOT_TIME);
    omap_rev_string(rev_string_buffer);
    printf("Texas Instruments %s
", rev_string_buffer);
} 

2.13.配置 DDR

   config_am335x_ddr();
 
《PATH ：》
/*  void DDR2_EMIF_Config(void); */
static void config_am335x_ddr(void)
{
    int data_macro_0 = 0;
    int data_macro_1 = 1;
 
    enable_ddr_clocks();
 
    config_vtp();
 
    Cmd_Macro_Config();
 
    Data_Macro_Config(data_macro_0);
    Data_Macro_Config(data_macro_1);
 
    __raw_writel(PHY_RANK0_DELAY, DATA0_RANK0_DELAYS_0);
    __raw_writel(PHY_RANK0_DELAY, DATA1_RANK0_DELAYS_0);
 
    __raw_writel(DDR_IOCTRL_VALUE, DDR_CMD0_IOCTRL);
    __raw_writel(DDR_IOCTRL_VALUE, DDR_CMD1_IOCTRL);
    __raw_writel(DDR_IOCTRL_VALUE, DDR_CMD2_IOCTRL);
    __raw_writel(DDR_IOCTRL_VALUE, DDR_DATA0_IOCTRL);
    __raw_writel(DDR_IOCTRL_VALUE, DDR_DATA1_IOCTRL);
 
    __raw_writel(__raw_readl(DDR_IO_CTRL) & 0xefffffff, DDR_IO_CTRL);
    __raw_writel(__raw_readl(DDR_CKE_CTRL) | 0x00000001, DDR_CKE_CTRL);
 
    config_emif_ddr2();
}
 
 
《PATH : /arm/include/asm/arch-ti81xx/cpu.h》
#define DATA0_RANK0_DELAYS_0        (DDR_PHY_BASE_ADDR + 0x134)
#define DATA1_RANK0_DELAYS_0        (DDR_PHY_BASE_ADDR + 0x1D8)
 
/* DDR offsets */
#define DDR_PHY_BASE_ADDR       0x44E12000
#define DDR_IO_CTRL         0x44E10E04
#define DDR_CKE_CTRL            0x44E1131C
#define CONTROL_BASE_ADDR       0x44E10000

2.14.设置 internal RAM 内存空间的栈指针，调用 board_init_f()函数

/* Set stackpointer in internal RAM to call board_init_f */
call_board_init_f:
    ldr sp, =(CONFIG_SYS_INIT_SP_ADDR)
    bic sp, sp, #7 /* 8-byte alignment for ABI compliance */
    ldr r0,=0x00000000
    bl  board_init_f

#define CONFIG_SYS_INIT_SP_ADDR     (CONFIG_SYS_INIT_RAM_ADDR + 
                     CONFIG_SYS_INIT_RAM_SIZE - 
                     GENERATED_GBL_DATA_SIZE)
 
#define CONFIG_SYS_INIT_RAM_ADDR    SRAM0_START
#define CONFIG_SYS_INIT_RAM_SIZE    SRAM0_SIZE
 
#ifdef CONFIG_AM335X
#define SRAM0_START         0x402F0400
#else
#define SRAM0_START         0x40300000
#endif
 
#if defined(CONFIG_AM335X) || defined(CONFIG_TI814X)
#define SRAM0_SIZE          (0x1B400) /* 109 KB */
#define SRAM_GPMC_STACK_SIZE        (0x40)
#endif
 
#define GENERATED_GBL_DATA_SIZE (128) /* (sizeof(struct global_data) + 15) & ~15 */ 

void board_init_f(ulong dummy)
{
    /*
     * We call relocate_code() with relocation target same as the
     * CONFIG_SYS_SPL_TEXT_BASE. This will result in relocation getting
     * skipped. Instead, only .bss initialization will happen. That's
     * all we need
     */
    debug(">>board_init_f()
");
    relocate_code(CONFIG_SPL_STACK, &gdata, CONFIG_SPL_TEXT_BASE);
}
 
