转载请注明来源：cuixiaolei的技术博客
这篇文章是lk启动流程分析（以高通为例），将会详细介绍下面的内容：
1).正常开机引导流程
2).recovery引导流程
3).fastboot引导流程
4).ffbm引导流程
5).lk向kernel传参
start----------------------------------------
在bootable/bootloader/lk/arch/arm/crt0.S文件中有下面代码，所以从kmain()开始介绍
bl kmain
kmain函数位于bootable/bootloader/lk/kernel/main.c
/* called from crt0.S */
void kmain(void) __NO_RETURN __EXTERNALLY_VISIBLE;
void kmain(void)
{
// get us into some sort of thread context
thread_init_early(); //初始化线程上下文
#ifdef FEATURE_AFTER_SALE_LOG_LK
// do console early init
console_init_early(); //初始化控制台
#endif
// early arch stuff
arch_early_init(); //架构初始化，如关闭cache，使能mmu
// do any super early platform initialization
platform_early_init(); //平台早期初始化
// do any super early target initialization
target_early_init(); //目标设备早期初始化,初始化串口
dprintf(INFO, "welcome to lk\n\n");
bs_set_timestamp(BS_BL_START);
// deal with any static constructors
dprintf(SPEW, "calling constructors\n");
call_constructors();
// bring up the kernel heap
dprintf(SPEW, "initializing heap\n");
heap_init(); //堆初始化
__stack_chk_guard_setup();
// initialize the threading system
dprintf(SPEW, "initializing threads\n");
thread_init(); //线程初始化
#ifdef FEATURE_AFTER_SALE_LOG_LK
// initialize the console layer
dprintf(SPEW, "initializing console layer\n");
console_init(); //初始化控制台
#endif
// initialize the dpc system
dprintf(SPEW, "initializing dpc\n");
dpc_init(); //lk系统控制器初始化
// initialize kernel timers
dprintf(SPEW, "initializing timers\n");
timer_init(); //kernel时钟初始化
#if (!ENABLE_NANDWRITE)
// create a thread to complete system initialization
dprintf(SPEW, "creating bootstrap completion thread\n");
thread_resume(thread_create("bootstrap2", &bootstrap2, NULL, DEFAULT_PRIORITY, DEFAULT_STACK_SIZE)); //创建一个线程初始化系统
// enable interrupts
exit_critical_section(); //使能中断
// become the idle thread
thread_become_idle(); //本线程切换成idle线程，idle为空闲线程，当没有更高优先级的线程时才执行
#else
bootstrap_nandwrite();
#endif
}
arch_early_init()负责使能内存管理单元mmu
bootable/bootloader/lk/arch/arm/arch.c
void arch_early_init(void)
{
/* turn off the cache */
arch_disable_cache(UCACHE); //关闭cache
/* set the vector base to our exception vectors so we dont need to double map at 0 */
#if ARM_CPU_CORTEX_A8
set_vector_base(MEMBASE); //设置异常向量基地址
#endif
#if ARM_WITH_MMU
arm_mmu_init(); //使能mmu
#endif
/* turn the cache back on */
arch_enable_cache(UCACHE); //打开cache
#if ARM_WITH_NEON
/* enable cp10 and cp11 */
uint32_t val;
__asm__ volatile("mrc p15, 0, %0, c1, c0, 2" : "=r" (val));
val |= (3<<22)|(3<<20);
__asm__ volatile("mcr p15, 0, %0, c1, c0, 2" :: "r" (val));
isb();
/* set enable bit in fpexc */
__asm__ volatile("mrc p10, 7, %0, c8, c0, 0" : "=r" (val));
val |= (1<<30);
__asm__ volatile("mcr p10, 7, %0, c8, c0, 0" :: "r" (val));
#endif
#if ARM_CPU_CORTEX_A8
/* enable the cycle count register */
uint32_t en;
__asm__ volatile("mrc p15, 0, %0, c9, c12, 0" : "=r" (en));
en &= ~(1<<3); /* cycle count every cycle */
en |= 1; /* enable all performance counters */
