bootm命令执行过程中调用了bootm_start函数，这个函数比较重要，所以先分析它。
1.common/cmd_bootm.c
static int bootm_start(cmd_tbl_t *cmdtp, int flag, int argc, char *argv[]) { void *os_hdr; int ret; memset ((void *)&images, 0, sizeof (images));//images是一个bootm_headers_t类型的全局变量。见下面的分析。 images.verify = getenv_yesno ("verify");//从环境变量中检查是否要对镜像的数据（不是镜像头）进行校验。 bootm_start_lmb();//不做任何有意义的工作，除了定义# define lmb_reserve(lmb, base, size) /* get kernel image header, start address and length */寻找可用的内核镜像，见下面的分析。主要根据传入的参数检查镜像的合法性并获取信息。 os_hdr = boot_get_kernel (cmdtp, flag, argc, argv, &images, &images.os.image_start, &images.os.image_len);//返回指向内存中镜像头的指针 if (images.os.image_len == 0) { puts ("ERROR: can't get kernel image!\n"); return 1; } /* get image parameters */ switch (genimg_get_format (os_hdr)) {//根据镜像魔数获取镜像类型 case IMAGE_FORMAT_LEGACY: images.os.type = image_get_type (os_hdr);//镜像类型 images.os.comp = image_get_comp (os_hdr);//压缩类型 images.os.os = image_get_os (os_hdr);//操作系统类型 images.os.end = image_get_image_end (os_hdr);//当前镜像的尾地址 images.os.load = image_get_load (os_hdr);//镜像数据的载入地址 break; default: puts ("ERROR: unknown image format type!\n"); return 1; } /* find kernel entry point */ if (images.legacy_hdr_valid) {//如果镜像已经通过验证 images.ep = image_get_ep (&images.legacy_hdr_os_copy);//获取入口地址，填充images.ep 。 } else { puts ("Could not find kernel entry point!\n"); return 1; } if (((images.os.type == IH_TYPE_KERNEL) || (images.os.type == IH_TYPE_MULTI)) && (images.os.os == IH_OS_LINUX)) { /* find ramdisk */3250的配置中这个函数不做任何工作 ret = boot_get_ramdisk (argc, argv, &images, IH_INITRD_ARCH, &images.rd_start, &images.rd_end); if (ret) { puts ("Ramdisk image is corrupt or invalid\n"); return 1; } } images.os.start = (ulong)os_hdr;//指向内存中镜像的头地址 images.state = BOOTM_STATE_START;//标记引导状态 return 0; }
总结一下这个函数的主要工作：第一步校验镜像的正确性，获取镜像的信息（根据镜像头），第二部将第一步获取的信息存入images（主要是填充image_info_t类型的os成员）
2. bootm_headers_t
typedef struct bootm_headers { /* * Legacy os image header, if it is a multi component image * then boot_get_ramdisk() and get_fdt() will attempt to get * data from second and third component accordingly. */ image_header_t *legacy_hdr_os; /* image header pointer */ image_header_t legacy_hdr_os_copy; /* header copy */ ulong legacy_hdr_valid; #ifndef USE_HOSTCC image_info_t os; /* os image info */ ulong ep; /* entry point of OS */ ulong rd_start, rd_end;/* ramdisk start/end */ ulong ft_len; /* length of flat device tree */ ulong initrd_start; ulong initrd_end; ulong cmdline_start; ulong cmdline_end; bd_t *kbd; #endif int verify; /* getenv("verify")[0] != 'n' */ #define BOOTM_STATE_START (0x00000001) #define BOOTM_STATE_LOADOS (0x00000002) #define BOOTM_STATE_RAMDISK (0x00000004) #define BOOTM_STATE_FDT (0x00000008) #define BOOTM_STATE_OS_CMDLINE (0x00000010) #define BOOTM_STATE_OS_BD_T (0x00000020) #define BOOTM_STATE_OS_PREP (0x00000040) #define BOOTM_STATE_OS_GO (0x00000080) int state; } bootm_headers_t; /* * Legacy format image header, * all data in network byte order (aka natural aka bigendian). */内核镜像头 include/image.h typedef struct image_header { uint32_t ih_magic; /* Image Header Magic Number */镜像头部幻数，为#define IH_MAGIC 0x27051956 uint32_t ih_hcrc; /* Image Header CRC Checksum */镜像头部crc校验码 uint32_t ih_time; /* Image Creation Timestamp */镜像创建时间戳 uint32_t ih_size; /* Image Data Size */镜像数据大小（不算头部）1828536 uint32_t ih_load; /* Data Load Address */数据将要载入的内存地址 80008000 uint32_t ih_ep; /* Entry Point Address */镜像入口地址 80008000 uint32_t ih_dcrc; /* Image Data CRC Checksum */镜像数据校验码 uint8_t ih_os; /* Operating System */操作系统类型 #define IH_OS_LINUX 5 uint8_t ih_arch; /* CPU architecture */CPU架构类型 #define IH_ARCH_ARM 2 uint8_t ih_type; /* Image Type */镜像类型 IH_TYPE_KERNEL uint8_t ih_comp; /* Compression Type */压缩类型 IH_COMP_NONE uint8_t ih_name[IH_NMLEN]; /* Image Name */镜像名字Linux-2.6.27.8，#define IH_NMLEN 32 } image_header_t; 镜像信息 include/image.h typedef struct image_info { ulong start, end; /* start/end of blob */镜像的起始地址和尾地址 ulong image_start, image_len; /* start of image within blob, len of image */镜像数据的开始地址和长度 ulong load; /* load addr for the image */镜像数据的装载地址 uint8_t comp, type, os; /* compression, type of image, os type */压缩类型，镜像类型和操作系统类型 } image_info_t;