#define CONFIG_SPL_TEXT_BASE        0x402F0400
#define CONFIG_SPL_MAX_SIZE     (46 * 1024)
#define CONFIG_SPL_STACK        LOW_LEVEL_SRAM_STACK

#define LOW_LEVEL_SRAM_STACK    0x4030B7FC 

/*
 * void relocate_code (addr_sp, gd, addr_moni)
 *
 * This "function" does not return, instead it continues in RAM
 * after relocating the monitor code.
 *
 */
    .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 0x402F0400*/</path>

2.15.代码重定位

代码重定向，它首先检测自己(MLO)是否已经在内存中：

如果是直接跳到下面的堆栈初始化代码 clear_bss。

如果不是就将自己从Nor Flash中拷贝到内存中。

Nor Flash 和Nand Flash 本质区别就在于是否进行代码拷贝，也就是下面代码所表述：无论
是Nor Flash 还是Nand Flash，核心思想就是将 uboot 代码搬运到内存中去运行，但是没有拷
贝bss 后面这段代码，只拷贝bss 前面的代码，bss 代码是放置全局变量的。Bss 段代码是为
了清零，拷贝过去再清零重复操作。

    /* Set up the stack                         */
stack_setup:
    mov sp, r4
 
    adr r0, _start
    cmp r0, r6
    moveq   r9, #0      /* no relocation. relocation offset(r9) = 0 */
    beq clear_bss       /* skip relocation */
    mov r1, r6          /* r1 <- scratch for copy_loop */
    ldr r3, _image_copy_end_ofs
    add r2, r0, r3      /* r2 <- source end address      */
 
copy_loop:                              /* 自拷贝 */
    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
    @a6@ 清空 bss 段

2.16.清空 bss 段

clear_bss:
 
    ldr r0, _bss_start_ofs
    ldr r1, _bss_end_ofs
    mov r4, r6          /* reloc addr */
    add r0, r0, r4
    add r1, r1, r4
 
    mov r2, #0x00000000     /* clear                */
 
clbss_l:str r2, [r0]        /* clear loop...            */
    add r0, r0, #4
    cmp r0, r1
    bne clbss_l
 
/*
 * These are defined in the board-specific linker script.
 */
.globl _bss_start_ofs
_bss_start_ofs:
    .word __bss_start - _start          /* __bss_start = 0x80000000 */

2.17.调用函数 board_init_r，用以完成 MLO（SPI）阶段的所有初始化，并跳转到 uboot.img 阶段

/*
 * We are done. Do not return, instead branch to second part of board
 * initialization, now running from RAM.
 */
jump_2_ram:
/*
 * If I-cache is enabled invalidate it
 */
#ifndef CONFIG_SYS_ICACHE_OFF
    mcr p15, 0, r0, c7, c5, 0   @ invalidate icache
    mcr     p15, 0, r0, c7, c10, 4  @ DSB
    mcr     p15, 0, r0, c7, c5, 4   @ ISB
#endif
    ldr r0, _board_init_r_ofs
    adr r1, _start
    add lr, r0, r1
    add lr, lr, r9
    /* setup parameters for board_init_r */
    mov r0, r5      /* gd_t */
    mov r1, r6      /* dest_addr */
    /* jump to it ... */
    mov pc, lr
 
_board_init_r_ofs:
    .word board_init_r - _start

《PATH : /arch/arm/cpu/armv7/omap-common/spl.c 》
void board_init_r(gd_t *id, ulong dummy)
{
    u32 boot_device;
    debug(">>spl:board_init_r()
");
 
    timer_init();
    i2c_init(CONFIG_SYS_I2C_SPEED, CONFIG_SYS_I2C_SLAVE);
 
