uboot之bootm命令分析

bootm要做的事情：

1. 讀取頭部，把內核拷貝到合適的地方

2. 把參數給內核准備好，並告訴內核參數的首地址
3. 引導內核

啟動內核：
do_bootm_linux: 1. 設置參數，跳到入口地址theKernel = (void (*)(int, int, uint))ntohl(hdr->ih_ep);

參數的傳遞：在某個地址按某種格式，存放好數據

uboot引導linux內核之do_bootm解析：
do_bootm
int do_bootm (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
{
ulong iflag;
ulong addr;
ulong data, len, checksum;
ulong  *len_ptr;
uint unc_len = CFG_BOOTM_LEN;
int i, verify;
char *name, *s;
int (*appl)(int, char *[]);
image_header_t *hdr = &header;

s = getenv ("verify");  //為是否對鏡像頭做校驗做准備，讀取uboot的環境變量verify，
如果環境變量verify等於’n’，則局部變量verify賦值成為0；如果環境變量verify為空（即沒有
定義環境變量verify）或者環境變量verify不等於’n’，則局部變量verify賦值成為1。
verify = (s && (*s == 'n')) ? 0 : 1;

if (argc < 2) { //獲取鏡像存放的內存首地址，如果參數個數小於2（即只是輸入了bootm），
//使用缺省加載地址CFG_LOAD_ADDR；否則使用第二個參數作為加載地址。
addr = load_addr;
} else {
addr = simple_strtoul(argv[1], NULL, 16);
}

SHOW_BOOT_PROGRESS (1);
printf ("## Booting image at %08lx ...\n", addr);

/* Copy header so we can blank CRC field for re-calculation */
#ifdef CONFIG_HAS_DATAFLASH
if (addr_dataflash(addr)){
read_dataflash(addr, sizeof(image_header_t), (char *)&header);
} else
#endif
memmove (&header, (char *)addr, sizeof(image_header_t));//從鏡像內存首地址讀取鏡像頭部，為下面的分析校驗做准備

if (ntohl(hdr->ih_magic) != IH_MAGIC) { //判斷文件頭中的幻數是否為IH_MAGIC，所以如果不是u-boot鏡像格式，會輸出提示信息”Bad Magic Number”
#ifdef __I386__ /* correct image format not implemented yet - fake it */
if (fake_header(hdr, (void*)addr, -1) != NULL) {
/* to compensate for the addition below */
addr -= sizeof(image_header_t);
/* turnof verify,
* fake_header() does not fake the data crc
*/
verify = 0;
} else
#endif /* __I386__ */
   {
puts ("Bad Magic Number\n");
SHOW_BOOT_PROGRESS (-1);
return 1;
   }
}
SHOW_BOOT_PROGRESS (2);

data = (ulong)&header;
len  = sizeof(image_header_t);

checksum = ntohl(hdr->ih_hcrc); //對鏡像頭做crc校驗
hdr->ih_hcrc = 0;

if (crc32 (0, (uchar *)data, len) != checksum) {
puts ("Bad Header Checksum\n");
SHOW_BOOT_PROGRESS (-2);
return 1;
}
SHOW_BOOT_PROGRESS (3);

#ifdef CONFIG_HAS_DATAFLASH
if (addr_dataflash(addr)){
len  = ntohl(hdr->ih_size) + sizeof(image_header_t);
read_dataflash(addr, len, (char *)CFG_LOAD_ADDR);
addr = CFG_LOAD_ADDR;
}
#endif

/* for multi-file images we need the data part, too */
print_image_hdr ((image_header_t *)addr);

data = addr + sizeof(image_header_t);
len  = ntohl(hdr->ih_size);

if (verify) {   //對鏡像的數據部分做crc校驗
puts ("   Verifying Checksum ... ");
if (crc32 (0, (uchar *)data, len) != ntohl(hdr->ih_dcrc)) {
printf ("Bad Data CRC\n");
SHOW_BOOT_PROGRESS (-3);
return 1;
}
puts ("OK\n");
}
SHOW_BOOT_PROGRESS (4);

len_ptr = (ulong *)data;