3。boot_get_kernel
/** * boot_get_kernel - find kernel image * @os_data: pointer to a ulong variable, will hold os data start address * @os_len: pointer to a ulong variable, will hold os data length * * boot_get_kernel() tries to find a kernel image, verifies its integrity * and locates kernel data. * * returns: * pointer to image header if valid image was found, plus kernel start * address and length, otherwise NULL */ 寻找可用的内核镜像 static void *boot_get_kernel (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[], bootm_headers_t *images, ulong *os_data, ulong *os_len) { image_header_t *hdr; ulong img_addr; /* find out kernel image address */ if (argc < 2) {//如果参数太少 img_addr = load_addr; //使用默认的镜像载入地址，这个地址是在配置头文件中定义的 ulong load_addr = CONFIG_SYS_LOAD_ADDR; debug ("* kernel: default image load address = 0x%08lx\n", load_addr); } else { img_addr = simple_strtoul(argv[1], NULL, 16);//参数足够的话，把第二个参数转化为16进制作为地址 debug ("* kernel: cmdline image address = 0x%08lx\n", img_addr); } show_boot_progress (1); /* copy from dataflash if needed */ img_addr = genimg_get_image (img_addr);//对于3250，这个函数什么都没做。因为没有dataflash. /* check image type, for FIT images get FIT kernel node */ *os_data = *os_len = 0; switch (genimg_get_format ((void *)img_addr)) {//根据上面得到的镜像的地址，获取镜像的类型。这个函数根据镜像头部的魔数返回类型。 case IMAGE_FORMAT_LEGACY: printf ("## Booting kernel from Legacy Image at %08lx ...\n", //打印引导消息 img_addr); hdr = image_get_kernel (img_addr, images->verify);//检验镜像的合法性（校验码、魔数、架构类型等），并打印信息。见下面的分析。 if (!hdr) return NULL; show_boot_progress (5); /* get os_data and os_len */ switch (image_get_type (hdr)) { case IH_TYPE_KERNEL: *os_data = image_get_data (hdr);//镜像数据的地址，也就是紧挨着镜像头的地址。 *os_len = image_get_data_size (hdr);//镜像数据部分大小 break; case IH_TYPE_MULTI: image_multi_getimg (hdr, 0, os_data, os_len); break; case IH_TYPE_STANDALONE: *os_data = image_get_data (hdr); *os_len = image_get_data_size (hdr); break; default: printf ("Wrong Image Type for %s command\n", cmdtp->name); show_boot_progress (-5); return NULL; } /* * copy image header to allow for image overwrites during kernel * decompression. */拷贝一份镜像的头部到images中。 memmove (&images->legacy_hdr_os_copy, hdr, sizeof(image_header_t)); /* save pointer to image header */ images->legacy_hdr_os = hdr;//images中指针指向镜像的头部 images->legacy_hdr_valid = 1;//镜像已经检验合格，置标志量 show_boot_progress (6); break; default: printf ("Wrong Image Format for %s command\n", cmdtp->name); show_boot_progress (-108); return NULL; } debug (" kernel data at 0x%08lx, len = 0x%08lx (%ld)\n", *os_data, *os_len, *os_len); return (void *)img_addr; }
4。image_get_kernel
common/cmd_bootm.c static image_header_t *image_get_kernel (ulong img_addr, int verify) { image_header_t *hdr = (image_header_t *)img_addr; if (!image_check_magic(hdr)) { //检查镜像头部的魔数是否等于 IH_MAGIC puts ("Bad Magic Number\n"); show_boot_progress (-1); return NULL; } show_boot_progress (2); if (!image_check_hcrc (hdr)) {//检查镜像头部的校验码（hcrc为0时镜像头的校验码） puts ("Bad Header Checksum\n"); show_boot_progress (-2); return NULL; } show_boot_progress (3); image_print_contents (hdr);//根据传入的镜像头地址，打印镜像的信息，见下面的分析。 /* Image Name: Linux-2.6.27.8 Image Type: ARM Linux Kernel Image (uncompressed) Data Size: 1828536 Bytes = 1.7 MiB Load Address: 80008000 Entry Point: 80008000 */ if (verify) {//是否要对镜像的数据部分进行校验 puts (" Verifying Checksum ... "); if (!image_check_dcrc (hdr)) { printf ("Bad Data CRC\n"); show_boot_progress (-3); return NULL; } puts ("OK\n"); } show_boot_progress (4); if (!image_check_target_arch (hdr)) {//检查是否是IH_ARCH_ARM架构 printf ("Unsupported Architecture 0x%x\n", image_get_arch (hdr)); show_boot_progress (-4); return NULL; } return hdr; }