嵌入式Linux2.6内核启动流程)

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1、真诚为您提供优质参考资料，若有不当之处，请指正。Linux内核构成(国嵌)Linux/arch/arm/boot/compressed/head.s1解压缩2初始化3启动应用程序1 arch/arm/boot/compressed/Makefile arch/arm/boot/compressed/vmlinux.lds2. arch/arm/kernel/vmlinux.ldsLinux内核启动流程（国嵌）arch/arm/boot/compressed/start.S（head.s负责解压缩）Start: .type start,#function .rept 8 mov r0, r0 .

2、endr b 1f .word 0x016f2818 Magic numbers to help the loader .word start absolute load/run zImage address .word _edata zImage end address1: mov r7, r1 save architecture ID mov r8, r2 save atags pointer这也标志着u-boot将系统完全的交给了OS，bootloader生命终止。之后代码在133行会读取cpsr并判断是否处理器处于supervisor模式从u-boot进入kernel，系统已经处于SV

3、C32模式；而利用angel进入则处于user模式，还需要额外两条指令。之后是再次确认中断关闭，并完成cpsr写入 mrs r2, cpsr get current mode tst r2, #3 not user? bne not_angel mov r0, #0x17 angel_SWIreason_EnterSVC swi 0x123456 angel_SWI_ARMnot_angel: mrs r2, cpsr turn off interrupts to orr r2, r2, #0xc0 prevent angel from running msr cpsr_c, r2 然后在LC

4、0地址处将分段信息导入r0-r6、ip、sp等寄存器，并检查代码是否运行在与链接时相同的目标地址，以决定是否进行处理。由于现在很少有人不使用loader和tags，将zImage烧写到rom直接从0x0位置执行，所以这个处理是必须的（但是zImage的头现在也保留了不用loader也可启动的能力）。arm架构下自解压头一般是链接在0x0地址而被加载到0x30008000运行，所以要修正这个变化。涉及到r5寄存器存放的zImage基地址 r6和r12（即ip寄存器）存放的got（global offset table） r2和r3存放的bss段起止地址 sp栈指针地址 很简单，这些寄存器统统被加

5、上一个你也能猜到的偏移地址 0x30008000。该地址是s3c2410相关的，其他的ARM处理器可以参考下表PXA2xx是0xa0008000 IXP2x00和IXP4xx是0x00008000 Freescale i.MX31/37是0x80008000 TI davinci DM64xx是0x80008000 TI omap系列是0x80008000 AT91RM/SAM92xx系列是0x20008000 Cirrus EP93xx是0x00008000 这些操作发生在代码172行开始的地方，下面只粘贴一部分 add r5, r5, r0 add r6, r6, r0 add ip, i

6、p, r0后面在211行进行bss段的清零工作not_relocated: mov r0, #01: str r0, r2, #4 clear bss str r0, r2, #4 str r0, r2, #4 str r0, r2, #4 cmp r2, r3 blo 1b 然后224行，打开cache，并为后面解压缩设置64KB的临时malloc空间 bl cache_on mov r1, sp malloc space above stack add r2, sp, #0x10000 64k max 接下来238行进行检查，确定内核解压缩后的Image目标地址是否会覆盖到zImage头，

7、如果是则准备将zImage头转移到解压出来的内核后面 cmp r4, r2 bhs wont_overwrite sub r3, sp, r5 compressed kernel size add r0, r4, r3, lsl #2 allow for 4x expansion cmp r0, r5 bls wont_overwrite mov r5, r2 decompress after malloc space mov r0, r5 mov r3, r7 bl decompress_kernel真实情况在大多数的应用中，内核编译都会把压缩的zImage和非压缩的Image链接到同样的地

8、址，s3c2410平台下即是0x30008000。这样做的好处是，人们不用关心内核是Image还是zImage，放到这个位置执行就OK，所以在解压缩后zImage头必须为真正的内核让路。在250行解压完毕，内核长度返回值存放在r0寄存器里。在内核末尾空出128字节的栈空间用，并且使其长度128字节对齐。 add r0, r0, #127 + 128 alignment + stack bic r0, r0, #127 align the kernel length算出搬移代码的参数：计算内核末尾地址并存放于r1寄存器，需要搬移代码原来地址放在r2，需要搬移的长度放在r3。然后执行搬移，并设置好

