6.3.4U-Boot与内核的关系

U-Boot作为Bootloader，具备多种引导内核启动的方式。常用的go和bootm命令可以直接引导内核映像启动。U-Boot与内核的关系主要是内核启动过程中参数的传递。

1．go命令的实现

/* common/cmd_boot.c  */

int do_go (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])

{

ulong addr, rc;

int     rcode = 0;

if (argc < 2) {

printf ("Usage:\n%s\n", cmdtp->usage);

return 1;

}

addr = simple_strtoul(argv[1], NULL, 16);

printf ("## Starting application at 0x%08lX ...\n", addr);

/*

* pass address parameter as argv[0] (aka command name),

* and all remaining args

*/

rc = ((ulong (*)(int, char *[]))addr) (--argc, &argv[1]);

if (rc != 0) rcode = 1;

printf ("## Application terminated, rc = 0x%lX\n", rc);

return rcode;

}

go命令调用do_go()函数，跳转到某个地址执行的。如果在这个地址准备好了自引导的内核映像，就可以启动了。尽管go命令可以带变参，实际使用时一般不用来传递参数。

2．bootm命令的实现

/* common/cmd_bootm.c */

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 = 0x400000;

int   i, verify;

char  *name, *s;

int   (*appl)(int, char *[]);

image_header_t *hdr = &header;

s = getenv ("verify");

verify = (s && (*s == 'n')) ? 0 : 1;

if (argc < 2) {

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 */

memmove (&header, (char *)addr, sizeof(image_header_t));

if (ntohl(hdr->ih_magic) != IH_MAGIC)

{

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);

hdr->ih_hcrc = 0;

if(crc32 (0, (char *)data, len) != checksum) {

puts ("Bad Header Checksum\n");

SHOW_BOOT_PROGRESS (-2);

return 1;

}

SHOW_BOOT_PROGRESS (3);

/* 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) {

puts ("   Verifying Checksum ... ");

if(crc32 (0, (char *)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;

……

switch (hdr->ih_os) {

default:                /* handled by (original) Linux case */

case IH_OS_LINUX:

do_bootm_linux  (cmdtp, flag, argc, argv,

addr, len_ptr, verify);

break;

……

}

bootm命令调用do_bootm函数。这个函数专门用来引导各种操作系统映像，可以支持引导Linux、vxWorks、QNX等操作系统。引导Linux的时候，调用do_bootm_linux()函数。

3．do_bootm_linux函数的实现

/* lib_arm/armlinux.c */

void do_bootm_linux (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[],

ulong addr, ulong *len_ptr, int verify)

{

DECLARE_GLOBAL_DATA_PTR;

ulong len = 0, checksum;

ulong initrd_start, initrd_end;

ulong data;

void (*theKernel)(int zero, int arch, uint params);

image_header_t *hdr = &header;

bd_t *bd = gd->bd;

#ifdef CONFIG_CMDLINE_TAG

char *commandline = getenv ("bootargs");

#endif

theKernel = (void (*)(int, int, uint))ntohl(hdr->ih_ep);

/* Check if there is an initrd image */

if(argc >= 3) {

SHOW_BOOT_PROGRESS (9);

addr = simple_strtoul (argv[2], NULL, 16);

printf ("## Loading Ramdisk Image at %08lx ...\n", addr);

/* Copy header so we can blank CRC field for re-calculation */

memcpy (&header, (char *) addr, sizeof (image_header_t));

if (ntohl (hdr->ih_magic) != IH_MAGIC) {

printf ("Bad Magic Number\n");

SHOW_BOOT_PROGRESS (-10);

do_reset (cmdtp, flag, argc, argv);

}

data = (ulong) & header;

len = sizeof (image_header_t);

checksum = ntohl (hdr->ih_hcrc);

hdr->ih_hcrc = 0;

if(crc32 (0, (char *) data, len) != checksum) {

printf ("Bad Header Checksum\n");

SHOW_BOOT_PROGRESS (-11);

do_reset (cmdtp, flag, argc, argv);

}

SHOW_BOOT_PROGRESS (10);

print_image_hdr (hdr);

data = addr + sizeof (image_header_t);

len = ntohl (hdr->ih_size);

if(verify) {

ulong csum = 0;

printf ("   Verifying Checksum ... ");

csum = crc32 (0, (char *) data, len);

if (csum != ntohl (hdr->ih_dcrc)) {

printf ("Bad Data CRC\n");

SHOW_BOOT_PROGRESS (-12);

do_reset (cmdtp, flag, argc, argv);

}

printf ("OK\n");

}

SHOW_BOOT_PROGRESS (11);

if ((hdr->ih_os != IH_OS_LINUX) ||

(hdr->ih_arch != IH_CPU_ARM) ||

(hdr->ih_type != IH_TYPE_RAMDISK)) {

printf ("No Linux ARM Ramdisk Image\n");

SHOW_BOOT_PROGRESS (-13);

do_reset (cmdtp, flag, argc, argv);

}

/* Now check if we have a multifile image */

} else if ((hdr->ih_type == IH_TYPE_MULTI) && (len_ptr[1])) {

ulong tail = ntohl (len_ptr[0]) % 4;

int i;

SHOW_BOOT_PROGRESS (13);

/* skip kernel length and terminator */

data = (ulong) (&len_ptr[2]);

/* skip any additional image length fields */

for (i = 1; len_ptr[i]; ++i)

data += 4;

/* add kernel length, and align */

data += ntohl (len_ptr[0]);

if (tail) {

data += 4 - tail;

}

len = ntohl (len_ptr[1]);

} else {

/* no initrd image */

SHOW_BOOT_PROGRESS (14);

len = data = 0;

}

if (data) {

initrd_start = data;

initrd_end = initrd_start + len;

} else {

initrd_start = 0;

initrd_end = 0;

}

SHOW_BOOT_PROGRESS (15);

debug ("## Transferring control to Linux (at address %08lx) ...\n",

(ulong) theKernel);

#if defined (CONFIG_SETUP_MEMORY_TAGS) || \

defined (CONFIG_CMDLINE_TAG) || \

defined (CONFIG_INITRD_TAG) || \

defined (CONFIG_SERIAL_TAG) || \

defined (CONFIG_REVISION_TAG) || \

defined (CONFIG_LCD) || \

defined (CONFIG_VFD)

setup_start_tag (bd);

#ifdef CONFIG_SERIAL_TAG

setup_serial_tag (&params);

#endif

#ifdef CONFIG_REVISION_TAG

setup_revision_tag (&params);

#endif

#ifdef CONFIG_SETUP_MEMORY_TAGS

setup_memory_tags (bd);

#endif

#ifdef CONFIG_CMDLINE_TAG

setup_commandline_tag (bd, commandline);

#endif

#ifdef CONFIG_INITRD_TAG

if (initrd_start && initrd_end)

setup_initrd_tag (bd, initrd_start, initrd_end);

#endif

setup_end_tag (bd);

#endif

/* we assume that the kernel is in place */

printf ("\nStarting kernel ...\n\n");

cleanup_before_linux ();

theKernel (0, bd->bi_arch_number, bd->bi_boot_params);

}

do_bootm_linux()函数是专门引导Linux映像的函数，它还可以处理ramdisk文件系统的映像。这里引导的内核映像和ramdisk映像，必须是U-Boot格式的。U-Boot格式的映像可以通过mkimage工具来转换，其中包含了U-Boot可以识别的符号。