__asm__ volatile("mcr p15, 0, %0, c9, c12, 0" :: "r" (en));
/* enable cycle counter */
en = (1<<31);
__asm__ volatile("mcr p15, 0, %0, c9, c12, 1" :: "r" (en));
#endif
}
platform_early_init()平台早期初始化，初始化平台的时钟和主板
bootable\bootloader\lk\platform\msm8952\platform.c
void platform_early_init(void){
board_init(); //主板初始化
platform_clock_init(); //时钟初始化
qgic_init();
qtimer_init();
}
从代码可知，会创建一个bootstrap2线程，并使能中断
static int bootstrap2(void *arg)
{
dprintf(SPEW, "top of bootstrap2()\n");
arch_init(); //架构初始化，此函数为空，什么都没做
// XXX put this somewhere else
#if WITH_LIB_BIO
bio_init();
#endif
#if WITH_LIB_FS
fs_init();
#endif
// initialize the rest of the platform
dprintf(SPEW, "initializing platform\n");
platform_init(); // 平台初始化,不同的平台要做的事情不一样，可以是初始化系统时钟,超频等
// initialize the target
dprintf(SPEW, "initializing target\n");
target_init(); //目标设备初始化,主要初始化Flash,整合分区表等
dprintf(SPEW, "calling apps_init()\n");
apps_init(); //应用功能初始化，主要调用boot_init，启动kernel，加载boot/recovery镜像等
return 0;
}
apps_init()通过下面方式进入aboot_init()函数
APP_START(aboot)
.init = aboot_init,
APP_END
bootable/bootloader/lk/app/app.cvoid apps_init(void)
{
const struct app_descriptor *app;
/* call all the init routines */
for (app = &__apps_start; app != &__apps_end; app++) {
if (app->init)
app->init(app);
}
/* start any that want to start on boot */
for (app = &__apps_start; app != &__apps_end; app++) {
if (app->entry && (app->flags & APP_FLAG_DONT_START_ON_BOOT) == 0) {
start_app(app);
}
}
}
从这里开始是这篇文章的重点，分析aboot.c文件。每个项目的文件可能会有不同，但是差别会很小。
bootable/bootloader/lk/app/aboot/aboot.c
void aboot_init(const struct app_descriptor *app)
{
unsigned reboot_mode = 0;
unsigned restart_reason = 0;
unsigned hard_reboot_mode = 0;
bool boot_into_fastboot = false;
uint8_t pon_reason = pm8950_get_pon_reason(); //pm8950_get_pon_reason() 获取开机原因
/* Setup page size information for nv storage */
if (target_is_emmc_boot()) //检测是emmc还是flash存储，并设置页大小，一般是2048
{
page_size = mmc_page_size();
page_mask = page_size - 1;
}
else
{
page_size = flash_page_size();
page_mask = page_size - 1;
}
ASSERT((MEMBASE + MEMSIZE) > MEMBASE); //断言，如果内存基地址+内存大小小于内存基地址，则直接终止错误
read_device_info(&device); //从devinfo分区表read data到device结构体
read_allow_oem_unlock(&device); //devinfo分区里记录了unlock状态，从device中读取此信息
/* Display splash screen if enabled */
if (!check_alarm_boot()) {
dprintf(SPEW, "Display Init: Start\n");
target_display_init(device.display_panel); //显示splash，Splash也就是应用程序启动之前先启动一个画面,上面简单的介绍应用程序的厂商,厂商的LOGO,名称和版本等信息,多为一张图片
dprintf(SPEW, "Display Init: Done\n");
}
#ifdef FEATURE_LOW_POWER_DISP_LK
if(is_low_voltage) { //如果电量低，则显示关机动画，并关闭设备
mdelay(2000);
//target_uninit();
target_display_shutdown();
shutdown_device();
}
#endif
is_alarm_boot = check_alarm_boot(); //检测开机原因是否是由于关机闹钟导致
target_serialno((unsigned char *) sn_buf);
dprintf(SPEW,"serial number: %s\n",sn_buf);
memset(display_panel_buf, '\0', MAX_PANEL_BUF_SIZE);
/*
* Check power off reason if user force reset,
* if yes phone will do normal boot.
