Lk启动流程分析

1 Lk概述

LK是(L)ittle(K)ernel的缩写。目前android平台普遍采用lk作为其bootloader，LK是一个开源项目。但是，LK只是整个系统的引导部分，所以它不是独立存在。LK是一个功能及其强大的bootloader，但现在只支持arm和x86平台。

LK的一个显著的特点就是它实现了一个简单的线程机制（thread），和对高通处理器的深度定制和使用。

2 源代码目录

app //主函数启动app执行的目录，第一个app在app/aboot/aboot.c中

arch //体系代码包含x86和arm

dev //设备目录，包含显示器，键盘，net,usb等设备的初始化代码

include //头文件

kernel //kernel/main.c主函数以及kernel/thread.c线程函数

lib //库文件

make //编译规则

platform //不同平台代码mdmxxx，msmxxx，apqxxx，qsdxxx，还有共享的目录msm_shared

project //整个工程的编译规则

target //通用init.c,具体目标板的初始化(主要为板子设备资源init.c代码中)

3 Lk入口

3.1 bootable\bootloader\lk\arch\arm\rule.mk文件下相关部分：

# potentially generated files that should be cleaned out with clean make rule
GENERATED += \
$(BUILDDIR)/system-onesegment.ld \
$(BUILDDIR)/system-twosegment.ld

# rules for generating the linker scripts

$(BUILDDIR)/trustzone-test-system-onesegment.ld: $(LOCAL_DIR)/trustzone-test-system-onesegment.ld $(LK_TOP_DIR)/target/$(TARGET)/rules.mk .FORCE
@echo generating $@
@$(MKDIR)
$(NOECHO)sed "s/%MEMBASE%/$(MEMBASE)/;s/%MEMSIZE%/$(MEMSIZE)/;s/%ROMLITE_PREFLASHED_DATA%/$(ROMLITE_PREFLASHED_DATA)/" < $< > $@

$(BUILDDIR)/trustzone-system-onesegment.ld: $(LOCAL_DIR)/trustzone-system-onesegment.ld $(LK_TOP_DIR)/target/$(TARGET)/rules.mk .FORCE
@echo generating $@
@$(MKDIR)
$(NOECHO)sed "s/%MEMBASE%/$(MEMBASE)/;s/%MEMSIZE%/$(MEMSIZE)/" < $< > $@

$(BUILDDIR)/system-onesegment.ld: $(LOCAL_DIR)/system-onesegment.ld $(LK_TOP_DIR)/target/$(TARGET)/rules.mk .FORCE
@echo generating $@
@$(MKDIR)
$(NOECHO)sed "s/%MEMBASE%/$(MEMBASE)/;s/%MEMSIZE%/$(MEMSIZE)/" < $< > $@

$(BUILDDIR)/system-twosegment.ld: $(LOCAL_DIR)/system-twosegment.ld $(LK_TOP_DIR)/target/$(TARGET)/rules.mk .FORCE
@echo generating $@
@$(MKDIR)
$(NOECHO)sed "s/%ROMBASE%/$(ROMBASE)/;s/%MEMBASE%/$(MEMBASE)/;s/%MEMSIZE%/$(MEMSIZE)/" < $< > $@

3.2 arch/arm/system-onesegment.ld

OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
OUTPUT_ARCH(arm)

ENTRY(_start)   /*跳入crt0.S文件执行代码*/
3.3 arch/arm/crt0.S
.section ".text.boot"
.globl _start
_start:
b   reset
b   arm_undefined
b   arm_syscall
b   arm_prefetch_abort
b   arm_data_abort
b   arm_reserved
b   arm_irq
b   arm_fiq

reset:
……

bl      kmain
b
……

4 kmain函数

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

// 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

// 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
}

4.1 arch_early_init函数

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();             //mmu初始化

#endif

/* turn the cache back on */
arch_enable_cache(UCACHE);  //使能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
}

4.2 platform_early_init函数

bootable/bootloader/lk/platform/msm8953/platform.c

void platform_early_init(void)
{
board_init();   //主板初始化
platform_clock_init();  //平台时钟初始化
qgic_init();    //中断控制器初始化
qtimer_init();  //定时器初始化
scm_init(); //
}

