嵌入式Linux2.6內核啟動流程蒼松書屋

1、linux內核構成(國嵌)linux/arch/arm/boot/compressed/head.s1解壓縮2初始化3啟動應用程序1 arch/arm/boot/compressed/makefile arch/arm/boot/compressed/vmlinux.lds2. arch/arm/kernel/vmlinux.ldslinux內核啟動流程（國嵌）arch/arm/boot/compressed/start.s（head.s負責解壓縮）start: .type start,#function .rept 8 mov r0, r0 .endr b 1f .word 0x016f28

2、18 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這也標志著u-boot將系統(tǒng)完全的交給了os，bootloader生命終止。之后代碼在133行會讀取cpsr并判斷是否處理器處于supervisor模式從u-boot進入kernel，系統(tǒng)已經處于svc32模式；而利用angel進入則處于user模

3、式，還需要額外兩條指令。之后是再次確認中斷關閉，并完成cpsr寫入 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lc0地址處將分段信息導入r0-r6、ip、sp等寄

4、存器，并檢查代碼是否運行在與鏈接時相同的目標地址，以決定是否進行處理。由于現(xiàn)在很少有人不使用loader和tags，將zimage燒寫到rom直接從0x0位置執(zhí)行，所以這個處理是必須的（但是zimage的頭現(xiàn)在也保留了不用loader也可啟動的能力）。arm架構下自解壓頭一般是鏈接在0x0地址而被加載到0x30008000運行，所以要修正這個變化。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棧指針地址 很簡單，這些寄存器統(tǒng)統(tǒng)被加上一個你也能猜到的偏移地址 0x30008000

5、。該地址是s3c2410相關的，其他的arm處理器可以參考下表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 這些操作發(fā)生在代碼172行開始的地方，下面只粘貼一部分 add r5, r5, r0 add r6, r6, r0 add ip, ip, r0后面在211行進行bss段的清零工作n

6、ot_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，打開cache，并為后面解壓縮設置64kb的臨時malloc空間 bl cache_on mov r1, sp malloc space above stack add r2, sp, #0x10000 64k max 接下來238行進行檢查，確定內核解壓縮后的image目標地址是否會覆蓋到zimage頭，如果是則準備將zimage頭轉移到解壓出來的內核

7、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真實情況在大多數(shù)的應用中，內核編譯都會把壓縮的zimage和非壓縮的image鏈接到同樣的地址，s3c2410平臺下即是0x300

8、08000。這樣做的好處是，人們不用關心內核是image還是zimage，放到這個位置執(zhí)行就ok，所以在解壓縮后zimage頭必須為真正的內核讓路。在250行解壓完畢，內核長度返回值存放在r0寄存器里。在內核末尾空出128字節(jié)的?？臻g用，并且使其長度128字節(jié)對齊。 add r0, r0, #127 + 128 alignment + stack bic r0, r0, #127 align the kernel length算出搬移代碼的參數(shù)：計算內核末尾地址并存放于r1寄存器，需要搬移代碼原來地址放在r2，需要搬移的長度放在r3。然后執(zhí)行搬移，并設置好sp指針指向新的棧（原來的棧也會被內核

9、覆蓋掉） 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，因為代碼地址變化了不能讓cache再命中被內核覆蓋的老地址。然后跳轉到新的

10、地址繼續(xù)執(zhí)行 bl cache_clean_flush add pc, r5, r0 call relocation code注意zimage在解壓后的搬移和跳轉會給gdb調試內核帶來麻煩。因為用來調試的符號表是在編譯是生成的，并不知道以后會被搬移到何處去，只有在內核解壓縮完成之后，根據(jù)計算出來的參數(shù)“告訴”調試器這個變化。以撰寫本文時使用的zimage為例，內核自解壓頭重定向后，reloc_start地址由0x30008360變?yōu)?x30533e60。故我們要把vmlinux的符號表也相應的從0x30008000后移到0x30533b00開始，這樣gdb就可以正確的對應源代碼和機器指令。隨著

11、頭部代碼移動到新的位置，不會再和內核的目標地址沖突，可以開始內核自身的搬移了。此時r0寄存器存放的是內核長度（嚴格的說是長度外加128byte的棧），r4存放的是內核的目的地址0x30008000，r5是目前內核存放地址，r6是cpu id，r7是machine id，r8是atags地址。代碼從501行開始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, r3, r10 - r14 reloca

12、te kernel stmia r1!, r0, r2, r3, r10 - r14 .endr cmp r5, r9 blo 1b add sp, r1, #128 relocate the stack接下來在516行清除并關閉cache，清零r0，將machine id存入r1，atags指針存入r2，再跳入0x30008000執(zhí)行真正的內核imagecall_kernel: bl cache_clean_flush bl cache_off mov r0, #0 must be zero mov r1, r7 restore architecture number mov r2, r8

13、restore atags pointer mov pc, r4 call kernel內核代碼入口在arch/arm/kernel/head.s文件的83行。首先進入svc32模式，并查詢cpu id，檢查合法性 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 r10, r5 invalid proc

