ICode9

精准搜索请尝试: 精确搜索
首页 > 系统相关> 文章详细

6.学习Camera之——linux内核启动流程

2021-11-29 19:02:46  阅读:172  来源: 互联网

标签:__ kernel 初始化 init Camera command 内核 linux


本文以Linux3.14版本源码为例分析其启动流程。各版本启动代码略有不同,但核心流程与思想万变不离
其宗。

内核映像被加载到内存并获得控制权之后,内核启动流程开始。通常,内核映像以压缩形式存储,并不是
一个可以执行的内核。因此,内核阶段的首要工作是自解压内核映像。

内核编译生成vmliunx后,通常会对其进行压缩,得到zImage(小内核,小于512KB)或bzImage(大内核,
大于512KB)。在它们的头部嵌有解压缩程序。

通过linux/arch/arm/boot/compressed目录下的Makefile寻找到vmlinux文件的链接脚本(vmlinux.lds),从中查找系统启动入口函数。

$(obj)/vmlinux: $(obj)/vmlinux.lds $(obj)/$(HEAD) $(obj)/piggy.$(suffix_y).o \
		$(addprefix $(obj)/, $(OBJS)) $(lib1funcs) $(ashldi3) \
		$(bswapsdi2) FORCE
	@$(check_for_multiple_zreladdr)
	$(call if_changed,ld)
	@$(check_for_bad_syms)
vmlinux.lds(linux/arch/arm/kernel/vmlinux.lds)链接脚本开头内容

OUTPUT_ARCH(arm)
ENTRY(stext)
jiffies = jiffies_64;
SECTIONS
{
	。
	。
	。
得到内核入口函数为 stext(linux/arch/arm/kernel/head.S)

内核引导阶段
ENTRY(stext)
	。
	。
	。
	bl	__lookup_processor_type	@ r5=procinfo r9=cpuid   //处理器是否支持
	movs	r10, r5				@ invalid processor (r5=0)?
 THUMB( it	eq )		@ force fixup-able long branch encoding
	beq	__error_p			@ yes, error 'p'           //不支持则打印错误信息
 
          。
	。
	。
	bl	__create_page_tables          //创建页表
 
	/*
	 * The following calls CPU specific code in a position independent
	 * manner.  See arch/arm/mm/proc-*.S for details.  r10 = base of
	 * xxx_proc_info structure selected by __lookup_processor_type
	 * above.  On return, the CPU will be ready for the MMU to be
	 * turned on, and r0 will hold the CPU control register value.
	 */
	ldr	r13, =__mmap_switched		@ address to jump to after   //保存MMU使能后跳转地址
						@ mmu has been enabled
	adr	lr, BSYM(1f)			@ return (PIC) address
	mov	r8, r4				@ set TTBR1 to swapper_pg_dir
 ARM(	add	pc, r10, #PROCINFO_INITFUNC	)
 THUMB(	add	r12, r10, #PROCINFO_INITFUNC	)
 THUMB(	mov	pc, r12				)
1:	b	__enable_mmu                                                                           //使能MMU后跳转到__mmap_switched
查找标签__mmap_switched所在位置:/linux/arch/arm/kernel/head-common.S
__mmap_switched:
	/*
	 * The following fragment of code is executed with the MMU on in MMU mode,
	 * and uses absolute addresses; this is not position independent.
	 *
	 *  r0  = cp#15 control register
	 *  r1  = machine ID
	 *  r2  = atags/dtb pointer
	 *  r9  = processor ID
	 */
	//保存设备信息、设备树及启动参数存储地址
	。
	。
	。
	b	start_kernel
 

内核初始化阶段
从start_kernel函数开始,内核进入C语言部分,完成内核的大部分初始化工作。

函数所在位置:/linux/init/Main.c

start_kernel涉及大量初始化工作,只例举重要的初始化工作。

asmlinkage void __init start_kernel(void)
{
	……                 //类型判断
	smp_setup_processor_id();                                                         //smp相关,返回启动CPU号
	……
	local_irq_disable();                                                                   //关闭当前CPU中断
	early_boot_irqs_disabled = true;
/*
 * Interrupts are still disabled. Do necessary setups, then
 * enable them
 */
	boot_cpu_init();
	page_address_init();             //初始化页地址
	pr_notice("%s", linux_banner);         //显示内核版本信息
	setup_arch(&command_line);
	mm_init_owner(&init_mm, &init_task);
	mm_init_cpumask(&init_mm);
	setup_command_line(command_line);
	setup_nr_cpu_ids();
	setup_per_cpu_areas();
	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */
 
	build_all_zonelists(NULL, NULL);
	page_alloc_init();                                                                            //页内存申请初始化
 
	pr_notice("Kernel command line: %s\n", boot_command_line);                                     //打印内核启动命令行参数
	parse_early_param();
	parse_args("Booting kernel", static_command_line, __start___param,
		   __stop___param - __start___param,
		   -1, -1, &unknown_bootoption);
 