9、sp指针指向新的栈（原来的栈也会被内核覆盖掉） add r1, r5, r0 end of decompressed kernel adr r2, reloc_start ldr r3, LC1 add r3, r2, r31: ldmia r2!, r9 - r14 copy relocation code stmia r1!, r9 - r14 ldmia r2!, r9 - r14 stmia r1!, r9 - r14 cmp r2, r3 blo 1b add sp, r1, #128 relocate the stack搬移完成后刷新cache，因为代码地址变化了不能让cache再

10、命中被内核覆盖的老地址。然后跳转到新的地址继续执行 bl cache_clean_flush add pc, r5, r0 call relocation code注意zImage在解压后的搬移和跳转会给gdb调试内核带来麻烦。因为用来调试的符号表是在编译是生成的，并不知道以后会被搬移到何处去，只有在内核解压缩完成之后，根据计算出来的参数“告诉”调试器这个变化。以撰写本文时使用的zImage为例，内核自解压头重定向后，reloc_start地址由0x30008360变为0x30533e60。故我们要把vmlinux的符号表也相应的从0x30008000后移到0x30533b00开始，这样gdb

11、就可以正确的对应源代码和机器指令。随着头部代码移动到新的位置，不会再和内核的目标地址冲突，可以开始内核自身的搬移了。此时r0寄存器存放的是内核长度（严格的说是长度外加128Byte的栈），r4存放的是内核的目的地址0x30008000，r5是目前内核存放地址，r6是CPU ID，r7是machine ID，r8是atags地址。代码从501行开始reloc_start: add r9, r5, r0 sub r9, r9, #128 do not copy the stack debug_reloc_start mov r1, r41: .rept 4 ldmia r5!, r0, r2, r

12、3, r10 - r14 relocate kernel stmia r1!, r0, r2, r3, r10 - r14 .endr cmp r5, r9 blo 1b add sp, r1, #128 relocate the stack接下来在516行清除并关闭cache，清零r0，将machine ID存入r1，atags指针存入r2，再跳入0x30008000执行真正的内核Imagecall_kernel: bl cache_clean_flush bl cache_off mov r0, #0 must be zero mov r1, r7 restore architecture

13、 number mov r2, r8 restore atags pointer mov pc, r4 call kernel内核代码入口在arch/arm/kernel/head.S文件的83行。首先进入SVC32模式，并查询CPU ID，检查合法性 msr cpsr_c, #PSR_F_BIT | PSR_I_BIT | SVC_MODE ensure svc mode and irqs disabled mrc p15, 0, r9, c0, c0 get processor id bl _lookup_processor_type r5=procinfo r9=cpuid movs r

14、10, r5 invalid processor (r5=0)? beq _error_p yes, error p接着在87行进一步查询machine ID并检查合法性 bl _lookup_machine_type r5=machinfo movs r8, r5 invalid machine (r5=0)? beq _error_a yes, error a其中_lookup_processor_type在linux-2.6.24-moko-linuxbj/arch/arm/kernel/head-common.S文件的149行，该函数首将标号3的实际地址加载到r3，然后将编译时生成的_

15、proc_info_begin虚拟地址载入到r5，_proc_info_end虚拟地址载入到r6，标号3的虚拟地址载入到r7。由于adr伪指令和标号3的使用，以及_proc_info_begin等符号在linux-2.6.24-moko-linuxbj/arch/arm/kernel/vmlinux.lds而不是代码中被定义，此处代码不是非常直观，想弄清楚代码缘由的读者请耐心阅读这两个文件和adr伪指令的说明。r3和r7分别存储的是同一位置标号3的物理地址（由于没有启用mmu，所以当前肯定是物理地址）和虚拟地址，所以儿者相减即得到虚拟地址和物理地址之间的offset。利用此offset，将r5

16、和r6中保存的虚拟地址转变为物理地址_lookup_processor_type: adr r3, 3f ldmda r3, r5 - r7 sub r3, r3, r7 get offset between virt&phys add r5, r5, r3 convert virt addresses to add r6, r6, r3 physical address space然后从proc_info中读出内核编译时写入的processor ID和之前从cpsr中读到的processor ID对比，查看代码和CPU硬件是否匹配（想在arm920t上运行为cortex-a8编译的内核？不让

17、！）。如果编译了多种处理器支持，如versatile板，则会循环每种type依次检验，如果硬件读出的ID在内核中找不到匹配，则r5置0返回1:ldmiar5, r3, r4 value, maskandr4, r4, r9 mask wanted bitsteqr3, r4beq2faddr5, r5, #PROC_INFO_SZ sizeof(proc_info_list)cmpr5, r6blo1bmovr5, #0 unknown processor2:movpc, lr _lookup_machine_type在linux-2.6.24-moko-linuxbj/arch/arm/ke

