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。