4.3 target_early_init函数

bootable/bootloader/lk/ target/msm8953/init.c

void target_early_init(void)
{
#if WITH_DEBUG_UART
uart_dm_init(1, 0, BLSP1_UART0_BASE);  //串口初始化
#endif
}
5. timer_init函数
bootable/bootloader/lk/ kernel/timer.c
void timer_init(void)
{
list_initialize(&timer_queue);

/* register for a periodic timer tick */
platform_set_periodic_timer(timer_tick, NULL, 10); /* 10ms */
}

4.4 bootstrap2函数分析

bootable/bootloader/lk/kernel/main.c

static int bootstrap2(void *arg)
{
dprintf(SPEW, "top of bootstrap2()\n");

arch_init();    //架构初始化，空函数

// 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();    //板级设备初始化，空函数

// initialize the target
dprintf(SPEW, "initializing target\n");
target_init();      //目标设备初始化，见1.1

dprintf(SPEW, "calling apps_init()\n");
apps_init();        //lk应用初始化，见1.2

return 0;
}

4.4.1 target_init函数

bootable/bootloader/lk/target/msm8953/init.c

void target_init(void)
{
#if VERIFIED_BOOT
#if !VBOOT_MOTA
int ret = 0;
#endif
#endif
dprintf(INFO, "target_init()\n");

spmi_init(PMIC_ARB_CHANNEL_NUM, PMIC_ARB_OWNER_ID); //初始化spmi 控制器

target_keystatus();

target_sdc_init();      //sd card 初始化，内部包含了mmc的初始化
if (partition_read_table())     //读取分区表
{
dprintf(CRITICAL, "Error reading the partition table info\n");
ASSERT(0);
}

#if LONG_PRESS_POWER_ON
shutdown_detect();
#endif

#if PON_VIB_SUPPORT
vib_timed_turn_on(VIBRATE_TIME);
#endif

if (target_use_signed_kernel())
target_crypto_init_params();

#if VERIFIED_BOOT
#if !VBOOT_MOTA
clock_ce_enable(CE1_INSTANCE);

/* Initialize Qseecom */
ret = qseecom_init();   //qse 初始化

if (ret < 0)
{
dprintf(CRITICAL, "Failed to initialize qseecom, error: %d\n", ret);
ASSERT(0);
}

/* Start Qseecom */
ret = qseecom_tz_init(); //qse tz初始化

if (ret < 0)
{
dprintf(CRITICAL, "Failed to start qseecom, error: %d\n", ret);
ASSERT(0);
}

if (rpmb_init() < 0)  //rpmb 初始化（Replay Protected Memory Block，emmc中的一个分区，总共五个分区：BOOT Area Partition 1，BOOT Area Partition 2，RPMB，User Data Area，Vender private area）
{
dprintf(CRITICAL, "RPMB init failed\n");
ASSERT(0);
}

/*
* Load the sec app for first time
*/
if (load_sec_app() < 0)         //加载安全app
{
dprintf(CRITICAL, "Failed to load App for verified\n");
ASSERT(0);
}
#endif
#endif

#if SMD_SUPPORT
rpm_smd_init(); //smd 初始化
#endif
}

4.4.2 apps_init函数

bootable/bootloader/lk/ app/app.c

void 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);
}
}
}
代码段：
for (app = &__apps_start; app != &__apps_end; app++) {
if (app->init)
app->init(app);
}
表示进入各自的app init函数，通过下面的代码段可以分析出具体有哪些app init函数。
bootable/bootloader/lk/ arch/arm/system-onesegment.ld
.rodata : {
……..
__apps_start = .;
KEEP (*(.apps))
__apps_end = .;
……..
}
bootable/bootloader/lk/ include/app.h
struct app_descriptor {
const char *name;
app_init  init;
app_entry entry;
unsigned int flags;
};
#define APP_START(appname) struct app_descriptor _app_##appname __SECTION(".apps") = { .name = #appname,
#define APP_END };
bootable/bootloader/lk/app/aboot/aboot.c
APP_START(aboot)
.init = aboot_init,
APP_END
上面的过程表示了如何将aboot模块的aboot_init函数加入到.apps代码段。这种方式也用在clocktests 、shell、pcitests、stringtests、tests模块中，请参见一下文件：
bootable/bootloader/lk/app/clocktests/clock_tests.c:APP_START(clocktests)
bootable/bootloader/lk/app/aboot/aboot.c:APP_START(aboot)
bootable/bootloader/lk/app/shell/shell.c:APP_START(shell)
bootable/bootloader/lk/app/pcitests/pci_tests.c:APP_START(pcitests)
bootable/bootloader/lk/app/stringtests/string_tests.c:APP_START(stringtests)
bootable/bootloader/lk/app/tests/tests.c:APP_START(tests)