#if defined(__PPC__)          //校驗cpu類型是否正確
if (hdr->ih_arch != IH_CPU_PPC)
#elif defined(__ARM__)
if (hdr->ih_arch != IH_CPU_ARM)
#elif defined(__I386__)
if (hdr->ih_arch != IH_CPU_I386)
#elif defined(__mips__)
if (hdr->ih_arch != IH_CPU_MIPS)
#elif defined(__nios__)
if (hdr->ih_arch != IH_CPU_NIOS)
#elif defined(__M68K__)
if (hdr->ih_arch != IH_CPU_M68K)
#elif defined(__microblaze__)
if (hdr->ih_arch != IH_CPU_MICROBLAZE)
#elif defined(__nios2__)
if (hdr->ih_arch != IH_CPU_NIOS2)
#elif defined(__blackfin__)
if (hdr->ih_arch != IH_CPU_BLACKFIN)
#elif defined(__avr32__)
if (hdr->ih_arch != IH_CPU_AVR32)
#else
# error Unknown CPU type
#endif
{
printf ("Unsupported Architecture 0x%x\n", hdr->ih_arch);
SHOW_BOOT_PROGRESS (-4);
return 1;
}
SHOW_BOOT_PROGRESS (5);

switch (hdr->ih_type) {
case IH_TYPE_STANDALONE:
name = "Standalone Application";
/* A second argument overwrites the load address */
if (argc > 2) {
hdr->ih_load = htonl(simple_strtoul(argv[2], NULL, 16));
}
break;
case IH_TYPE_KERNEL:
name = "Kernel Image";
break;
case IH_TYPE_MULTI:
name = "Multi-File Image";
len  = ntohl(len_ptr[0]);
/* OS kernel is always the first image */
data += 8; /* kernel_len + terminator */
for (i=1; len_ptr[i]; ++i)
data += 4;
break;
default: printf ("Wrong Image Type for %s command\n", cmdtp->name);
SHOW_BOOT_PROGRESS (-5);
return 1;
}
SHOW_BOOT_PROGRESS (6);

/*
* We have reached the point of no return: we are going to
* overwrite all exception vector code, so we cannot easily
* recover from any failures any more...
*/

iflag = disable_interrupts();

#ifdef CONFIG_AMIGAONEG3SE
/*
* We've possible left the caches enabled during
* bios emulation, so turn them off again
*/
icache_disable(); 
invalidate_l1_instruction_cache();
flush_data_cache();
dcache_disable();
#endif

switch (hdr->ih_comp) { //根據鏡像的壓縮類型把內核鏡像解壓到指定的地址，一般是mkimage時-a指定的
那個地址，-a指定的那個地址也就是存儲在鏡像頭裡面的hdr->ih_load變量中，
什麼時候指定的呢，其實就是在執行mkimage指令的時候，-a輸入的時候，賦給的數值
case IH_COMP_NONE:
if(ntohl(hdr->ih_load) == addr) { //如果image header中指示的加載地址和bootm命令中參數2指定的地址相同，則表示不需要copy，可以就地執行。
printf ("   XIP %s ... ", name);
} else {
#if defined(CONFIG_HW_WATCHDOG) || defined(CONFIG_WATCHDOG)
size_t l = len;
void *to = (void *)ntohl(hdr->ih_load);
void *from = (void *)data;

printf ("   Loading %s ... ", name);

while (l > 0) {
size_t tail = (l > CHUNKSZ) ? CHUNKSZ : l;
WATCHDOG_RESET();
memmove (to, from, tail);
to += tail;
from += tail;
l -= tail;
}
#else /* !(CONFIG_HW_WATCHDOG || CONFIG_WATCHDOG) */如果image header中指示的加載地址和bootm命令中參數2指定的地址不相同，則表示要從image header中指示的加載地址處把image data copy到bootm命令中參數2指定的地址處，然後再執行。