18、rnel/head-common.S文件的197行，编码方法与检查processor ID完全一样，请参考前段_lookup_machine_type:adrr3, 3bldmiar3, r4, r5, r6subr3, r3, r4 get offset between virt&physaddr5, r5, r3 convert virt addresses toaddr6, r6, r3 physical address space1:ldrr3, r5, #MACHINFO_TYPE get machine typeteqr3, r1 matches loader number?beq

19、2f foundaddr5, r5, #SIZEOF_MACHINE_DESC next machine_desccmpr5, r6blo1bmovr5, #0 unknown machine2:movpc, lr代码回到head.S第92行，检查atags合法性，然后创建初始页表bl_vet_atagsbl_create_page_tables 创建页表的代码在218行，首先将内核起始地址-0x4000到内核起始地址之间的16K存储器清0_create_page_tables:pgtblr4 page table address/* * Clear the 16K level 1 swapp

20、er page table */movr0, r4movr3, #0addr6, r0, #0x40001:strr3, r0, #4strr3, r0, #4strr3, r0, #4strr3, r0, #4teqr0, r6bne1b 然后在234行将proc_info中的mmu_flags加载到r7ldrr7, r10, #PROCINFO_MM_MMUFLAGS mm_mmuflags在242行将PC指针右移20位，得到内核第一个1MB空间的段地址存入r6，在s3c2410平台该值是0x300。接着根据此值存入映射标识movr6, pc, lsr #20 start of kerne

21、l sectionorrr3, r7, r6, lsl #20 flags + kernel basestrr3, r4, r6, lsl #2 identity mapping完成页表设置后回到102行，为打开虚拟地址映射作准备。设置sp指针，函数返回地址lr指向_enable_mmu，并跳转到linux-2.6.24-moko-linuxbj/arch/arm/mm/proc-arm920.S的386行，清除I-cache、D-cache、write buffer和TLB_arm920_setup:movr0, #0mcrp15, 0, r0, c7, c7 invalidate I,D

22、caches on v4mcrp15, 0, r0, c7, c10, 4 drain write buffer on v4#ifdef CONFIG_MMUmcrp15, 0, r0, c8, c7 invalidate I,D TLBs on v4#endif然后返回head.S的158行，加载domain和页表，跳转到_turn_mmu_on_enable_mmu:#ifdef CONFIG_ALIGNMENT_TRAPorrr0, r0, #CR_A#elsebicr0, r0, #CR_A#endif#ifdef CONFIG_CPU_DCACHE_DISABLEbicr0, r0,

23、 #CR_C#endif#ifdef CONFIG_CPU_BPREDICT_DISABLEbicr0, r0, #CR_Z#endif#ifdef CONFIG_CPU_ICACHE_DISABLEbicr0, r0, #CR_I#endifmovr5, #(domain_val(DOMAIN_USER, DOMAIN_MANAGER) | domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | domain_val(DOMAIN_TABLE, DOMAIN_MANAGER) | domain_val(DOMAIN_IO, DOMAIN_CLIENT)mcrp

24、15, 0, r5, c3, c0, 0 load domain access registermcrp15, 0, r4, c2, c0, 0 load page table pointerb_turn_mmu_on在194行把mmu使能位写入mmu，激活虚拟地址。然后将原来保存在sp中的地址载入pc，跳转到head-common.S的_mmap_switched，至此代码进入虚拟地址的世界movr0, r0mcrp15, 0, r0, c1, c0, 0 write control regmrcp15, 0, r3, c0, c0, 0 read id regmovr3, r3movr3,

25、 r3movpc, r13在head-common.S的37行开始清除内核bss段，processor ID保存在r9，machine ID报存在r1，atags地址保存在r2，并将控制寄存器保存到r7定义的内存地址。接下来跳入linux-2.6.24-moko-linuxbj/init/main.c的507行，start_kernel函数。这里只粘贴部分代码（第一个C语言函数，作一系列的初始化）_mmap_switched:adrr3, _switch_data + 4ldmiar3!, r4, r5, r6, r7cmpr4, r5 Copy data segment if needed1

26、:cmpner5, r6ldrnefp, r4, #4strnefp, r5, #4bne1basmlinkage void _init start_kernel(void)char * command_line;extern struct kernel_param _start_param, _stop_param;smp_setup_processor_id();/* * Need to run as early as possible, to initialize the * lockdep hash: */lockdep_init();debug_objects_early_init(