5 aboot_init函数分析

/bootable/bootloader/lk/app/aboot/aboot.c

void aboot_init(const struct app_descriptor *app)
{
unsigned reboot_mode = 0;

/* Initialise wdog to catch early lk crashes */
#if WDOG_SUPPORT
msm_wdog_init();        //看门狗初始化
#endif

/* 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;
mmc_blocksize = mmc_get_device_blocksize();
mmc_blocksize_mask = mmc_blocksize - 1;
}
else
{
page_size = flash_page_size();
page_mask = page_size - 1;
}

ASSERT((MEMBASE + MEMSIZE) > MEMBASE);

read_device_info(&device);          //读取device info分区的信息到device结构体中
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 DISPLAY_SPLASH_SCREEN
#if NO_ALARM_DISPLAY
if (!check_alarm_boot()) {      //判断是否是因为闹钟导致的重启
#endif
dprintf(SPEW, "Display Init: Start\n");
#if ENABLE_WBC
/* Wait if the display shutdown is in progress */
while(pm_app_display_shutdown_in_prgs());
if (!pm_appsbl_display_init_done())
target_display_init(device.display_panel);
else
display_image_on_screen();
#else
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
#endif
dprintf(SPEW, "Display Init: Done\n");
#if NO_ALARM_DISPLAY
}
#endif
#endif

target_serialno((unsigned char *) sn_buf);      //读取序列号
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())      //判断是否是用户设置的重启
goto normal_boot;

/* Check if we should do something other than booting up */
//判断是否进入各种模式
if (keys_get_state(KEY_VOLUMEUP) && keys_get_state(KEY_VOLUMEDOWN))
{
dprintf(ALWAYS,"dload mode key sequence detected\n");
reboot_device(EMERGENCY_DLOAD);
dprintf(CRITICAL,"Failed to reboot into dload mode\n");

boot_into_fastboot = true;
}
if (!boot_into_fastboot)
{
if (keys_get_state(KEY_HOME) || keys_get_state(KEY_VOLUMEUP))
boot_into_recovery = 1;
if (!boot_into_recovery &&
(keys_get_state(KEY_BACK) || keys_get_state(KEY_VOLUMEDOWN)))
boot_into_fastboot = true;
}
#if NO_KEYPAD_DRIVER
if (fastboot_trigger())
boot_into_fastboot = true;
#endif

#if USE_PON_REBOOT_REG
reboot_mode = check_hard_reboot_mode();
#else
reboot_mode = check_reboot_mode();
#endif
if (reboot_mode == RECOVERY_MODE)
{
boot_into_recovery = 1;
}
else if(reboot_mode == FASTBOOT_MODE)
{
boot_into_fastboot = true;
}
else if(reboot_mode == ALARM_BOOT)
{
boot_reason_alarm = true;
}
#if VERIFIED_BOOT
#if !VBOOT_MOTA
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);
}
#endif
#endif

normal_boot:
if (!boot_into_fastboot)
{
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();  //跳入linux kernel启动，后面深入分析
}
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();    //跳入linux kernel启动，后面深入分析
}
dprintf(CRITICAL, "ERROR: Could not do normal boot. Reverting "
"to fastboot mode.\n");
}

/* We are here means regular boot did not happen. Start fastboot. */

/* register aboot specific fastboot commands */
aboot_fastboot_register_commands();

/* dump partition table for debug info */
partition_dump();