*/
if (is_user_force_reset()) //如果强制重启，直接进入normal_boot
goto normal_boot;
dprintf(ALWAYS, "pon_reason=0x%02x\n", pon_reason);
/* Check if we should do something other than booting up */
if ( (pon_reason & USB_CHG) //启动原因是插上USB，并且用户同时按住了音量上下键，进入下载模式
&& (keys_get_state(KEY_VOLUMEUP) && keys_get_state(KEY_VOLUMEDOWN)))
{
display_dloadimage_on_screen(); //显示下载模式图片
volume_keys_init(); //初始化音量按键
int i = 0;
int j = 0;
int k = 0;
dload_flag = 1 ;
while(1) //进入下载模式后，通过不同的按键组合进入不同的模式，下面的代码逻辑很简单，就不介绍了
{
thread_sleep(200);
//dprintf(ALWAYS, "in while circle\n");
if ( check_volume_up_key() && !check_volume_down_key() && !check_power_key() )
{
/* Hold volume_up_key 3 sec to download mode, if not enough, need to hold another 3 sec. */
for(i = 0;i < 15;++i)
{
thread_sleep(200);
if (!check_volume_up_key())
{
dprintf(ALWAYS, "press volume_up not enough time\n");
break;
}
}
if(i == 15)
{
break;
}
}
else if (check_power_key() && !check_volume_up_key() && !check_volume_down_key())
{
/* Hold power_key 1 sec to normal boot, if not enough, need to hold another 1 sec. */
for(j = 0;j < 5;++j)
{
thread_sleep(200);
if (!check_power_key())
{
//dprintf(ALWAYS, "press power_key not enough time\n");
break;
}
}
if(j == 5)
{
goto normal_boot;
}
}
else if (!check_volume_down_key() && !check_volume_up_key() && !check_power_key())
{
/* Hold no key and go to normal boot 30 sec later. */
for(k = 0;k < 150;++k)
{
thread_sleep(200);
if (check_power_key() || check_volume_up_key())
{
//dprintf(ALWAYS, "press nothing\n");
break;
}
}
if(k == 150)
{
//dprintf(ALWAYS, "goto normal_boot\n");
goto normal_boot;
}
}
}
dprintf(CRITICAL,"dload mode key sequence detected\n");
if (set_download_mode(EMERGENCY_DLOAD))
{
dprintf(CRITICAL,"dload mode not supported by target\n");
}
else
{
reboot_device(DLOAD);
dprintf(ALWAYS,"Failed to reboot into dload mode\n");
}
boot_into_fastboot = true; //下载模式本质上是进入fastboot
}
if (!boot_into_fastboot) //如果不是通过usb+上下键进入下载模式
{
if (keys_get_state(KEY_HOME) || (keys_get_state(KEY_VOLUMEUP) && !keys_get_state(KEY_VOLUMEDOWN))) //上键+电源键 进入recovery模式
{
boot_into_recovery = 1;
struct recovery_message msg;
strcpy(msg.recovery, "recovery\n--show_text");
}
if (!boot_into_recovery &&
(keys_get_state(KEY_BACK) || (keys_get_state(KEY_VOLUMEDOWN) && !keys_get_state(KEY_VOLUMEUP)))) //下键+back键进入fastboot模式，我的手机是有back实体键的
boot_into_fastboot = true;
}
reboot_mode = check_reboot_mode(); //检测开机原因，并且修改相应的标志位
hard_reboot_mode = check_hard_reboot_mode();
if (reboot_mode == RECOVERY_MODE ||
hard_reboot_mode == RECOVERY_HARD_RESET_MODE) {
boot_into_recovery = 1;
} else if(reboot_mode == FASTBOOT_MODE ||
hard_reboot_mode == FASTBOOT_HARD_RESET_MODE) {
boot_into_fastboot = true;
} else if(reboot_mode == ALARM_BOOT ||
hard_reboot_mode == RTC_HARD_RESET_MODE) {
boot_reason_alarm = true;
}
else if (reboot_mode == DM_VERITY_ENFORCING)
{
device.verity_mode = 1;
write_device_info(&device);
} else if(reboot_mode == DM_VERITY_LOGGING) {
device.verity_mode = 0;
write_device_info(&device);
} else if(reboot_mode == DM_VERITY_KEYSCLEAR) {
if(send_delete_keys_to_tz())
ASSERT(0);
}
normal_boot:
if(dload_flag){
display_image_on_screen(); //显示界面，上面提到过
