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.cvoid 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.cvoid 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 4096struct 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