全志 a64 linux sdk采用了android 的boot image，这里顺着该sdk系统理解android的引导镜像。
如下linux-3.10/scripts/build.sh代码片段显示，在完成内核编译后，改脚本直接将kernel的二进制可执行文件和cpio的gzip压缩包rootfs.cpio.gz 一起合并进入boot.img。
#linux-3.10/scripts/build.sh line.27
export MKBOOTIMG=${LICHEE_TOOLS_DIR}/pack/pctools/linux/android/mkbootimg
local BIMAGE="output/bImage";
local RAMDISK="output/rootfs.cpio.gz";
#linux-3.10/scripts/build.sh line.287
${MKBOOTIMG} --kernel ${BIMAGE} \
--ramdisk ${RAMDISK} \
--board ${CHIP} \
--base ${BASE} \
--kernel_offset ${KERNEL_OFFSET} \
--ramdisk_offset ${RAMDISK_OFFSET} \
-o output/boot.img对于如上 mkbootimg 通过review github mkbootimg 代码片段确定该应用功能。
//mkbootimg.c line466
if(write(fd, &hdr, sizeof(hdr)) != sizeof(hdr)) goto fail;
if(write_padding(fd, pagesize, sizeof(hdr))) goto fail;
if(write(fd, kernel_data, hdr.kernel_size) != (ssize_t) hdr.kernel_size) goto fail;
if(write_padding(fd, pagesize, hdr.kernel_size)) goto fail;
if(write(fd, ramdisk_data, hdr.ramdisk_size) != (ssize_t) hdr.ramdisk_size) goto fail;
if(write_padding(fd, pagesize, hdr.ramdisk_size)) goto fail;整合传递参数后，mkbootimg 依次将header（metadata）、kernel、ramdisk、等信息到一个文件。
//mkboot.h line.166
/* When the boot image header has a version of BOOT_HEADER_VERSION_ONE, the structure of the boot
* image is as follows:
*
* +-----------------+
* | boot header | 1 page
* +-----------------+
* | kernel | n pages
* +-----------------+
* | ramdisk | m pages
* +-----------------+
* | second stage | o pages
* +-----------------+
* | recovery dtbo | 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 = (recovery_dtbo_size + page_size - 1) / page_size
*
* 0. all entities are page_size aligned in flash
* 1. kernel and ramdisk are required (size != 0)
* 2. recovery_dtbo is required for recovery.img in non-A/B devices(recovery_dtbo_size != 0)
* 3. second is optional (second_size == 0 -> no second)
* 4. load each element (kernel, ramdisk, second) at
* the specified physical address (kernel_addr, etc)
* 5. If booting to recovery mode in a non-A/B device, extract recovery dtbo and
* apply the correct set of overlays on the base device tree depending on the
* hardware/product revision.
* 6. prepare tags at tag_addr. kernel_args[] is
* appended to the kernel commandline in the tags.
* 7. r0 = 0, r1 = MACHINE_TYPE, r2 = tags_addr
* 8. if second_size != 0: jump to second_addr
* else: jump to kernel_addr
*/这里需要深入理解metadataboot_img_hdr_v1,它描述这个引导镜像的kernel、ramfs 大小基地址。
//bootimg.h line 121
struct boot_img_hdr_v1 {
uint8_t magic[BOOT_MAGIC_SIZE];
uint32_t kernel_size; /* size in bytes */
uint32_t kernel_addr; /* physical load addr */
uint32_t ramdisk_size; /* size in bytes */
uint32_t ramdisk_addr; /* physical load addr */
uint32_t second_size; /* size in bytes */
uint32_t second_addr; /* physical load addr */
uint32_t tags_addr; /* physical addr for kernel tags */
uint32_t page_size; /* flash page size we assume */
/*
* version for the boot image header.
* alternately this is used as dt_size on some hardware.
*/
union {
uint32_t header_version;
uint32_t dt_size; /* device tree in bytes */
};
/* operating system version and security patch level; for
* version "A.B.C" and patch level "Y-M-D":
* ver = A << 14 | B << 7 | C (7 bits for each of A, B, C)
* lvl = ((Y - 2000) & 127) << 4 | M (7 bits for Y, 4 bits for M)
* os_version = ver << 11 | lvl */
uint32_t os_version;
uint8_t name[BOOT_NAME_SIZE]; /* asciiz product name */
uint8_t cmdline[BOOT_ARGS_SIZE];
uint32_t id[8]; /* timestamp / checksum / sha1 / etc */
/* Supplemental command line data; kept here to maintain
* binary compatibility with older versions of mkbootimg */
uint8_t extra_cmdline[BOOT_EXTRA_ARGS_SIZE];
uint32_t recovery_dtbo_size; /* size in bytes for recovery DTBO image */
uint64_t recovery_dtbo_offset; /* offset to recovery dtbo in boot image */
uint32_t header_size;
} __attribute__((packed));直接dump boot.img 验证如上首地址的metadata。
jay@mxj-build:~/a64_v4l2$ hexdump -n 100 -C /home/jay/a64_v4l2/out/sun50iw1p1/debian/common/boot.img
00000000 41 4e 44 52 4f 49 44 21 50 4b e8 00 00 00 08 41 |ANDROID!PK.....A|
00000010 69 d0 29 00 00 00 00 42 00 00 00 00 00 00 f0 41 |i.)....B.......A|
00000020 00 01 00 41 00 08 00 00 00 00 00 00 00 00 00 00 |...A............|
00000030 73 75 6e 35 30 69 00 00 00 00 00 00 00 00 00 00 |sun50i..........|
00000040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000064