#ifdef CONFIG_SPL_BOARD_INIT
    spl_board_init();
#endif
 
    boot_device = omap_boot_device();
    debug("boot device - %d
", boot_device);
    switch (boot_device) {
#ifdef CONFIG_SPL_MMC_SUPPORT
    case BOOT_DEVICE_MMC1:
    case BOOT_DEVICE_MMC2:
        spl_mmc_load_image();
        break;
#endif
#ifdef CONFIG_SPL_NAND_SUPPORT
    case BOOT_DEVICE_NAND:
        spl_nand_load_image();
        break;
#endif
#ifdef CONFIG_SPL_YMODEM_SUPPORT
    case BOOT_DEVICE_UART:
        spl_ymodem_load_image();
        break;
#endif
    default:
        printf("SPL: Un-supported Boot Device - %d!!!
", boot_device);
        hang();
        break;
    }
 
    switch (spl_image.os) {
    case IH_OS_U_BOOT:
        debug("Jumping to U-Boot
");
        jump_to_image_no_args();
        break;
    default:
        puts("Unsupported OS image.. Jumping nevertheless..
");
        jump_to_image_no_args();
    }
}

3，第三级 bootloader：uboot.img 做了哪些事情？

uboot.img 内存分布如下：

访问 /arch/arm/lib/board.c 中 的 board_init_f() 函数：

在 uboot.img 运行过程中，有两个非常重要的结构体：gd_t 和 bd_t 。

其中 gd_t ：global_data 数据结构的定义，位于：/arch/arm/include/asm/global_data.h 中。

                 其成员主要是一些全局的系统初始化参数。

其中 bd_t ：bd_info 数据结构的定义，位于：/arch/arm/include/asm/u-boot.h 中。

                 其成员是开发板的相关参数。

/*
 * 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)
 */
 
typedef struct  global_data {
    bd_t        *bd;
    unsigned long   flags;
    unsigned long   baudrate;
    unsigned long   have_console;   /* serial_init() was called */
    unsigned long   env_addr;   /* Address  of Environment struct */
    unsigned long   env_valid;  /* Checksum of Environment valid? */
    unsigned long   fb_base;    /* base address of frame buffer */
#ifdef CONFIG_FSL_ESDHC
    unsigned long   sdhc_clk;
#endif
#ifdef CONFIG_AT91FAMILY
    /* "static data" needed by at91's clock.c */
    unsigned long   cpu_clk_rate_hz;
    unsigned long   main_clk_rate_hz;
    unsigned long   mck_rate_hz;
    unsigned long   plla_rate_hz;
    unsigned long   pllb_rate_hz;
    unsigned long   at91_pllb_usb_init;
#endif
#ifdef CONFIG_ARM
    /* "static data" needed by most of timer.c on ARM platforms */
    unsigned long   timer_rate_hz;
    unsigned long   tbl;
    unsigned long   tbu;
    unsigned long long  timer_reset_value;
    unsigned long   lastinc;
#endif
#ifdef CONFIG_IXP425
    unsigned long   timestamp;
#endif
    unsigned long   relocaddr;  /* Start address of U-Boot in RAM */
    phys_size_t ram_size;   /* RAM size */
    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;
#if !(defined(CONFIG_SYS_ICACHE_OFF) && defined(CONFIG_SYS_DCACHE_OFF))
    unsigned long   tlb_addr;
#endif
    void        **jt;       /* jump table */
    char        env_buf[32];    /* buffer for getenv() before reloc. */
} gd_t;
 
#define DECLARE_GLOBAL_DATA_PTR     register volatile gd_t *gd asm ("r8")
 
 
typedef struct bd_info {
    int         bi_baudrate;    /* serial console baudrate */
    unsigned long   bi_ip_addr; /* IP Address */
    ulong           bi_arch_number; /* unique id for this board */
    ulong           bi_boot_params; /* where this board expects params */
    struct              /* RAM configuration */
    {
    ulong start;
    ulong size;
    }           bi_dram[CONFIG_NR_DRAM_BANKS];
} bd_t;

