[IMX6Q]u-boot启动kernel流程

u-boot版本：v2009.08

u-boot利用了env里的cmd来实现调用boot linux的接口, 效果等同于在u-boot中敲"booti xxx..."

start_armboot -> board_late_init:

int board_late_init(void)
{
int ret = 0;
#ifdef MX6Q_SABRESD_ANDROID_H
switch (get_boot_device()) {
case SD_BOOT:
if (!getenv("fastboot_dev"))
setenv("fastboot_dev", "mmc2");
/*本例的boot device是SD,并且device number是2*/
if (!getenv("bootcmd"))
setenv("bootcmd", "booti mmc2");
break;
......
}
#endif
......
return 0;
}

start_armboot ->main_loop:

void main_loop (void)
{
......
/*获取前面set的bootcmd,值是"booti mmc2"*/
s = getenv ("bootcmd");

debug ("### main_loop: bootcmd=\"%s\"\n", s ? s : "<UNDEFINED>");
/*如果u-boot开机阶段没有收到按键事件，那么就去启动kernel*/
if (bootdelay >= 0 && s && !abortboot (bootdelay)) {
......
# ifndef CONFIG_SYS_HUSH_PARSER
/*调用booti mmc2去boot linux*/
run_command (s, 0);
# else
......
}

booti命令对应的定义如下：

U_BOOT_CMD(
booti,	3,	1,	do_booti,
"booti   - boot android bootimg from memory\n",
"[<addr> | mmc0 | mmc1 | mmc2 | mmcX] [<partition>] \n    - boot application image stored in memory or mmc\n"
"\t'addr' should be the address of boot image which is zImage+ramdisk.img\n"
"\t'mmcX' is the mmc device you store your boot.img, which will read the boot.img from 1M offset('/boot' partition)\n"
"\t 'partition' (optional) is the partition id of your device, if no partition give, will going to 'boot' partition\n"
);

do_booti:

int do_booti(cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
{
......
if (mmcc != -1) {
#ifdef CONFIG_MMC
......
/*获取device信息*/
mmc = find_mmc_device(mmcc);
if (!mmc) {
printf("booti: cannot find '%d' mmc device\n", mmcc);
goto fail;
}
......
#ifdef CONFIG_ANDROID_BOOT_PARTITION_MMC
#ifdef CONFIG_ANDROID_RECOVERY_PARTITION_MMC
/*开机可能是进入正常boot模式，也可以是进入recovery模式，取决于传进来的参数,本例是kernel*/
if (!strcmp(ptn, "boot"))
partno = CONFIG_ANDROID_BOOT_PARTITION_MMC;
if (!strcmp(ptn, "recovery"))
partno = CONFIG_ANDROID_RECOVERY_PARTITION_MMC;
/*获取kernel对应的partition info*/
if (get_partition_info(dev_desc, partno, &info)) {
printf("booti: device don't have such partition:%s\n", ptn);
goto fail;
}
#endif
#endif

#ifdef CONFIG_FASTBOOT
......
#else
/*获取kernel的hdr，此信息是在编译boot.img的时候就存放在它起始的某个固定位置。*/
if (mmc->block_dev.block_read(mmcc, info.start,
1, (void *)hdr) < 0) {
printf("booti: mmc failed to read bootimg header\n");
goto fail;
}
......
sector = info.start + (hdr->page_size / 512);
#endif
/*读取kernel到RAM中的kernel_addr*/
if (mmc->block_dev.block_read(mmcc, sector,
(hdr->kernel_size / 512) + 1,
(void *)hdr->kernel_addr) < 0) {
printf("booti: mmc failed to read kernel\n");
goto fail;
}
/* flush cache after read */
flush_cache((ulong)hdr->kernel_addr, hdr->kernel_size); /* FIXME */
sector += ALIGN_SECTOR(hdr->kernel_size, hdr->page_size) / 512;
/*读取ramdisk到kernel的ramdisk_addr*/
if (mmc->block_dev.block_read(mmcc, sector,
(hdr->ramdisk_size / 512) + 1,
(void *)hdr->ramdisk_addr) < 0) {
printf("booti: mmc failed to read kernel\n");
goto fail;
}
/* flush cache after read */
flush_cache((ulong)hdr->ramdisk_addr, hdr->ramdisk_size); /* FIXME */
#else
return -1;
#endif
} else {
......
}
......
/*根据hdr信息来boot linux*/
do_booti_linux(hdr);
......
}

void do_booti_linux (boot_img_hdr *hdr)
{
ulong initrd_start, initrd_end;
#ifdef CONFIG_SERIAL_TAG
char appended_cmd_line[512];
#endif
void (*theKernel)(int zero, int arch, uint params);
bd_t *bd = gd->bd;
#ifdef CONFIG_CMDLINE_TAG
char *commandline = getenv("bootargs");

/* If no bootargs env, just use hdr command line */
/*本例bootargs没有定义*/
if (!commandline) {
/*获取cmdline,此值在device/fsl/imx6/BoardConfigCommon.mk中定义。*/
commandline = (char *)hdr->cmdline;
......
}
#endif
/*kernel address就是boot kernel的函数指针地址了*/
theKernel = (void (*)(int, int, uint))(hdr->kernel_addr);
/*获取ramdisk start 和 end地址*/
initrd_start = hdr->ramdisk_addr;
initrd_end = initrd_start + hdr->ramdisk_size;

/*将memory,serial,revision,commandline等都放在各自
的struct tag中，kernel启动之后会解析它们。*/
#if defined (CONFIG_SETUP_MEMORY_TAGS)
setup_start_tag(bd);
#ifdef CONFIG_SERIAL_TAG
setup_serial_tag (¶ms);
#endif
#ifdef CONFIG_REVISION_TAG
setup_revision_tag (¶ms);
#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 (hdr->ramdisk_size)
setup_initrd_tag (bd, initrd_start, initrd_end);
#endif
......
setup_end_tag (bd);
#endif
......
/*启动kernel, never return back!*/
theKernel (0, bd->bi_arch_number, bd->bi_boot_params);
}

注意theKernel()的参数，第二个是machine type,kernel会去匹配，如果没有找到对应的type id，那么就无法正常启动kernel.

int board_init(void)
{
......
/* board id for linux */
gd->bd->bi_arch_number = MACH_TYPE_MX6Q_SABRESD;
......
}

第三个参数就是前面的那些tags.

到此,kernel就启动起来了。