u-boot的SPL源码流程分析

　　上次梳理了一下SPL的基本概念和代码总体思路，这次就针对代码跑的流程做个梳理。SPL中，入口在u-boot-spl.lds中

ENTRY(_start)
SECTIONS
{
.text :
{
__start = .;
*(.vectors)                          //进入中断向量表，对应的跳转到U-boot/arch/arm/lib/vectors.S文件处理
arch/arm/cpu/armv7/start.o (.text*) //跳转到对应的启动加载项，后续针对这个做处理。
*(.text*)
} >.sram
. = ALIGN(4);
.rodata : { *(SORT_BY_ALIGNMENT(.rodata*)) } >.sram
. = ALIGN(4);
.data : { *(SORT_BY_ALIGNMENT(.data*)) } >.sram
. = ALIGN(4); 
.u_boot_list : {
KEEP(*(SORT(.u_boot_list*_i2c_*)));
} >.sram
. = ALIGN(4); 
__image_copy_end = .;

　　在这里，启动加载会跳转到文件arch/arm/cpu/armv7/start.S中，这个是怎么处理的呢？在这里，文件的主要工作有下面几种：

A 重启保存启动参数：

reset:
        /* Allow the board to save important registers */
        b       save_boot_params
save_boot_params_ret:
        /*
         * disable interrupts (FIQ and IRQ), also set the cpu to SVC32 mode,
         * except if in HYP mode already
         */
        mrs     r0, cpsr
        and     r1, r0, #0x1f           @ mask mode bits
        teq     r1, #0x1a               @ test for HYP mode
        bicne   r0, r0, #0x1f           @ clear all mode bits
        orrne   r0, r0, #0x13           @ set SVC mode
        orr     r0, r0, #0xc0           @ disable FIQ and IRQ
        msr     cpsr,r0

B 设置向量表并跳转：

/*
 * Setup vector:
 * (OMAP4 spl TEXT_BASE is not 32 byte aligned.
 * Continue to use ROM code vector only in OMAP4 spl)
 */
#if !(defined(CONFIG_OMAP44XX) && defined(CONFIG_SPL_BUILD))
        /* Set V=0 in CP15 SCTLR register - for VBAR to point to vector */
        mrc     p15, 0, r0, c1, c0, 0   @ Read CP15 SCTLR Register
        bic     r0, #CR_V               @ V = 0
        mcr     p15, 0, r0, c1, c0, 0   @ Write CP15 SCTLR Register

        /* Set vector address in CP15 VBAR register */
        ldr     r0, =_start
        mcr     p15, 0, r0, c12, c0, 0  @Set VBAR
#endif

        /* the mask ROM code should have PLL and others stable */
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
        bl      cpu_init_cp15
        bl      cpu_init_crit
#endif

        bl      _main

C 针对CP15协处理器做优化，并关闭Icache MMU和TLBS，具体代码如下：

ENTRY(cpu_init_cp15)
        /*
         * Invalidate L1 I/D
         */
        mov     r0, #0                  @ set up for MCR
        mcr     p15, 0, r0, c8, c7, 0   @ invalidate TLBs
        mcr     p15, 0, r0, c7, c5, 0   @ invalidate icache
        mcr     p15, 0, r0, c7, c5, 6   @ invalidate BP array
        mcr     p15, 0, r0, c7, c10, 4  @ DSB
        mcr     p15, 0, r0, c7, c5, 4   @ ISB

        /*
         * disable MMU stuff and caches
         */
        mrc     p15, 0, r0, c1, c0, 0
        bic     r0, r0, #0x00002000     @ clear bits 13 (--V-)
        bic     r0, r0, #0x00000007     @ clear bits 2:0 (-CAM)
        orr     r0, r0, #0x00000002     @ set bit 1 (--A-) Align
        orr     r0, r0, #0x00000800     @ set bit 11 (Z---) BTB
#ifdef CONFIG_SYS_ICACHE_OFF
        bic     r0, r0, #0x00001000     @ clear bit 12 (I) I-cache
#else
        orr     r0, r0, #0x00001000     @ set bit 12 (I) I-cache
#endif
        mcr     p15, 0, r0, c1, c0, 0

为什么要关闭这些特性呢？ 
/******************************************************* 　　
*1、为什么要关闭mmu？ 　
　*因为MMU是把虚拟地址转化为物理地址得作用 　
　*而我们现在是要设置控制寄存器，而控制寄存器本来就是实地址（物理地址）， 　
　*再使能MMU，不就是多此一举了吗？ 　　
*2、为什么要关闭cache？ 　　　　　
　*catch和MMU是通过CP15管理的，刚上电的时候，CPU还不能管理他们。 
  *所以上电的时候MMU必须关闭，指令cache可关闭，可不关闭，但数据cache一定要关闭
  *否则可能导致刚开始的代码里面，去取数据的时候，从catch里面取， 
  *而这时候RAM中数据还没有cache过来，导致数据预取异常 　　　　　
*******************************************************************/

　D 系统的重要寄存器和内存时钟初始化。

/*************************************************************************
 *
 * CPU_init_critical registers
 *
 * setup important registers
 * setup memory timing
 *
 *************************************************************************/
ENTRY(cpu_init_crit)
        /*
         * Jump to board specific initialization...
         * The Mask ROM will have already initialized
         * basic memory. Go here to bump up clock rate and handle
         * wake up conditions.
         */
        b       lowlevel_init           @ go setup pll,mux,memory
ENDPROC(cpu_init_crit)

　　下面系统就跳转到了函数lowlevel_init去执行了，这里完成了什么任务呢？接下来要进行追踪arch/arm/cpu/armv7/lowlevel_init.S

进去看一看了。

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_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)

　　其实，这里面做的事情是比较简单的，就是完成一些简单的初始化动作。主要的工作还是在board_init_f()函数里面完成。

文字里面解释很清楚了，这里就不做赘述了。

　　在最后的跳转__main()中，函数跳转到了文件arch/arm/lib/crt0.S中来，这里做了什么工作呢？

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 */
    .........................................................................................

