mkbootimg是Android project的一部分，用來封裝boot image的，其使用參數如下：
usage: mkbootimg
--kernel <filename>
--ramdisk <filename>
[ --second <2ndbootloader-filename> ]
[ --cmdline <kernel-commandline> ]
[ --board <boardname> ]
[ --base <address> ]
[ --pagesize <pagesize> ]
[ --ramdisk_offset <ramdisk_offset> ]
[ --dt <filename> ]
[ --tags-addr <address> ]
-o|--output <filename>
基本上就是將kenrnel、ramdisk、device tree等封裝成一個檔案，讓boot loader能將其載入RAM中，並正確執行。檔案的layout如下(就如bootimg.h中註解所提到的，https://www.codeaurora.org/cgit/quic/femto/platform/system/core/tree/mkbootimg/bootimg.h?h=LNX.LE.5.0.1-57023-9x40)
/*
** +-----------------+
** | boot header | 1 page
** +-----------------+
** | kernel | n pages
** +-----------------+
** | ramdisk | m pages
** +-----------------+
** | second stage | o pages
** +-----------------+
** | device tree | p pages
** +-----------------+
**
** n = (kernel_size + page_size - 1) / page_size
** m = (ramdisk_size + page_size - 1) / page_size
** o = (second_size + page_size - 1) / page_size
** p = (dt_size + page_size - 1) / page_size
**
** 0. all entities are page_size aligned in flash
** 1. kernel and ramdisk are required (size != 0)
** 2. second is optional (second_size == 0 -> no second)
** 3. load each element (kernel, ramdisk, second) at
** the specified physical address (kernel_addr, etc)
** 4. prepare tags at tag_addr. kernel_args[] is
** appended to the kernel commandline in the tags.
** 5. r0 = 0, r1 = MACHINE_TYPE, r2 = tags_addr
** 6. if second_size != 0: jump to second_addr
** else: jump to kernel_addr
*/
其中boot header會有一些information，讓LK能正確的將各個section load到正確的Address上，boot header定義如下
typedef struct boot_img_hdr boot_img_hdr;
#define BOOT_MAGIC "ANDROID!"
#define BOOT_MAGIC_SIZE 8
#define BOOT_NAME_SIZE 16
#define BOOT_ARGS_SIZE 512
struct boot_img_hdr
{
unsigned char magic[BOOT_MAGIC_SIZE];
unsigned kernel_size; /* size in bytes */
unsigned kernel_addr; /* physical load addr */
unsigned ramdisk_size; /* size in bytes */
unsigned ramdisk_addr; /* physical load addr */
unsigned second_size; /* size in bytes */
unsigned second_addr; /* physical load addr */
unsigned tags_addr; /* physical addr for kernel tags */
unsigned page_size; /* flash page size we assume */
unsigned dt_size; /* device tree in bytes */
unsigned unused; /* future expansion: should be 0 */
unsigned char name[BOOT_NAME_SIZE]; /* asciiz product name */
unsigned char cmdline[BOOT_ARGS_SIZE];
unsigned id[8]; /* timestamp / checksum / sha1 / etc */
};
第一個欄位magic用以識別這個image是否為有效，後續會帶著kernel、ramdisk、device tree跟second的size大小，以及要load的address。
以下是我的platform的FLASH與其RAM的layout，如下圖
基本上，main function就是將這些檔案pack成一個檔案，並做page alignment，有興趣再去看code吧。
下面這段是我寫的umkbootimg，讀取boot header資訊後印出。
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include "bootimg.h"
static void *load_file(const char *fn, unsigned sz)
{
char *data;
int fd;
data = 0;
fd = open(fn, O_RDONLY);
if(fd < 0) return 0;
data = (char*) malloc(sz);
if(data == 0) goto oops;
if(read(fd, data, sz) != sz) goto oops;
close(fd);
return data;
oops:
close(fd);
if(data != 0) free(data);
return 0;
}
int usage(void)
{
fprintf(stderr,"usage: umkbootimg <filename>\n");
return 1;
}
int main(int argc, char **argv)
{
boot_img_hdr *hdr;
char *img_name = 0;
void *img_data = 0;
unsigned sz;
if (argc < 2) {
return usage();
}
hdr = (boot_img_hdr *) load_file(argv[1], sizeof(boot_img_hdr));
hdr->magic[BOOT_MAGIC_SIZE] = 0;
printf("magic:%s\n", hdr->magic);
printf("kernel_size:%u/0x%08x\n", hdr->kernel_size, hdr->kernel_size);
printf("kernel_addr:%u/0x%08x\n", hdr->kernel_addr, hdr->kernel_addr);
printf("ramdisk_size:%u/0x%08x\n", hdr->ramdisk_size, hdr->ramdisk_size);
printf("ramdisk_addr:%u/0x%08x\n", hdr->ramdisk_addr, hdr->ramdisk_addr);
printf("second_size:%u/0x%08x\n", hdr->second_size, hdr->second_size);
printf("second_addr:%u/0x%08x\n", hdr->second_addr, hdr->second_addr);
printf("tags_addr:%u/0x%08x\n", hdr->tags_addr, hdr->tags_addr);
printf("page_size:%u/0x%08x\n", hdr->page_size, hdr->page_size);
printf("dt_size:%u/0x%08x\n", hdr->dt_size, hdr->dt_size);
printf("name:%s\n", hdr->name);
printf("cmdline:%s\n", hdr->cmdline);
return 0;
}
參考資料：
- https://www.codeaurora.org/cgit/quic/femto/platform/system/core/tree/mkbootimg/bootimg.h?h=LNX.LE.5.0.1-57023-9x40