	……
	/*
	 * Set up the scheduler prior starting any interrupts (such as the
	 * timer interrupt). Full topology setup happens at smp_init()
	 * time - but meanwhile we still have a functioning scheduler.
	 */
	sched_init();                                                                                    //进程调度器初始化

	/*
	 * Disable preemption - early bootup scheduling is extremely
	 * fragile until we cpu_idle() for the first time.
	 */
	preempt_disable();                                                                                    //禁止内核抢占

	if (WARN(!irqs_disabled(), "Interrupts were enabled *very* early, fixing it\n"))
		local_irq_disable();                                                                      //检查关闭CPU中断	
	
          /*大量初始化内容 见名知意*/
	idr_init_cache();
	rcu_init();
	tick_nohz_init();
	context_tracking_init();
	radix_tree_init();
	/* init some links before init_ISA_irqs() */
	early_irq_init();
	init_IRQ();
	tick_init();
	init_timers();
	hrtimers_init();
	softirq_init();
	timekeeping_init();
	time_init();
	sched_clock_postinit();
	perf_event_init();
	profile_init();
	call_function_init();
	WARN(!irqs_disabled(), "Interrupts were enabled early\n");
	early_boot_irqs_disabled = false;
	local_irq_enable();                                                                            //本地中断可以使用了
 
	kmem_cache_init_late();
 
	/*
	 * HACK ALERT! This is early. We're enabling the console before
	 * we've done PCI setups etc, and console_init() must be aware of
	 * this. But we do want output early, in case something goes wrong.
	 */
	console_init();                                                                            //初始化控制台,可以使用printk了

	if (panic_later)
		panic("Too many boot %s vars at `%s'", panic_later,
		      panic_param);
 
	lockdep_info();
 
	/*
	 * Need to run this when irqs are enabled, because it wants
	 * to self-test [hard/soft]-irqs on/off lock inversion bugs
	 * too:
	 */
	locking_selftest();
 
#ifdef CONFIG_BLK_DEV_INITRD
	if (initrd_start && !initrd_below_start_ok &&
	    page_to_pfn(virt_to_page((void *)initrd_start)) < min_low_pfn) {
		pr_crit("initrd overwritten (0x%08lx < 0x%08lx) - disabling it.\n",
		    page_to_pfn(virt_to_page((void *)initrd_start)),
		    min_low_pfn);
		initrd_start = 0;
	}
#endif
	page_cgroup_init();
	debug_objects_mem_init();
	kmemleak_init();
	setup_per_cpu_pageset();
	numa_policy_init();
	if (late_time_init)
		late_time_init();
	sched_clock_init();
	calibrate_delay();
	pidmap_init();
	anon_vma_init();
	acpi_early_init();
#ifdef CONFIG_X86
	if (efi_enabled(EFI_RUNTIME_SERVICES))
		efi_enter_virtual_mode();
#endif
#ifdef CONFIG_X86_ESPFIX64
	/* Should be run before the first non-init thread is created */
	init_espfix_bsp();
#endif
	thread_info_cache_init();
	cred_init();
	fork_init(totalram_pages);                                                             //初始化fork

	proc_caches_init();
	buffer_init();
	key_init();
	security_init();
	dbg_late_init();
	vfs_caches_init(totalram_pages);                                                      //虚拟文件系统初始化

	signals_init();
	/* rootfs populating might need page-writeback */
	page_writeback_init();
#ifdef CONFIG_PROC_FS
	proc_root_init();
#endif
	cgroup_init();
	cpuset_init();
	taskstats_init_early();
	delayacct_init();
 
	check_bugs();
 
	sfi_init_late();
 
	if (efi_enabled(EFI_RUNTIME_SERVICES)) {
		efi_late_init();
		efi_free_boot_services();
	}
 
	ftrace_init();
 
	/* Do the rest non-__init'ed, we're now alive */
	rest_init();  //函数最后调用rest_init()函数
}

/*最重要使命:创建kernel_init进程,并进行后续初始化*/
static noinline void __init_refok rest_init(void)
{
	int pid;
 
	rcu_scheduler_starting();
	/*
	 * We need to spawn init first so that it obtains pid 1, however
	 * the init task will end up wanting to create kthreads, which, if
	 * we schedule it before we create kthreadd, will OOPS.
	 */
	
	kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);                             //创建kernel_init进程
	
	numa_default_policy();
	pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
	rcu_read_lock();
	kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
	rcu_read_unlock();
	complete(&kthreadd_done);
 
	/*
	 * The boot idle thread must execute schedule()
	 * at least once to get things moving:
	 */
	init_idle_bootup_task(current);
	schedule_preempt_disabled();
	/* Call into cpu_idle with preempt disabled */
	//cpu_idle就是在系统闲置时用来降低电力的使用和减少热的产生的空转函数,函数至此不再返回,
其余工作从kernel_init进程处发起
	cpu_startup_entry(CPUHP_ONLINE);
}
kernel_init函数将完成设备驱动程序的初始化,并调用init_post函数启动用户进程

