题外话：
汇编部分
之前一直看的是ARM9的u-boot，AM335X系列为代表的ARMv7系列处理器为了使得加载方式尽可能的灵活，就将原来的u-boot分解成了两个部分：SPL和u-boot，而实际上在重启后，到SPL被执行之间，还有一段片上ROM内的代码被运行。该ROM内的代码根据启动时的配置引脚的配置，自动生成一个加载优先列表，比如说首先加载MMC1，在考虑MMC0，最后再考虑uart等等。比如说用户正确的将SPL（SPL加上一个包头变成MLO）和u-boot放置到SD中后，那么ROM就会在启动：1.按照加载列表的boot优先级逐个尝试，2.判断是MMC1上拥有有效设备（比如说SD卡已经插入）后，继续判断MMC上挂的是SD卡还是MMC内存，3. 查看格式 4. 加载SPL（MLO）到片内的SRAM， 5.cpu将pc指到SRAM，SPL正式开始运行。
下面开始分析，SPL是如何工作的，首先是程序的入口，在arch/arm/cpu/armv7/start.S中的程序的入口，reset，下面开始分析，源码如下：
- reset:
- bl save_boot_params /*lowlevel_init.S (arch\arm\cpu\armv7\omap-common)*/
- /*
- * 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
- /*
- * 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 SCTRL register - for VBAR to point to vector */
- mrc p15, 0, r0, c1, c0, 0 @ Read CP15 SCTRL Register
- bic r0, #CR_V @ V = 0
- mcr p15, 0, r0, c1, c0, 0 @ Write CP15 SCTRL 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/*this branch will only work in SPL*/
- bl cpu_init_cp15 /*wlg: find out in this file, we do not explain in detial*/
- bl cpu_init_crit /*wlg: find out in this file, please jump*/
- #endif
reset:
bl save_boot_params /*lowlevel_init.S (arch\arm\cpu\armv7\omap-common)*/
/*
* 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
/*
* Setup vector:
* (OMAP4 spl TEXT_BASE is not 32 byte aligned.
* Continue to use ROM code vector only in OMAP4 spl)
*/
可以看到reset进去后，1. 首先需要执行save_boot_paras，它的入口就在lowlevel_init.S (arch\arm\cpu\armv7\omap-common)中，它的具体形式如下
- ENTRY(save_boot_params)
- ldr r1, =OMAP_SRAM_SCRATCH_BOOT_PARAMS
- str r0, [r1]
- bx lr
- ENDPROC(save_boot_params)
ENTRY(save_boot_params)
ldr r1, =OMAP_SRAM_SCRATCH_BOOT_PARAMS
str r0, [r1]
bx lr
ENDPROC(save_boot_params)2.下面的代码请结合相应的书册，其原始注释已经写得很清楚了，一路往下走，来到了#ifndef CONFIG_SKIP_LOWLEVEL_INIT，这个东西我们简单的介绍下，像这种大的开源工程，其包含了很多的宏开关，相当于用户实际需需要什么，定义什么，就会有相应的驱动代码与之匹配。就像此处，实际上u-boot和SPL是共享start.S的，而实际上他们执行的东西是不一样的，如何让编译器正确的理解用户实际想要包含的代码段，一方面靠的是宏做条件编译，另一方面靠的是makefile，后者就更加复杂了；在这里我们分析的就是SPL程序，所以我们会在相应的文件中定义CONFIG_SKIP_LOWLEVEL_INIT这个宏，那么这段代码实际上就被包含在了SPL程序中；继续往下看：
3. 跳到cpu_init_cp15，这段代码就在本文中，不再展开
4.执行bl cpu_init_crit ，这个程序的入口就在本文档中，以下展开讨论：
- #ifndef CONFIG_SKIP_LOWLEVEL_INIT
- /*************************************************************************
- *
- * 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/*wlg: jump to /arch/arm/cpu/armv7/lowlevel_init.s*/
- ENDPROC(cpu_init_crit)
- #endif
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
/*************************************************************************
*
* 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/*wlg: jump to /arch/arm/cpu/armv7/lowlevel_init.s*/
ENDPROC(cpu_init_crit)
首先还是宏打头，表示条件编译，在SPL程序中，下面的代码段实际被包含！看注释，已经大致了解它的作用，整个代码段只有一句，直接转跳到下一个程序的入口，在/arch/arm/cpu/armv7/lowlevel_init.s中定义了lowlevel_init。注意到，之前转跳都是用的bl，表示会返回，但是这里却用的是b？我个人的理解是bl没法子应对二次转跳，也就是说在lowlevel_init中还会做更复杂的转跳，甚至开始进入C函数中，那么bl我觉得是无力的。那么这里到底需不需要返回呢？
答案是需要的，但是会利用stack进行返回指针保存，利用push和pop来实现。 拿下一个问题就是，我们还没有初始化sp呢，你就敢乱用push？嘿嘿，我们继续往下看：
- ENTRY(lowlevel_init)
- /*
- * Setup a temporary stack wlg: we set a temp stack in SRAM for building
- a enveriment for C program */
- ldr sp, =CONFIG_SYS_INIT_SP_ADDR /*wlg: this is a temp stack built in SRAM
- bic sp, sp, #7 /* 8-byte alignment for ABI compliance */
- #ifdef CONFIG_SPL_BUILD