/* initialize and start fastboot */
fastboot_init(target_get_scratch_address(), target_get_max_flash_size());
#if FBCON_DISPLAY_MSG
display_fastboot_menu();    //显示fastboot菜单
#endif
}

5.1 read_device_info函数分析

device_info结构体

bootable/bootloader/lk/ app/aboot/devinfo.h

struct device_info
{
unsigned char magic[DEVICE_MAGIC_SIZE];
bool is_unlocked;
bool is_tampered;
bool is_unlock_critical;
bool charger_screen_enabled;
char display_panel[MAX_PANEL_ID_LEN];
char bootloader_version[MAX_VERSION_LEN];
char radio_version[MAX_VERSION_LEN];
};

2 read_device_info函数

bootable/bootloader/lk/ app/aboot/aboot.c

void read_device_info(device_info *dev)
{
if(target_is_emmc_boot())
{
struct device_info *info = memalign(PAGE_SIZE, ROUNDUP(BOOT_IMG_MAX_PAGE_SIZE, PAGE_SIZE));     //开辟一片内存，其大小是对齐的倍数，必须是2的幂
if(info == NULL)
{
dprintf(CRITICAL, "Failed to allocate memory for device info struct\n");
ASSERT(0);
}
info_buf = info;

#if USE_RPMB_FOR_DEVINFO            //从rpmb中读取devinfo
if (is_secure_boot_enable()) {
if((read_device_info_rpmb((void*) info, PAGE_SIZE)) < 0)
ASSERT(0);
}
else
read_device_info_mmc(info);
#else
read_device_info_mmc(info);     //从mmc中读取devinfo
#endif

if (memcmp(info->magic, DEVICE_MAGIC, DEVICE_MAGIC_SIZE))
{
memcpy(info->magic, DEVICE_MAGIC, DEVICE_MAGIC_SIZE);
if (is_secure_boot_enable()) {  //是否为安全启动
info->is_unlocked = 0;
#if !VBOOT_MOTA
info->is_unlock_critical = 0;
#endif
} else {
info->is_unlocked = 1;
#if !VBOOT_MOTA
info->is_unlock_critical = 1;
#endif
}
info->is_tampered = 0;
info->charger_screen_enabled = 0;
#if !VBOOT_MOTA
info->verity_mode = 1; //enforcing by default
#endif
write_device_info(info);
}
memcpy(dev, info, sizeof(device_info)); //回写devinfo
free(info);
}
else
{
read_device_info_flash(dev);        //从flash中读取devinfo
}
}

5.2 boot_linux_from_mmc分析

/bootable/bootloader/lk/app/aboot/aboot.c

int boot_linux_from_mmc(void)
{
// buf由前面BUF_DMA_ALIGN(buf, BOOT_IMG_MAX_PAGE_SIZE); 表示声明一个buf数组，并将boot img copy到其中
//Equal to max-supported pagesize
struct boot_img_hdr *hdr = (void*) buf;
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，并判断其是否存在相应的标志位，否则返回false
boot_into_recovery = 1;

if (!boot_into_recovery) {
memset(ffbm_mode_string, '\0', sizeof(ffbm_mode_string));
rcode = get_ffbm(ffbm_mode_string, sizeof(ffbm_mode_string));
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_ffbm = false;
//有前面定义确定uhdr的位置
uhdr = (struct boot_img_hdr *)EMMC_BOOT_IMG_HEADER_ADDR;
if (!memcmp(uhdr->magic, BOOT_MAGIC, BOOT_MAGIC_SIZE)) {//校验magic是否为"ANDROID! "
dprintf(INFO, "Unified boot method!\n");
hdr = uhdr; //将uhdr的值赋给hdr，即buf
goto unified_boot;
}
if (!boot_into_recovery) {  //正常启动
index = partition_get_index("boot");
ptn = partition_get_offset(index);
if(ptn == 0) {
dprintf(CRITICAL, "ERROR: No boot partition found\n");
return -1;
}
}
else {
index = partition_get_index("recovery");//进入recovery模式
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)) {  //将bootimg读取到buf中
dprintf(CRITICAL, "ERROR: Cannot read boot image header\n");
return -1;
}

if (memcmp(hdr->magic, BOOT_MAGIC, BOOT_MAGIC_SIZE)) {//校验
dprintf(CRITICAL, "ERROR: Invalid boot image header\n");
return -1;
}

if (hdr->page_size && (hdr->page_size != page_size)) {

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所占页的总大小
ramdisk_actual = ROUND_TO_PAGE(hdr->ramdisk_size, page_mask);   //ramdisk所占页的总大小

image_addr = (unsigned char *)target_get_scratch_address();

