mkbootimg -- pack boot images utils

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