}
if (!boot_into_fastboot) //如果不是fastboot模式
{
if (target_is_emmc_boot())
{
if(emmc_recovery_init())
dprintf(ALWAYS,"error in emmc_recovery_init\n");
if(target_use_signed_kernel())
{
if((device.is_unlocked) || (device.is_tampered))
{
#ifdef TZ_TAMPER_FUSE
set_tamper_fuse_cmd();
#endif
#if USE_PCOM_SECBOOT
set_tamper_flag(device.is_tampered);
#endif
}
}
boot_linux_from_mmc(); //程序会跑到这里，又一个重点内容，下面会独立分析这个函数。
}
else
{
recovery_init();
#if USE_PCOM_SECBOOT
if((device.is_unlocked) || (device.is_tampered))
set_tamper_flag(device.is_tampered);
#endif
boot_linux_from_flash();
}
dprintf(CRITICAL, "ERROR: Could not do normal boot. Reverting "
"to fastboot mode.\n");
}
//下面的代码是fastboot的准备工作，从中可以看出，进入fastboot模式是不启动kernel的
/* We are here means regular boot did not happen. Start fastboot. */
/* register aboot specific fastboot commands */
aboot_fastboot_register_commands(); //注册fastboot命令，建议看下此函数的源码，此函数是fastboot支持的命令，如flash、erase等等
/* dump partition table for debug info */
partition_dump();
/* initialize and start fastboot */
fastboot_init(target_get_scratch_address(), target_get_max_flash_size()); //初始化fastboot
#if FBCON_DISPLAY_MSG
display_fastboot_menu_thread(); //显示fastboot界面
#endif
}
关于device_info,这里多说一点
devinfo Device information including:iis_unlocked, is_tampered, is_verified, charger_screen_enabled, display_panel, bootloader_version, radio_version
All these attirbutes are set based on some specific conditions and written on devinfo partition.
devinfo是一个独立的分区，里面存放了下面的一些信息，上面是高通对这个分区的介绍。
struct device_info
{
unsigned char magic[DEVICE_MAGIC_SIZE];
bool is_unlocked;
bool is_tampered;
bool is_verified;
bool charger_screen_enabled;
char display_panel[MAX_PANEL_ID_LEN];
char bootloader_version[MAX_VERSION_LEN];
char radio_version[MAX_VERSION_LEN];
};
从上面的分析，我们大致可以知道boot_init()主要工作
1).确定page_size大小;
2).从devinfo分区获取devinfo信息;
3).通过不同按键选择设置对应标志位boot_into_xxx;
4).如果进入fastboot模式，初始化fastboot命令等。
5).进入boot_linux_from_mmc()函数。
下面分析lk启动过程中另一个重要的函数boot_linux_from_mmc();它主要负责根据boot_into_xxx从对应的分区内读取相关信息并传给kernel，然后引导kernel。
程序走到这，说成没有进入fastboot模式，可能的情况有：正常启动，进入recovery，开机闹钟启动。
boot_linux_from_mmc()主要做下面的事情
1).程序会从boot分区或者recovery分区的header中读取地址等信息，然后把kernel、ramdisk加载到内存中。
2).程序会从misc分区中读取bootloader_message结构体，如果有boot-recovery，则进入recovery模式
3).更新cmdline，然后把cmdline写到tags_addr地址，把参数传给kernel，kernel起来以后会到这个地址读取参数。
int boot_linux_from_mmc(void)
{
struct boot_img_hdr *hdr = (void*) buf; //************buf和hdr指向相同的地址，可以理解为buf就是hdr
struct boot_img_hdr *uhdr;
unsigned offset = 0;
int rcode;
unsigned long long ptn = 0;
int index = INVALID_PTN;
unsigned char *image_addr = 0;
unsigned kernel_actual;
unsigned ramdisk_actual;
unsigned imagesize_actual;
unsigned second_actual = 0;
unsigned int dtb_size = 0;
unsigned int out_len = 0;
unsigned int out_avai_len = 0;
unsigned char *out_addr = NULL;
uint32_t dtb_offset = 0;
unsigned char *kernel_start_addr = NULL;
unsigned int kernel_size = 0;
int rc;
#if DEVICE_TREE
struct dt_table *table;
struct dt_entry dt_entry;
unsigned dt_table_offset;
uint32_t dt_actual;
uint32_t dt_hdr_size;
unsigned char *best_match_dt_addr = NULL;
#endif
struct kernel64_hdr *kptr = NULL;
