从零写bootloader--第二阶段启动的实现

说明：第一阶段和第二阶段具体细节未做说明，旨在整个启动流程，即如何自己实现bootloader的整体思维。代码附带贴出，可以参考。

在第一阶段start.s中未完成的工作只剩下main函数。main函数的工作如下：

1、从NandFlash中把内核读入内存

2、设置需要传递的参数

3、跳转执行

第一阶段代码与下面代码一起便可启动内核，具体不在此分析，如不理解，可以参看uboot源码

boot.c（串口的初始化在第一阶段的init.c中）

#include "setup.h"

extern void uart0_init(void);
extern void nand_read(unsigned int addr, unsigned charchar *buf, unsigned int len);
extern void puts(charchar *str);
extern void puthex(unsigned int val);


static struct tag *params;

void setup_start_tag(void)
{
    params = (struct tag *)0x30000100;

    params->hdr.tag = ATAG_CORE;
    params->hdr.size = tag_size (tag_core);

    params->u.core.flags = 0;
    params->u.core.pagesize = 0;
    params->u.core.rootdev = 0;

    params = tag_next (params);
}

void setup_memory_tags(void)
{
    params->hdr.tag = ATAG_MEM;
    params->hdr.size = tag_size (tag_mem32);

    params->u.mem.start = 0x30000000;
    params->u.mem.size  = 64*1024*1024;

    params = tag_next (params);
}

int strlen(charchar *str)
{
    int i = 0;
    while (str[i])
    {
        i++;
    }
    return i;
}

void strcpy(charchar *dest, charchar *src)
{
    while ((*dest++ = *src++) != '\0');
}

void setup_commandline_tag(charchar *cmdline)
{
    int len = strlen(cmdline) + 1;

    params->hdr.tag  = ATAG_CMDLINE;
    params->hdr.size = (sizeof (struct tag_header) + len + 3) >> 2;

    strcpy (params->u.cmdline.cmdline, cmdline);

    params = tag_next (params);
}

void setup_end_tag(void)
{
    params->hdr.tag = ATAG_NONE;
    params->hdr.size = 0;
}


int main(void)
{
    void (*theKernel)(int zero, int arch, unsigned int params);
    volatile unsigned intint *p = (volatile unsigned intint *)0x30008000;

    /* 0. 帮内核设置串口: 内核启动的开始部分会从串口打印一些信息,但是内核一开始没有初始化串口 */
    uart0_init();

    /* 1. 从NAND FLASH里把内核读入内存 */
    puts("Copy kernel from nand\n\r");
    nand_read(0x60000+64, (unsigned charchar *)0x30008000, 0x200000);  //uImage = 64字节头部 + zImage，内核存放在0x60000的位置
    puthex(0x1234ABCD);                                            //用作测试
    puts("\n\r");
    puthex(*p);                                                    //打印出zImage前4字节
    puts("\n\r");

    /* 2. 设置参数 */
    puts("Set boot params\n\r");
    setup_start_tag();
    setup_memory_tags();
    setup_commandline_tag("noinitrd root=/dev/mtdblock3 init=/linuxrc console=ttySAC0");
    setup_end_tag();

    /* 3. 跳转执行 */
    puts("Boot kernel\n\r");
    theKernel = (void (*)(int, int, unsigned int))0x30008000;
    theKernel(0, 362, 0x30000100);           //要传递的参数约定放在0x30000100的位置，内核放在0x30008000的位置。中间放参数
    /*
     *  mov r0, #0
     *  ldr r1, =362
     *  ldr r2, =0x30000100
     *  mov pc, #0x30008000
     */

    puts("Error!\n\r");
    /* 如果一切正常, 不会执行到这里 */

    return -1;
}

Makefile

CC      = arm-linux-gcc
LD      = arm-linux-ld
AR      = arm-linux-ar
OBJCOPY = arm-linux-objcopy
OBJDUMP = arm-linux-objdump

CFLAGS      := -Wall -O2
CPPFLAGS    := -nostdinc -nostdlib -fno-builtin

objs := start.o init.o boot.o

boot.bin: $(objs)
    ${LD} -Tboot.lds -o boot.elf $^
    ${OBJCOPY} -O binary -S boot.elf $@
    ${OBJDUMP} -D -m arm boot.elf > boot.dis

%.o:%.c
    ${CC} $(CPPFLAGS) $(CFLAGS) -c -o $@ $<

%.o:%.S
    ${CC} $(CPPFLAGS) $(CFLAGS) -c -o $@ $<

clean:
    rm -f *.o *.bin *.elf *.dis