#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 */
        ldr     pc, =board_init_r       /* this is auto-relocated! */

        /* we should not return here. */
#endif

ENDPROC(_main)

　　这里做的工作还挺多，主要就是初始化C的运行环境和调用单板board_init_f(0)初始化操作。

下面就要看board_init_r的工作了。在文件arch/arm/lib/spl.c中可以看到：

/*
 * In the context of SPL, board_init_f must ensure that any clocks/etc for
 * DDR are enabled, ensure that the stack pointer is valid, clear the BSS
 * and call board_init_f.  We provide this version by default but mark it
 * as __weak to allow for platforms to do this in their own way if needed.
 */
void __weak board_init_f(ulong dummy)
{       
        /* Clear the BSS. */
        memset(__bss_start, 0, __bss_end - __bss_start);

#ifndef CONFIG_DM
        /* TODO: Remove settings of the global data pointer here */
        gd = &gdata;
#endif
        
        board_init_r(NULL, 0);
}

　　这里又调用了common/spl/spl.c文件中的board_init_r(),让我们看看这个都做了什么改动呢？

void board_init_r(gd_t *dummy1, ulong dummy2)
{
        u32 boot_device;
        int ret;

        debug(">>spl:board_init_r()
");

#if defined(CONFIG_SYS_SPL_MALLOC_START)
        mem_malloc_init(CONFIG_SYS_SPL_MALLOC_START,
                        CONFIG_SYS_SPL_MALLOC_SIZE);
        gd->flags |= GD_FLG_FULL_MALLOC_INIT;
#elif defined(CONFIG_SYS_MALLOC_F_LEN)
        gd->malloc_limit = CONFIG_SYS_MALLOC_F_LEN;
        gd->malloc_ptr = 0;
#endif
        if (IS_ENABLED(CONFIG_OF_CONTROL) &&
                        !IS_ENABLED(CONFIG_SPL_DISABLE_OF_CONTROL)) {
                ret = fdtdec_setup();
                if (ret) {
                        debug("fdtdec_setup() returned error %d
", ret);
                        hang();
                                      }
        }

#ifndef CONFIG_PPC
        /*
         * timer_init() does not exist on PPC systems. The timer is initialized
         * and enabled (decrementer) in interrupt_init() here.
         */
        timer_init();
#endif

#ifdef CONFIG_SPL_BOARD_INIT
        spl_board_init();
#endif

        boot_device = spl_boot_device();
        debug("boot device - %d
", boot_device);
        switch (boot_device) {
#ifdef CONFIG_SPL_RAM_DEVICE
        case BOOT_DEVICE_RAM:
                spl_ram_load_image();
                break;
#endif
#ifdef CONFIG_SPL_MMC_SUPPORT
        case BOOT_DEVICE_MMC1:
        case BOOT_DEVICE_MMC2:
        case BOOT_DEVICE_MMC2_2:
                spl_mmc_load_image();
                break;
#endif
#ifdef CONFIG_SPL_NAND_SUPPORT
        case BOOT_DEVICE_NAND:
                spl_nand_load_image();
                break;
#endif
#ifdef CONFIG_SPL_ONENAND_SUPPORT
        case BOOT_DEVICE_ONENAND:
                spl_onenand_load_image();
                break;
#endif
#ifdef CONFIG_SPL_NOR_SUPPORT
        case BOOT_DEVICE_NOR:
                spl_nor_load_image();
                break;
                                               
#endif
#ifdef CONFIG_SPL_YMODEM_SUPPORT
        case BOOT_DEVICE_UART:
                spl_ymodem_load_image();
                break;
#endif
#ifdef CONFIG_SPL_SPI_SUPPORT
        case BOOT_DEVICE_SPI:
                spl_spi_load_image();
                break;
#endif
#ifdef CONFIG_SPL_ETH_SUPPORT
        case BOOT_DEVICE_CPGMAC:
#ifdef CONFIG_SPL_ETH_DEVICE
                spl_net_load_image(CONFIG_SPL_ETH_DEVICE);
#else
                spl_net_load_image(NULL);
#endif
                break;
#endif
#ifdef CONFIG_SPL_USBETH_SUPPORT
        case BOOT_DEVICE_USBETH:
                spl_net_load_image("usb_ether");
                break;
#endif
#ifdef CONFIG_SPL_USB_SUPPORT
#endif
        default:
                debug("Unsupported OS image.. Jumping nevertheless..
");
        }
#if defined(CONFIG_SYS_MALLOC_F_LEN) && !defined(CONFIG_SYS_SPL_MALLOC_SIZE)
        debug("SPL malloc() used %#lx bytes (%ld KB)
", gd->malloc_ptr,
              gd->malloc_ptr / 1024);
#endif

        jump_to_image_no_args(&spl_image);

                                                                                                                                                                                   

　　其实，这里面做了那么多，整体的思路就是按照不同的启动模式，调用不同的image来下载，到此，SPL的使命基本结束了。