if (check_format_bit()) //查找bootselect分区，查看分区表，没有此分区，所以返回值为false
boot_into_recovery = 1;
if (!boot_into_recovery) { //此时有两种可能，正常开机/进入ffbm工厂测试模式，进入工厂测试模式是正行启动，但是向kernel传参会多一个字符串"androidboot.mode='ffbm_mode_string'"
memset(ffbm_mode_string, '\0', sizeof(ffbm_mode_string)); //ffbm_mode_string = ""
rcode = get_ffbm(ffbm_mode_string, sizeof(ffbm_mode_string)); //从misc分区0地址中读取sizeof(ffbm_mode_string)的内容，如果内容是"ffbm-"，返回1，否则返回0
if (rcode <= 0) {
boot_into_ffbm = false;
if (rcode < 0)
dprintf(CRITICAL,"failed to get ffbm cookie");
} else
boot_into_ffbm = true;
} else //boot_into_recovery=true
boot_into_ffbm = false;
uhdr = (struct boot_img_hdr *)EMMC_BOOT_IMG_HEADER_ADDR; //uhdr指向boot分区header地址，header是什么东西，下面会详细介绍
if (!memcmp(uhdr->magic, BOOT_MAGIC, BOOT_MAGIC_SIZE)) { //检查uhdr->magic 是否等于 "ANDROID!",不知到为什么要这么做，觉的没有什么作用
dprintf(INFO, "Unified boot method!\n");
hdr = uhdr;
goto unified_boot;
}
if (!boot_into_recovery) { //如果不是recovery模式，可能是正常启动或者进入ffbm，再次生命ffbm和正常启动流程一样启动kernel，只是kernel起来以后，init.c文件会读取是否有"ffbm-"
index = partition_get_index("boot"); //读取boot分区
ptn = partition_get_offset(index); //读取boot分区的偏移量
if(ptn == 0) {
dprintf(CRITICAL, "ERROR: No boot partition found\n");
return -1;
}
}
else {
index = partition_get_index("recovery"); //进入recovery模式，读取recovery分区，并获得recovery分区的偏移量。recovery.img和boot.img的组成是一样的，下面有介绍
ptn = partition_get_offset(index);
if(ptn == 0) {
dprintf(CRITICAL, "ERROR: No recovery partition found\n");
return -1;
}
}
/* Set Lun for boot & recovery partitions */
mmc_set_lun(partition_get_lun(index));
if (mmc_read(ptn + offset, (uint32_t *) buf, page_size)) { //从boot/recovery分区读取1字节的内容到buf(hdr)中，我们知道在boot/recovery中开始的1字节存放的是hdr的内容，下面有详细的介绍。
dprintf(CRITICAL, "ERROR: Cannot read boot image header\n");
return -1;
}
if (memcmp(hdr->magic, BOOT_MAGIC, BOOT_MAGIC_SIZE)) { //上面已经从boot/recovery分区读取了header到hdr，这里对比magic是否等于"ANDROID！",如果不是，则表明读取的header是错误的，也算是校验吧
dprintf(CRITICAL, "ERROR: Invalid boot image header\n");
return -1;
}
if (hdr->page_size && (hdr->page_size != page_size)) { //比较也的大小是否相同，应该都是相同的2048字节
if (hdr->page_size > BOOT_IMG_MAX_PAGE_SIZE) {
dprintf(CRITICAL, "ERROR: Invalid page size\n");
return -1;
}
page_size = hdr->page_size;
page_mask = page_size - 1;
}
/* ensure commandline is terminated */
hdr->cmdline[BOOT_ARGS_SIZE-1] = 0;
kernel_actual = ROUND_TO_PAGE(hdr->kernel_size, page_mask); //kernel所占的页的总大小 例如kernel大小0x01,kernel_actual = 2048
ramdisk_actual = ROUND_TO_PAGE(hdr->ramdisk_size, page_mask); //ramdisk所占的页的总大小
image_addr = (unsigned char *)target_get_scratch_address();
#if DEVICE_TREE
dt_actual = ROUND_TO_PAGE(hdr->dt_size, page_mask); //dt所占的页的大小
imagesize_actual = (page_size + kernel_actual + ramdisk_actual + dt_actual); //image占的页的总大小
#else
imagesize_actual = (page_size + kernel_actual + ramdisk_actual);
#endif
#if VERIFIED_BOOT
boot_verifier_init(); //校验boot
#endif
if (check_aboot_addr_range_overlap((uint32_t) image_addr, imagesize_actual)) //校验image_addr是否被覆盖
{
dprintf(CRITICAL, "Boot image buffer address overlaps with aboot addresses.\n");
return -1;
}
/*
* Update loading flow of bootimage to support compressed/uncompressed
* bootimage on both 64bit and 32bit platform.