部分书籍介绍的内核启动流程基于经典的2.6版本,kernel_init函数还会调用init_post函数专门负责
_init进程的启动,现版本已经被整合到了一起。

static int __ref kernel_init(void *unused)
{
	int ret;
 
	kernel_init_freeable();     //该函数中完成smp开启  驱动初始化 共享内存初始化等工作
	/* need to finish all async __init code before freeing the memory */
	async_synchronize_full();
	free_initmem();         //初始化尾声,清除内存无用数据
	mark_rodata_ro();
	system_state = SYSTEM_RUNNING;
	numa_default_policy();
 
	flush_delayed_fput();
 
	if (ramdisk_execute_command) {
		ret = run_init_process(ramdisk_execute_command);
		if (!ret)
			return 0;
		pr_err("Failed to execute %s (error %d)\n",
		       ramdisk_execute_command, ret);
	}
 
	/*
	 * We try each of these until one succeeds.
	 *
	 * The Bourne shell can be used instead of init if we are
	 * trying to recover a really broken machine.
     * 寻找init函数,创建一号进程_init (第一个用户空间进程)*/
	if (execute_command) {
		ret = run_init_process(execute_command);
		if (!ret)
			return 0;
		pr_err("Failed to execute %s (error %d).  Attempting defaults...\n",
			execute_command, ret);
	}
	if (!try_to_run_init_process("/sbin/init") ||
	    !try_to_run_init_process("/etc/init") ||
	    !try_to_run_init_process("/bin/init") ||
	    !try_to_run_init_process("/bin/sh"))
		return 0;
 
	panic("No working init found.  Try passing init= option to kernel. "
	      "See Linux Documentation/init.txt for guidance.");
}
static int __ref kernel_init(void *unused)
{
	int ret;
 
	kernel_init_freeable(); //该函数中完成smp开启  驱动初始化 共享内存初始化等工作
	/* need to finish all async __init code before freeing the memory */
	async_synchronize_full();
	free_initmem();        //初始化尾声,清除内存无用数据
	mark_rodata_ro();
	system_state = SYSTEM_RUNNING;
	numa_default_policy();
 
	flush_delayed_fput();
 
	if (ramdisk_execute_command) {
		ret = run_init_process(ramdisk_execute_command);
		if (!ret)
			return 0;
		pr_err("Failed to execute %s (error %d)\n",
		       ramdisk_execute_command, ret);
	}
 
	/*
	 * We try each of these until one succeeds.
	 *
	 * The Bourne shell can be used instead of init if we are
	 * trying to recover a really broken machine.
	 * 寻找init函数,创建一号进程_init (第一个用户空间进程)*/
	if (execute_command) {
		ret = run_init_process(execute_command);
		if (!ret)
			return 0;
		pr_err("Failed to execute %s (error %d).  Attempting defaults...\n",
			execute_command, ret);
	}
	if (!try_to_run_init_process("/sbin/init") ||
	    !try_to_run_init_process("/etc/init") ||
	    !try_to_run_init_process("/bin/init") ||
	    !try_to_run_init_process("/bin/sh"))
		return 0;
 
	panic("No working init found.  Try passing init= option to kernel. "
	      "See Linux Documentation/init.txt for guidance.");
}
 

到此,内核初始化已经接近尾声,所有的初始化函数都已经调用,因此free_initmem函数可以舍弃内存的__init_begin至__init_end之间的数据。

当内核被引导并进行初始化后,内核启动了自己的第一个用户空间应用程序_init,这是调用的第一个使用
标准C库编译的程序,其进程编号时钟为1.

_init负责出发其他必须的进程,以使系统进入整体可用的状态。

以下为内核启动流程图:

 

标签:__,kernel,初始化,init,Camera,command,内核,linux
来源: https://blog.csdn.net/Linux_zhicheng/article/details/121616782

本站声明: 1. iCode9 技术分享网(下文简称本站)提供的所有内容,仅供技术学习、探讨和分享;
2. 关于本站的所有留言、评论、转载及引用,纯属内容发起人的个人观点,与本站观点和立场无关;
3. 关于本站的所有言论和文字,纯属内容发起人的个人观点,与本站观点和立场无关;
4. 本站文章均是网友提供,不完全保证技术分享内容的完整性、准确性、时效性、风险性和版权归属;如您发现该文章侵犯了您的权益,可联系我们第一时间进行删除;
5. 本站为非盈利性的个人网站,所有内容不会用来进行牟利,也不会利用任何形式的广告来间接获益,纯粹是为了广大技术爱好者提供技术内容和技术思想的分享性交流网站。

专注分享技术,共同学习,共同进步。侵权联系[81616952@qq.com]

Copyright (C)ICode9.com, All Rights Reserved.

ICode9版权所有