#if DEVICE_TREE
dt_actual = ROUND_TO_PAGE(hdr->dt_size, page_mask); //dt所占页的总大小
imagesize_actual = (page_size + kernel_actual + ramdisk_actual + dt_actual);    //image所占页的总大小
#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((uintptr_t) image_addr, imagesize_actual))   //aboot和bootimage地址是否重合
{
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 (%s) image (%d): start\n",
(!boot_into_recovery ? "boot" : "recovery"),imagesize_actual);
bs_set_timestamp(BS_KERNEL_LOAD_START);

if ((target_get_max_flash_size() - page_size) < imagesize_actual)//判断image能否完全放入ddr
{
dprintf(CRITICAL, "booimage  size is greater than DDR can hold\n");
return -1;
}

/* Read image without signature */
if (mmc_read(ptn + offset, (void *)image_addr, imagesize_actual))
{
dprintf(CRITICAL, "ERROR: Cannot read boot image\n");
return -1;
}

dprintf(INFO, "Loading (%s) image (%d): done\n",
(!boot_into_recovery ? "boot" : "recovery"),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);

/* Change the condition a little bit to include the test framework support.
* We would never reach this point if device is in fastboot mode, even if we did
* that means we are in test mode, so execute kernel authentication part for the
* tests */
if((target_use_signed_kernel() && (!device.is_unlocked)) || is_test_mode_enabled())
{
offset = imagesize_actual;
if (check_aboot_addr_range_overlap((uintptr_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, (void *)(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);//校验bootimg
/* The purpose of our test is done here */
if(is_test_mode_enabled() && auth_kernel_img)
return 0;
} else {
second_actual  = ROUND_TO_PAGE(hdr->second_size,  page_mask);   //second_size所占页的总大小
#ifdef TZ_SAVE_KERNEL_HASH
aboot_save_boot_hash_mmc((uint32_t) image_addr, imagesize_actual);
#endif /* TZ_SAVE_KERNEL_HASH */

#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(boot_verify_get_state() == ORANGE)
{
#if FBCON_DISPLAY_MSG
display_bootverify_menu(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
}
#endif

#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.
*/
if (is_gzip_package((unsigned char *)(image_addr + page_size), hdr->kernel_size))   //判断内核格式并解压
{
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_size = hdr->kernel_size;
}

/*
* 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/ramdisk/tags地址

/* Get virtual addresses since the hdr saves physical addresses. */
hdr->kernel_addr = VA((addr_t)(hdr->kernel_addr));  //转换为虚拟地址
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) ||
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); //移动kernel, ramdisk and device tree到相应的地址
memmove((void*) hdr->ramdisk_addr, (char *)(image_addr + page_size + kernel_actual), hdr->ramdisk_size);

#if DEVICE_TREE
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;
}

/* Its Error if, dt_hdr_size (table->num_entries * dt_entry size + Dev_Tree Header)
goes beyound hdr->dt_size*/
if (dt_hdr_size > ROUND_TO_PAGE(hdr->dt_size,hdr->page_size)) {
dprintf(CRITICAL, "ERROR: Invalid Device Tree size \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(dt_entry.offset > (UINT_MAX - dt_entry.size)) {
dprintf(CRITICAL, "ERROR: Device tree contents are Invalid\n");
return -1;
}