* 1. Load bootimage from emmc partition onto DDR.
* 2. Check if bootimage is gzip format. If yes, decompress compressed kernel
* 3. Check kernel header and update kernel load addr for 64bit and 32bit
* platform accordingly.
* 4. Sanity Check on kernel_addr and ramdisk_addr and copy data.
*/
dprintf(INFO, "Loading boot image (%d): start\n", imagesize_actual);
bs_set_timestamp(BS_KERNEL_LOAD_START);
/* Read image without signature */
if (mmc_read(ptn + offset, (void *)image_addr, imagesize_actual)) //读取boot/recovery分区到image_addr
{
dprintf(CRITICAL, "ERROR: Cannot read boot image\n");
return -1;
}
dprintf(INFO, "Loading boot image (%d): done\n", imagesize_actual);
bs_set_timestamp(BS_KERNEL_LOAD_DONE);
/* Authenticate Kernel */
dprintf(INFO, "use_signed_kernel=%d, is_unlocked=%d, is_tampered=%d.\n",
(int) target_use_signed_kernel(),
device.is_unlocked,
device.is_tampered);
if(target_use_signed_kernel() && (!device.is_unlocked)) //这里是false ，感兴趣可以追target_use_signed_kernel(),会发现这个函数返回的是0
{
offset = imagesize_actual;uhdr->magic
if (check_aboot_addr_range_overlap((uint32_t)image_addr + offset, page_size))
{
dprintf(CRITICAL, "Signature read buffer address overlaps with aboot addresses.\n");
return -1;
}
/* Read signature */
if(mmc_read(ptn + offset, (voidffbm_mode_string *)(image_addr + offset), page_size))
{
dprintf(CRITICAL, "ERROR: Cannot read boot image signature\n");
return -1;
}
verify_signed_bootimg((uint32_t)image_addr, imagesize_actual);
} else {
second_actual = ROUND_TO_PAGE(hdr->second_size, page_mask);
#ifdef TZ_SAVE_KERNEL_HASH
aboot_save_boot_hash_mmc((uint32_t) image_addr, imagesize_actual);
#endif /* TZ_SAVE_KERNEL_HASH */
#if VERIFIED_BOOT
if(boot_verify_get_state() == ORANGE) //校验boot
{
#if FBCON_DISPLAY_MSG
display_bootverify_menu_thread(DISPLAY_MENU_ORANGE);
wait_for_users_action();
#else
dprintf(CRITICAL,
"Your device has been unlocked and can't be trusted.\nWait for 5 seconds before proceeding\n");
mdelay(5000);
#endif
set_root_flag(ORANGE,1);
}
#endif
#ifdef MDTP_SUPPORT
{
/* Verify MDTP lock.
* For boot & recovery partitions, MDTP will use boot_verifier APIs,
* since verification was skipped in aboot. The signature is not part of the loaded image.
*/
mdtp_ext_partition_verification_t ext_partition;
ext_partition.partition = boot_into_recovery ? MDTP_PARTITION_RECOVERY : MDTP_PARTITION_BOOT;
ext_partition.integrity_state = MDTP_PARTITION_STATE_UNSET;
ext_partition.page_size = page_size;
ext_partition.image_addr = (uint32)image_addr;
ext_partition.image_size = imagesize_actual;
ext_partition.sig_avail = FALSE;
mdtp_fwlock_verify_lock(&ext_partition);
}
#endif /* MDTP_SUPPORT */
}
#if VERIFIED_BOOT
#if !VBOOT_MOTA
// send root of trust
if(!send_rot_command((uint32_t)device.is_unlocked))
ASSERT(0);
#endif
#endif
/*
* Check if the kernel image is a gzip package. If yes, need to decompress it.
* If not, continue booting.