- ldr r9, =gdata /*wlg: we set a temp gdata(global_data) in SRAM(.data section), and */
- #else /*wlg: r9 is point to that region in SRAM on chip ///////SPL*/
- sub sp, sp, #GD_SIZE
- bic sp, sp, #7 /*wlg: cause uboot will define skip_lowlevel_init, so this branch will*/
- mov r9, sp /*wlg: not active, even on stage of uboot!////////////uboot*/
- #endif
- /*
- * Save the old lr(passed in ip) and the current lr to stack
- */
- push {ip, lr} /*wlg: save the address return*/
- /*
- * go setup pll, mux, memory
- */
- bl s_init /*wlg: jump to arch/arm/cpu/armv7/am335x/board.c- s_init()*/
- pop {ip, pc} /*wlg: to make SDRAM initialition, then make a return to _reset*/
- ENDPROC(lowlevel_init)
ENTRY(lowlevel_init)
/*
* Setup a temporary stack wlg: we set a temp stack in SRAM for building
a enveriment for C program */
ldr sp, =CONFIG_SYS_INIT_SP_ADDR /*wlg: this is a temp stack built in SRAM
bic sp, sp, #7 /* 8-byte alignment for ABI compliance */
1. 一开始就把sp指向了SRAM的高位CONFIG_SYS_INIT_SP_ADDR（因为低位拿来保存text程序了，高位用来保存数据bss，stack等等）
2.做了对齐，下面又是一个条件编译，这里我们仍然执行的是ldr r9, =gdata；这个我们要好好分析一下，去寻找gdata的定义，会发现在某个c文件中定义了一个全局变量，即global_data的结构体数据，那么在实际运行的时候，gdata是在SRAM中的。所以这里仅需知道，用户将sp和r9都指向了SRAM中某处。大家都知道sp是专门拿来做堆栈的，那么r9在这里有什么用处呢？我们简单的介绍下，这里的r9，和后面会出面的gd是同一个东西，也就是说，当用用需要修改或读取gdata里的元素的数据时，直接可以使用gd.a,gd.b即可，因为r9实际指向了gdata，而gd实际上就是r9的一个别名（宏定义）；
3.再往下push {ip, lr}，就是之前所说的用stack的方式保存lr（返回指针），所以我们在前面给sp赋值；同时，给sp赋值也相当于是为进入C语言函数做准备
4. bl s_init，再次作了转跳，这是一个重点的部分，函数定义在arch/arm/cpu/armv7/am335x/board.c- s_init()
C语言
实际上在之前已经完成了sp的初始化，我们可以开始调用C语言函数了。同时，我们需要看到在文档board.c的最下方
#define DECLARE_GLOBAL_DATA_PTR
register volatile gd_t *gd asm (“r9”)
这实际上就是说，以后用户调用gd的地方，实际上就是用了r9里的数作为指针，而经过上面的分析，r9实际上指向了gdata，用户定义的一个global_data的结构体。一句话说就是，gd不是一个野指针，而是实际上指向了gdata；以后不在展开讨论；
继续往下看，首先是一个条件编译，这部分我觉得是跳过的，也就是说没有被编译进来，继续往下看
进入了save_omap_boot_params（）函数，该函数在arch/arm/cpu/armv7/Omap-common/Boot-common.c中定义；我们之前提及过boot参数，之前一直没有保存，就是因为当时才启动，没有合适的保存boot参数的区域（gd指针还没有实际指向有效的数据区域）。当用户完成所有的准备工作后，那么boot参数的保存就必须要开始了：
- void s_init(void)
- {
- /*
- * The ROM will only have set up sufficient pinmux to allow for the
- * first 4KiB NOR to be read, we must finish doing what we know of
- * the NOR mux in this space in order to continue.
- */
- #ifdef CONFIG_NOR_BOOT
- enable_norboot_pin_mux();
- #endif
- /*
- * Save the boot parameters passed from romcode.
- * We cannot delay the saving further than this,
- * to prevent overwrites.
- */
- #ifdef CONFIG_SPL_BUILD
- save_omap_boot_params();//wlg: should record the boot parament now, arch/arm/cpu/armv7/Omap-common/Boot-common.c
void s_init(void)
{
/*
* The ROM will only have set up sufficient pinmux to allow for the
* first 4KiB NOR to be read, we must finish doing what we know of
* the NOR mux in this space in order to continue.
*/
- void save_omap_boot_params(void)
- {
- u32 rom_params = *((u32 *)OMAP_SRAM_SCRATCH_BOOT_PARAMS);//wlg: look in TRM P4954
- u8 boot_device;//wlg:
- u32 dev_desc, dev_data;
- if ((rom_params < NON_SECURE_SRAM_START) ||
- (rom_params > NON_SECURE_SRAM_END))
- return;
- /*
- * rom_params can be type casted to omap_boot_parameters and
- * used. But it not correct to assume that romcode structure
- * encoding would be same as u-boot. So use the defined offsets.