27、);cgroup_init_early();local_irq_disable();early_boot_irqs_off();early_init_irq_lock_class();/* * Interrupts are still disabled. Do necessary setups, then * enable them */lock_kernel();tick_init();boot_cpu_init();page_address_init();printk(KERN_NOTICE);printk(linux_banner);setup_arch(&command_line);m

28、m_init_owner(&init_mm, &init_task);setup_command_line(command_line);setup_per_cpu_areas();setup_nr_cpu_ids();smp_prepare_boot_cpu();/* arch-specific boot-cpu hooks */* * Set up the scheduler prior starting any interrupts (such as the * timer interrupt). Full topology setup happens at smp_init() * ti

29、me - but meanwhile we still have a functioning scheduler. */sched_init();/* * Disable preemption - early bootup scheduling is extremely * fragile until we cpu_idle() for the first time. */preempt_disable();build_all_zonelists();page_alloc_init();printk(KERN_NOTICE Kernel command line: %sn, boot_comm

30、and_line);parse_early_param();parse_args(Booting kernel, static_command_line, _start_param, _stop_param - _start_param, &unknown_bootoption);if (!irqs_disabled() printk(KERN_WARNING start_kernel(): bug: interrupts were enabled *very* early, fixing itn);local_irq_disable();sort_main_extable();trap_in

31、it();rcu_init();/* init some links before init_ISA_irqs() */early_irq_init();init_IRQ();pidhash_init();init_timers();hrtimers_init();softirq_init();timekeeping_init();time_init();sched_clock_init();profile_init();if (!irqs_disabled()printk(KERN_CRIT start_kernel(): bug: interrupts were enabled early

32、n);early_boot_irqs_on();local_irq_enable();/* * HACK ALERT! This is early. Were enabling the console before * weve 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();if (panic_later)panic(panic_later, panic_par

33、am);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_INITRDif (initrd_start & !initrd_below_start_ok & page_to_pfn(virt_to_page(void *)initrd_start) min_low_pfn) pri

34、ntk(KERN_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;#endifvmalloc_init();vfs_caches_init_early();cpuset_init_early();page_cgroup_init();mem_init();enable_debug_pagealloc();cpu_hotplug_init();kmem_cache_init

35、();debug_objects_mem_init();idr_init_cache();setup_per_cpu_pageset();numa_policy_init();if (late_time_init)late_time_init();calibrate_delay();pidmap_init();pgtable_cache_init();prio_tree_init();anon_vma_init();#ifdef CONFIG_X86if (efi_enabled)efi_enter_virtual_mode();#endifthread_info_cache_init();c

36、red_init();fork_init(num_physpages);proc_caches_init();buffer_init();key_init();security_init();vfs_caches_init(num_physpages);radix_tree_init();signals_init();/* rootfs populating might need page-writeback */page_writeback_init();#ifdef CONFIG_PROC_FSproc_root_init();#endifcgroup_init();cpuset_init

37、();taskstats_init_early();delayacct_init();check_bugs();acpi_early_init(); /* before LAPIC and SMP init */ftrace_init();/* Do the rest non-_inited, were now alive */rest_init();tatic noinline void _init_refok rest_init(void)_releases(kernel_lock)int pid;kernel_thread(kernel_init, NULL, CLONE_FS | CL

38、ONE_SIGHAND);numa_default_policy();pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);unlock_kernel();/* * The boot idle thread must execute schedule() * at least once to get things moving: */init_idle_bootup_task(current);rcu_scheduler

39、_starting();preempt_enable_no_resched();schedule();preempt_disable();/* Call into cpu_idle with preempt disabled */cpu_idle();static noinline int init_post(void)/* need to finish all async _init code before freeing the memory */async_synchronize_full();free_initmem();unlock_kernel();mark_rodata_ro()

40、;system_state = SYSTEM_RUNNING;numa_default_policy();if (sys_open(const char _user *) /dev/console, O_RDWR, 0) signal-flags |= SIGNAL_UNKILLABLE;if (ramdisk_execute_command) run_init_process(ramdisk_execute_command);printk(KERN_WARNING Failed to execute %sn,ramdisk_execute_command);/* * We try each

41、of these until one succeeds. * * The Bourne shell can be used instead of init if we are * trying to recover a really broken machine. */if (execute_command) run_init_process(execute_command);printk(KERN_WARNING Failed to execute %s. Attempting defaults.n, execute_command);run_init_process(/sbin/init); 根文件系统命令run_init_process(/etc/init);run_init_process(/bin/init);run_init_process(/bin/sh);panic(No init found. Try passing init= option to kernel.);14 / 14