*/
//检测kernel image是否是gzip的包，如果是，解压，如果不是，继续boot。得到kernel的起始地址和大小
if (is_gzip_package((unsigned char *)(image_addr + page_size), hdr->kernel_size))
{
out_addr = (unsigned char *)(image_addr + imagesize_actual + page_size);
out_avai_len = target_get_max_flash_size() - imagesize_actual - page_size;
dprintf(INFO, "decompressing kernel image: start\n");
rc = decompress((unsigned char *)(image_addr + page_size),
hdr->kernel_size, out_addr, out_avai_len,
&dtb_offset, &out_len);
if (rc)
{
dprintf(CRITICAL, "decompressing kernel image failed!!!\n");
ASSERT(0);
}
dprintf(INFO, "decompressing kernel image: done\n");
kptr = (struct kernel64_hdr *)out_addr;
kernel_start_addr = out_addr;
kernel_size = out_len;
} else {
kptr = (struct kernel64_hdr *)(image_addr + page_size);
kernel_start_addr = (unsigned char *)(image_addr + page_size); //kernel_start起始地址
kernel_size = hdr->kernel_size; //kernel大小
}
/*
* Update the kernel/ramdisk/tags address if the boot image header
* has default values, these default values come from mkbootimg when
* the boot image is flashed using fastboot flash:raw
*/
update_ker_tags_rdisk_addr(hdr, IS_ARM64(kptr)); //更新kernel/tags/ramdisk地址
/* Get virtual addresses since the hdr saves physical addresses. */
hdr->kernel_addr = VA((addr_t)(hdr->kernel_addr)); //保存虚拟地址（mmu）
hdr->ramdisk_addr = VA((addr_t)(hdr->ramdisk_addr));
hdr->tags_addr = VA((addr_t)(hdr->tags_addr));
kernel_size = ROUND_TO_PAGE(kernel_size, page_mask);
/* Check if the addresses in the header are valid. */
if (check_aboot_addr_range_overlap(hdr->kernel_addr, kernel_size) || //检测kernel/ramdisk/tags地址是否超出emmc地址
check_aboot_addr_range_overlap(hdr->ramdisk_addr, ramdisk_actual))
{
dprintf(CRITICAL, "kernel/ramdisk addresses overlap with aboot addresses.\n");
return -1;
}
#ifndef DEVICE_TREE
if (check_aboot_addr_range_overlap(hdr->tags_addr, MAX_TAGS_SIZE))
{
dprintf(CRITICAL, "Tags addresses overlap with aboot addresses.\n");
return -1;
}
#endif
/* Move kernel, ramdisk and device tree to correct address */
memmove((void*) hdr->kernel_addr, kernel_start_addr, kernel_size); //把kernel/ramdisk放在相应的地址上
memmove((void*) hdr->ramdisk_addr, (char *)(image_addr + page_size + kernel_actual), hdr->ramdisk_size);
#if DEVICE_TREE //读取设备树信息，放在相应的地址上
if(hdr->dt_size) {
dt_table_offset = ((uint32_t)image_addr + page_size + kernel_actual + ramdisk_actual + second_actual);
table = (struct dt_table*) dt_table_offset;
if (dev_tree_validate(table, hdr->page_size, &dt_hdr_size) != 0) {
dprintf(CRITICAL, "ERROR: Cannot validate Device Tree Table \n");
return -1;
}
/* Find index of device tree within device tree table */
if(dev_tree_get_entry_info(table, &dt_entry) != 0){
dprintf(CRITICAL, "ERROR: Getting device tree address failed\n");
return -1;
}
if (is_gzip_package((unsigned char *)dt_table_offset + dt_entry.offset, dt_entry.size))
{
unsigned int compressed_size = 0;
out_addr += out_len;
out_avai_len -= out_len;
dprintf(INFO, "decompressing dtb: start\n");
rc = decompress((unsigned char *)dt_table_offset + dt_entry.offset,
dt_entry.size, out_addr, out_avai_len,
&compressed_size, &dtb_size);
if (rc)
{
dprintf(CRITICAL, "decompressing dtb failed!!!\n");
ASSERT(0);
}
dprintf(INFO, "decompressing dtb: done\n");
best_match_dt_addr = out_addr;
} else {
best_match_dt_addr = (unsigned char *)dt_table_offset + dt_entry.offset;
dtb_size = dt_entry.size;
}
/* Validate and Read device device tree in the tags_addr */
if (check_aboot_addr_range_overlap(hdr->tags_addr, dtb_size))
{
dprintf(CRITICAL, "Device tree addresses overlap with aboot addresses.\n");
return -1;
}
memmove((void *)hdr->tags_addr, (char *)best_match_dt_addr, dtb_size);
} else {
/* Validate the tags_addr */
if (check_aboot_addr_range_overlap(hdr->tags_addr, kernel_actual))
{
dprintf(CRITICAL, "Device tree addresses overlap with aboot addresses.\n");
return -1;
}
/*
* If appended dev tree is found, update the atags with
* memory address to the DTB appended location on RAM.
