Andriod生成bootImage

Boot.img

在Android eMMC 分区详解 以Boot.img为例，详述了Android eMMC分区。

为了理解Linux Kernel 的Boot过程，了解Boot.img的生成，以及Bootloader（LK）如何解析Boot.img是非常必要的。

In file system/core/mkbootimg/bootimg.h

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 */
};

/*
** +-----------------+
** | 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
*/

在文件system/core/mkbootimg/Android.mk

LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)

LOCAL_SRC_FILES := mkbootimg.c
LOCAL_STATIC_LIBRARIES := libmincrypt

LOCAL_MODULE := mkbootimg

include $(BUILD_HOST_EXECUTABLE)

$(call dist-for-goals,dist_files,$(LOCAL_BUILT_MODULE))

可以看到这个模块要生成一个mkbootimg的可执行文件。

如何使用mkbootimg 

system/core/mkbootimg/mkbootimg.c

int usage(void)
{
    fprintf(stderr,"usage: mkbootimg\n"
            "       --kernel <filename>\n"
            "       --ramdisk <filename>\n"
            "       [ --second <2ndbootloader-filename> ]\n"
            "       [ --cmdline <kernel-commandline> ]\n"
            "       [ --board <boardname> ]\n"
            "       [ --base <address> ]\n"
            "       [ --pagesize <pagesize> ]\n"
            "       [ --dt <filename> ]\n"
            "       -o|--output <filename>\n"
            );
    return 1;
}

mkbootimg --kernel <filename>  --ramdisk <filename> [ --dt <filename> ]  -o boot.img

经过最后一步就生成了boot.img.