/* Ensure we are not overshooting dt_size with the dt_entry selected */
if ((dt_entry.offset + dt_entry.size) > hdr->dt_size) {
dprintf(CRITICAL, "ERROR: Device tree contents are Invalid\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,
(const char *)hdr->cmdline, board_machtype(),
(void *)hdr->ramdisk_addr, hdr->ramdisk_size);

return 0;
}

主要完成的工作：

1.将uhdr存入hdr/buf

2.读取bootimg到内存buf/hdr中

3.判断内核是否为gzip格式，如果是则解压

4.boot linux并通过boot_linux函数传递内核的地址，tags的地址，命令行参数，ramdisk地址和大小

5.2.1 boot_img_hdr结构分析

boot_img_hdr主要存储了bootimg、ramdisk、dt的地址

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;
};

5.2.2 boot_linux函数分析

void boot_linux(void *kernel, unsigned *tags,
const char *cmdline, unsigned machtype,
void *ramdisk, unsigned ramdisk_size)
{
unsigned char *final_cmdline;
#if DEVICE_TREE
int ret = 0;
#endif

void (*entry)(unsigned, unsigned, unsigned*) = (entry_func_ptr*)(PA((addr_t)kernel));//获取kernel的入口地址
uint32_t tags_phys = PA((addr_t)tags);
struct kernel64_hdr *kptr = ((struct kernel64_hdr*)(PA((addr_t)kernel)));//获取hdr

ramdisk = (void *)PA((addr_t)ramdisk);

final_cmdline = update_cmdline((const char*)cmdline);//更新cmdline，返回的final_cmdline是各个cmdline的字符数组

#if DEVICE_TREE
dprintf(INFO, "Updating device tree: start\n");

/* Update the Device Tree */
ret = update_device_tree((void *)tags,(const char *)final_cmdline, ramdisk, ramdisk_size);//将更新的cmdline存入dt
if(ret)
{
dprintf(CRITICAL, "ERROR: Updating Device Tree Failed \n");
ASSERT(0);
}
dprintf(INFO, "Updating device tree: done\n");
#else
/* Generating the Atags */
generate_atags(tags, final_cmdline, ramdisk, ramdisk_size);//将更新的cmdline、ramdisk存入tags对应的地址
#endif

free(final_cmdline);//释放在update_cmdline中申请的内存空间

#if VERIFIED_BOOT
#if !VBOOT_MOTA
if (device.verity_mode == 0) {
#if FBCON_DISPLAY_MSG
display_bootverify_menu(DISPLAY_MENU_LOGGING);
wait_for_users_action();
#else
dprintf(CRITICAL,
"The dm-verity is not started in enforcing mode.\nWait for 5 seconds before proceeding\n");
mdelay(5000);
#endif
}

#endif
#endif

#if VERIFIED_BOOT
/* Write protect the device info */
if (!boot_into_recovery && target_build_variant_user() && devinfo_present && mmc_write_protect("devinfo", 1))
{
dprintf(INFO, "Failed to write protect dev info\n");
ASSERT(0);
}
#endif

/* Turn off splash screen if enabled */
#if DISPLAY_SPLASH_SCREEN
target_display_shutdown();  //显示关机图像
#endif

/* Perform target specific cleanup */
target_uninit();    //执行目标特定清除

dprintf(INFO, "booting linux @ %p, ramdisk @ %p (%d), tags/device tree @ %p\n",
entry, ramdisk, ramdisk_size, (void *)tags_phys);

enter_critical_section(); //进入临界区

/* do any platform specific cleanup before kernel entry */
platform_uninit();  //执行平台特定清除

arch_disable_cache(UCACHE);

#if ARM_WITH_MMU
arch_disable_mmu();//禁止mmu
#endif
bs_set_timestamp(BS_KERNEL_ENTRY);

if (IS_ARM64(kptr))//判断是否是64位的内核
/* Jump to a 64bit kernel */
scm_elexec_call((paddr_t)kernel, tags_phys);//执行内核启动
else
/* Jump to a 32bit kernel */
entry(0, machtype, (unsigned*)tags_phys);
}

主要功能：

1. 获取kernel、ramdisk的地址

2. 更新cmdline，并存储在dt/tags分区中

3. 回收平台特定资源

4. 进入kernel

5.2.3 ffbm分析

如果boot_linux_from_xxx函数中check_format_bit()=0，且misc分区的0地址为”ffbm-“,则boot_into_ffbm=true。该过程通过get_ffbm函数实现。

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;
}