* Else update with the atags address in the kernel header
*/
void *dtb;
dtb = dev_tree_appended((void*)(image_addr + page_size),
hdr->kernel_size, dtb_offset,
(void *)hdr->tags_addr);
if (!dtb) {
dprintf(CRITICAL, "ERROR: Appended Device Tree Blob not found\n");
return -1;
}
}
#endif
if (boot_into_recovery && !device.is_unlocked && !device.is_tampered)
target_load_ssd_keystore();
unified_boot:
boot_linux((void *)hdr->kernel_addr, (void *)hdr->tags_addr, //进入boot_linux函数，此函数比较简单，更新cmdline。
(const char *)hdr->cmdline, board_machtype(),
(void *)hdr->ramdisk_addr, hdr->ramdisk_size);
return 0;
}
如果misc分区的0地址内容是"ffbm-"，则boot_into_ffbm=true
int get_ffbm(char *ffbm, unsigned size)
{
const char *ffbm_cmd = "ffbm-";
uint32_t page_size = get_page_size();
char *ffbm_page_buffer = NULL;
int retval = 0;
if (size < FFBM_MODE_BUF_SIZE || size >= page_size)
{
dprintf(CRITICAL, "Invalid size argument passed to get_ffbm\n");
retval = -1;
goto cleanup;
}
ffbm_page_buffer = (char*)malloc(page_size);
if (!ffbm_page_buffer)
{
dprintf(CRITICAL, "Failed to alloc buffer for ffbm cookie\n");
retval = -1;
goto cleanup;
}
if (read_misc(0, ffbm_page_buffer, page_size))
{
dprintf(CRITICAL, "Error reading MISC partition\n");
retval = -1;
goto cleanup;
}
ffbm_page_buffer[size] = '\0';
if (strncmp(ffbm_cmd, ffbm_page_buffer, strlen(ffbm_cmd)))
{
retval = 0;
goto cleanup;
}
else
{
if (strlcpy(ffbm, ffbm_page_buffer, size) <
FFBM_MODE_BUF_SIZE -1)
{
dprintf(CRITICAL, "Invalid string in misc partition\n");
retval = -1;
}
else
retval = 1;
}
cleanup:
if(ffbm_page_buffer)
free(ffbm_page_buffer);
return retval;
}
boot.img和recovery.img的组成是一样的，所以lk加载方式一样，只是读取的地址和大小不同而已。
我们看下boot.img和recovery.img镜像里有什么，理解了这个再看lk加载boot.img/recovery.img就知道是怎么回事了:
** +-----------------+
** | boot header | 1 page
** +-----------------+
** | kernel | n pages
** +-----------------+
** | ramdisk | m pages
** +-----------------+
** | second stage | o pages
** +-----------------+
** | device tree | p pages
** +-----------------+
分析boot_img_hdr结构提
kernel_size kernel表示zImage的实际大小
kernel_addr kernel的zImage载入内存的物理地址，也是bootloader要跳转的地址
ramdisk_size ramdisk的实际大小
ramdisk_addr ramdisk加载到内存的实际物理地址，之后kernel会解压并把它挂载成根文件系统，我们的中枢神经－init.rc就隐藏于内
tags_addr tags_addr是传参数用的物理内存地址，它作用是把bootloader中的参数传递给kernel,参数放在这个地址上
page_size page_size是存储芯片（ram/emmc）的页大小，取决与存储芯片
cmdline command line它可以由bootloader向kernel传参的内容，存放在tag_addr地址
second 可选
bootable/bootloader/lk/app/aboot/bootimg.h
#ifndef _BOOT_IMAGE_H_
#define _BOOT_IMAGE_H_
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
#define BOOT_IMG_MAX_PAGE_SIZE 4096
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
*/
boot_img_hdr *mkbootimg(void *kernel, unsigned kernel_size,
void *ramdisk, unsigned ramdisk_size,
void *second, unsigned second_size,
unsigned page_size,
unsigned *bootimg_size);
void bootimg_set_cmdline(boot_img_hdr *hdr, const char *cmdline);
#define KERNEL64_HDR_MAGIC 0x644D5241 /* ARM64 */
struct kernel64_hdr
{
uint32_t insn;
uint32_t res1;
uint64_t text_offset;
uint64_t res2;
uint64_t res3;
uint64_t res4;
uint64_t res5;
uint64_t res6;
uint32_t magic_64;
uint32_t res7;
};
#endif