Android Kernel - Boot Loader

最新推荐文章于 2024-12-27 20:18:30 发布

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最新推荐文章于 2024-12-27 20:18:30 发布 · 1.2w 阅读

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#android #thread #descriptor #struct #timer #list

本文详细解析了Android Bootloader的核心组件LK（Little Kernel）的主要代码流程，包括kmain函数、线程初始化、架构早期初始化、平台初始化、定时器初始化等关键步骤。通过阅读源代码，介绍了内存管理、中断处理、调试机制、设备初始化等方面的知识，为理解Android启动流程提供了深入的洞察。

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Android Boot loader 的 code 在 bootable/bootloader/lk 底下, LK 是 Little Kernel 的缩写, 是 andriod bootloader 的核心精神.

入口函数在 kernel/main.c 中的 kmain(), 以下就来读读这一段 code.

void kmain(void) { // get us into some sort of thread context thread_init_early(); // early arch stuff arch_early_init(); // do any super early platform initialization platform_early_init(); // do any super early target initialization target_early_init(); dprintf(INFO, "welcome to lk/n/n"); // deal with any static constructors dprintf(SPEW, "calling constructors/n"); call_constructors(); // bring up the kernel heap dprintf(SPEW, "initializing heap/n"); heap_init(); // initialize the threading system dprintf(SPEW, "initializing threads/n"); thread_init(); // initialize the dpc system dprintf(SPEW, "initializing dpc/n"); dpc_init(); // initialize kernel timers dprintf(SPEW, "initializing timers/n"); timer_init(); #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)); // enable interrupts exit_critical_section(); // become the idle thread thread_become_idle(); #else bootstrap_nandwrite(); #endif }

In include/debug.h: 我们可以看到 dprintf 的第一个参数是代表 debug level.

/* debug levels */ #define CRITICAL 0 #define ALWAYS 0 #define INFO 1 #define SPEW 2

In include/debug.h:

#define dprintf(level, x...) do { if ((level) <= DEBUGLEVEL) { _dprintf(x); } } while (0)

所以 dprintf 会依 DEBUGLEVEL 来判断是否输出信息.

来看第一个 call 的函数: thread_init_early, define in thread.c

void thread_init_early(void) { int i; /* initialize the run queues */ for (i=0; i < NUM_PRIORITIES; i++) list_initialize(&run_queue[i]); /* initialize the thread list */ list_initialize(&thread_list); /* create a thread to cover the current running state */ thread_t *t = &bootstrap_thread; init_thread_struct(t, "bootstrap"); /* half construct this thread, since we're already running */ t->priority = HIGHEST_PRIORITY; t->state = THREAD_RUNNING; t->saved_critical_section_count = 1; list_add_head(&thread_list, &t->thread_list_node); current_thread = t; }

#define NUM_PRIORITIES 32 in include/kernel/thread.h

list_initialize() defined in include/list.h: initialized a list

static inline void list_initialize(struct list_node *list) { list->prev = list->next = list; }

run_queue 是 static struct list_node run_queue[NUM_PRIORITIES]

thread_list 是 static struct list_node thread_list

再来要 call 的函数是: arch_early_init() defined in arch/arm/arch.c

void arch_early_init(void) { /* turn off the cache */ arch_disable_cache(UCACHE); /* 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); #endif #if ARM_WITH_MMU arm_mmu_init(); platform_init_mmu_mappings(); #endif /* turn the cache back on */ arch_enable_cache(UCACHE); #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)); /* set enable bit in fpexc */ val = (1<<30); __asm__ volatile("mcr p10, 7, %0, c8, c0, 0" :: "r" (val)); #endif }

现代操作系统普遍采用虚拟内存管理（Virtual Memory Management）机制，这需要处理器中的MMU（Memory Management Unit，

内存管理单元）提供支持。

CPU执行单元发出的内存地址将被MMU截获，从CPU到MMU的地址称为虚拟地址（Virtual Address，以下简称VA），而MMU将这个地

址翻译成另一个地址发到CPU芯片的外部地址引脚上，也就是将VA映射成PA

MMU将VA映射到PA是以页（Page）为单位的，32位处理器的页尺寸通常是4KB。例如，MMU可以通过一个映射项将VA的一页

0xb7001000~0xb7001fff映射到PA的一页0x2000~