其中 DECLARE_GLOBAL_DATA_PTR 宏定义在系统初始化过程中会被频繁调用，

其的作用是，声明gd这么一个全局的指针，这个指针指向gd_t结构体类型，并且这个gd指针是保存在ARM的r8这个寄存器里面的。

uboot.img 第一个运行的文件还是 start.o，其在运行访问的 board_init_f() 函数定义在 /arch/arm/lib/board.c 中：

void board_init_f(ulong bootflag)
{
    bd_t *bd;
    init_fnc_t **init_fnc_ptr;
    gd_t *id;
    ulong addr, addr_sp;
 
    /* Pointer is writable since we allocated a register for it */
    gd = (gd_t *) ((CONFIG_SYS_INIT_SP_ADDR) & ~0x07);
    /* compiler optimization barrier needed for GCC >= 3.4 */
    __asm__ __volatile__("": : :"memory");
 
    memset((void *)gd, 0, sizeof(gd_t));
 
        ...
}

#define CONFIG_SYS_INIT_RAM_ADDR    SRAM0_START
#define CONFIG_SYS_INIT_RAM_SIZE    SRAM0_SIZE
#define CONFIG_SYS_INIT_SP_ADDR     (CONFIG_SYS_INIT_RAM_ADDR + 
                     CONFIG_SYS_INIT_RAM_SIZE - 
                     GENERATED_GBL_DATA_SIZE)
 
#define SRAM0_START         0x402F0400
 
#define SRAM0_SIZE          (0x1B400) /* 109 KB */
 
#define GENERATED_GBL_DATA_SIZE (128) /* (sizeof(struct global_data) + 15) & ~15 */

因此，系统初始化参数将会被保存在 （保存 MLO（SPL）文件的内存空间的）末尾 2 KB 处。

通过计算的 gb 指针指向的内存空间地址为 gb = 0x4030B000

gb_t 结构体中某些元素的值是来自于 uboot.img's header，这个header的数据保存在内存的0x807FFFCO，大小为 64字节

/*
 * Legacy format image header,
 * all data in network byte order (aka natural aka bigendian).
 */
typedef struct image_header {
    uint32_t    ih_magic;   /* Image Header Magic Number    */
    uint32_t    ih_hcrc;    /* Image Header CRC Checksum    */
    uint32_t    ih_time;    /* Image Creation Timestamp */
    uint32_t    ih_size;    /* Image Data Size      */
    uint32_t    ih_load;    /* Data  Load  Address      */
    uint32_t    ih_ep;      /* Entry Point Address      */
    uint32_t    ih_dcrc;    /* Image Data CRC Checksum  */
    uint8_t     ih_os;      /* Operating System     */
    uint8_t     ih_arch;    /* CPU architecture     */
    uint8_t     ih_type;    /* Image Type           */
    uint8_t     ih_comp;    /* Compression Type     */
    uint8_t     ih_name[IH_NMLEN];  /* Image Name       */
} image_header_t;

/*
 * 8MB into the SDRAM to allow for SPL's bss at the beginning of SDRAM.
 * 64 bytes before this address should be set aside for u-boot.img's
 * header. That is 0x807FFFC0--0x80800000 should not be used for any
 * other needs.
 */
#define CONFIG_SYS_TEXT_BASE        0x80800000</path>

void board_init_f(ulong bootflag)
{
    bd_t *bd;
    init_fnc_t **init_fnc_ptr;
    gd_t *id;
    ulong addr, addr_sp;
 
    /* Pointer is writable since we allocated a register for it */
    gd = (gd_t *) ((CONFIG_SYS_INIT_SP_ADDR) & ~0x07);
    /* compiler optimization barrier needed for GCC >= 3.4 */
    __asm__ __volatile__("": : :"memory");
 
    memset((void *)gd, 0, sizeof(gd_t));
 
        ...
}