- */
- gd->arch.omap_boot_params.omap_bootdevice = boot_device =
- *((u8 *)(rom_params + BOOT_DEVICE_OFFSET));//wlg: P4954, offset = 0x8,
- //wlg: it point to Current Booting Device!the parament will be use to copy uboot
- gd->arch.omap_boot_params.ch_flags =
- *((u8 *)(rom_params + CH_FLAGS_OFFSET));
- if ((boot_device >= MMC_BOOT_DEVICES_START) &&//wlg: boot device list from 0-6,XIP,MMC0,MMC1 and so on
- (boot_device <= MMC_BOOT_DEVICES_END)) {
- #if !defined(CONFIG_AM33XX) && !defined(CONFIG_TI81XX) && \//wlg: skip this
- !defined(CONFIG_AM43XX)
- if ((omap_hw_init_context() ==
- OMAP_INIT_CONTEXT_UBOOT_AFTER_SPL)) {
- gd->arch.omap_boot_params.omap_bootmode =
- *((u8 *)(rom_params + BOOT_MODE_OFFSET));
- } else
- #endif
- {
- dev_desc = *((u32 *)(rom_params + DEV_DESC_PTR_OFFSET));//wlg: point to memory device descriptor
- dev_data = *((u32 *)(dev_desc + DEV_DATA_PTR_OFFSET));
- gd->arch.omap_boot_params.omap_bootmode =
- *((u32 *)(dev_data + BOOT_MODE_OFFSET));//wlg: record the boot mode into global_data(temp)
- }
- }
- #ifdef CONFIG_DRA7XX
- /*
- * We get different values for QSPI_1 and QSPI_4 being used, but
- * don’t actually care about this difference. Rather than
- * mangle the later code, if we’re coming in as QSPI_4 just
- * change to the QSPI_1 value.
- */
- if (gd->arch.omap_boot_params.omap_bootdevice == 11)
- gd->arch.omap_boot_params.omap_bootdevice = BOOT_DEVICE_SPI;
- #endif//wlg: return to s_init()
- }
void save_omap_boot_params(void)
{
u32 rom_params = *((u32 *)OMAP_SRAM_SCRATCH_BOOT_PARAMS);//wlg: look in TRM P4954
u8 boot_device;//wlg:
u32 dev_desc, dev_data;
if ((rom_params < NON_SECURE_SRAM_START) ||
(rom_params > NON_SECURE_SRAM_END))
return;
/*
* rom_params can be type casted to omap_boot_parameters and
* used. But it not correct to assume that romcode structure
* encoding would be same as u-boot. So use the defined offsets.
*/
gd->arch.omap_boot_params.omap_bootdevice = boot_device =
*((u8 *)(rom_params + BOOT_DEVICE_OFFSET));//wlg: P4954, offset = 0x8,
//wlg: it point to Current Booting Device!the parament will be use to copy uboot
gd->arch.omap_boot_params.ch_flags =
*((u8 *)(rom_params + CH_FLAGS_OFFSET));
if ((boot_device >= MMC_BOOT_DEVICES_START) &&//wlg: boot device list from 0-6,XIP,MMC0,MMC1 and so on
(boot_device <= MMC_BOOT_DEVICES_END)) {
if !defined(CONFIG_AM33XX) && !defined(CONFIG_TI81XX) && \//wlg: skip this
!defined(CONFIG_AM43XX)
if ((omap_hw_init_context() ==
OMAP_INIT_CONTEXT_UBOOT_AFTER_SPL)) {
gd->arch.omap_boot_params.omap_bootmode =
*((u8 *)(rom_params + BOOT_MODE_OFFSET));
} else
endif
{
dev_desc = *((u32 *)(rom_params + DEV_DESC_PTR_OFFSET));//wlg: point to memory device descriptor
dev_data = *((u32 *)(dev_desc + DEV_DATA_PTR_OFFSET));
gd->arch.omap_boot_params.omap_bootmode =
*((u32 *)(dev_data + BOOT_MODE_OFFSET));//wlg: record the boot mode into global_data(temp)
}
}
ifdef CONFIG_DRA7XX
/*
* We get different values for QSPI_1 and QSPI_4 being used, but
* don't actually care about this difference. Rather than
* mangle the later code, if we're coming in as QSPI_4 just
* change to the QSPI_1 value.
*/
if (gd->arch.omap_boot_params.omap_bootdevice == 11)
gd->arch.omap_boot_params.omap_bootdevice = BOOT_DEVICE_SPI;
endif//wlg: return to s_init()
}
boot参数请查看TRM P4954，也就是技术文档的4954页，里面记录了参数保存的顺序以及名称，将其用指针（rom_params）取出后一次放入到gd所指向的gdata中，一般有：
1.boot原因
2.SPL文件来源等等
这些参数在后期启动uboot都有作用，这里不展开先，过几天在续写一下！2017.01.22夜