14、essor (r5=0)? beq _error_p yes, error 'p'接著在87行進一步查詢machine id并檢查合法性 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，該函數(shù)首將標號3的實際地址加載到r3，然后將編譯時生成的

15、_proc_info_begin虛擬地址載入到r5，_proc_info_end虛擬地址載入到r6，標號3的虛擬地址載入到r7。由于adr偽指令和標號3的使用，以及_proc_info_begin等符號在linux-2.6.24-moko-linuxbj/arch/arm/kernel/vmlinux.lds而不是代碼中被定義，此處代碼不是非常直觀，想弄清楚代碼緣由的讀者請耐心閱讀這兩個文件和adr偽指令的說明。r3和r7分別存儲的是同一位置標號3的物理地址（由于沒有啟用mmu，所以當前肯定是物理地址）和虛擬地址，所以兒者相減即得到虛擬地址和物理地址之間的offset。利用此offset，將r

16、5和r6中保存的虛擬地址轉變?yōu)槲锢淼刂穇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然后從proc_info中讀出內核編譯時寫入的processor id和之前從cpsr中讀到的processor id對比，查看代碼和cpu硬件是否匹配（想在arm920t上運行為cortex-a8編譯的

17、內核？不讓?。?。如果編譯了多種處理器支持，如versatile板，則會循環(huán)每種type依次檢驗，如果硬件讀出的id在內核中找不到匹配，則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/a

18、rm/kernel/head-common.s文件的197行，編碼方法與檢查processor id完全一樣，請參考前段_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 n

19、umber?beq2f foundaddr5, r5, #sizeof_machine_desc next machine_desccmpr5, r6blo1bmovr5, #0 unknown machine2:movpc, lr代碼回到head.s第92行，檢查atags合法性，然后創(chuàng)建初始頁表bl_vet_atagsbl_create_page_tables 創(chuàng)建頁表的代碼在218行，首先將內核起始地址-0x4000到內核起始地址之間的16k存儲器清0_create_page_tables:pgtblr4 page table address/* * clear the 16k leve

20、l 1 swapper page table */movr0, r4movr3, #0addr6, r0, #0x40001:strr3, r0, #4strr3, r0, #4strr3, r0, #4strr3, r0, #4teqr0, r6bne1b 然后在234行將proc_info中的mmu_flags加載到r7ldrr7, r10, #procinfo_mm_mmuflags mm_mmuflags在242行將pc指針右移20位，得到內核第一個1mb空間的段地址存入r6，在s3c2410平臺該值是0x300。接著根據(jù)此值存入映射標識movr6, pc, lsr #20 start

21、 of kernel sectionorrr3, r7, r6, lsl #20 flags + kernel basestrr3, r4, r6, lsl #2 identity mapping完成頁表設置后回到102行，為打開虛擬地址映射作準備。設置sp指針，函數(shù)返回地址lr指向_enable_mmu，并跳轉到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 invali

22、date i,d 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，加載domain和頁表，跳轉到_turn_mmu_on_enable_mmu:#ifdef config_alignment_traporrr0, r0, #cr_a#elsebicr0, r0, #cr_a#endif#ifdef config_cpu_dcache_disableb

23、icr0, r0, #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_cl

24、ient)mcrp15, 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使能位寫入mmu，激活虛擬地址。然后將原來保存在sp中的地址載入pc，跳轉到head-common.s的_mmap_switched，至此代碼進入虛擬地址的世界movr0, r0mcrp15, 0, r0, c1, c0, 0 write control regmrcp15, 0, r3, c0, c0, 0 read id regmovr3,

25、 r3movr3, r3movpc, r13在head-common.s的37行開始清除內核bss段，processor id保存在r9，machine id報存在r1，atags地址保存在r2，并將控制寄存器保存到r7定義的內存地址。接下來跳入linux-2.6.24-moko-linuxbj/init/main.c的507行，start_kernel函數(shù)。這里只粘貼部分代碼（第一個c語言函數(shù)，作一系列的初始化）_mmap_switched:adrr3, _switch_data + 4ldmiar3!, r4, r5, r6, r7cmpr4, r5 copy data segment i

26、f needed1: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_ea

27、rly_init();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(&co

28、mmand_line);mm_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 happe

29、ns at smp_init() * time - 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 c

30、ommand line: %sn", boot_command_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*

31、 early, fixing itn");local_irq_disable();sort_main_extable();trap_init();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_disa

32、bled()printk(kern_crit "start_kernel(): bug: interrupts were " "enabled earlyn");early_boot_irqs_on();local_irq_enable();/* * 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 wa

33、nt output early, in case something goes wrong. */console_init();if (panic_later)panic(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_init

34、rdif (initrd_start && !initrd_below_start_ok && page_to_pfn(virt_to_page(void *)initrd_start) < min_low_pfn) printk(kern_crit "initrd overwritten (0x%08lx < 0x%08lx) - " "disabling it.n", page_to_pfn(virt_to_page(void *)initrd_start), min_low_pfn);initrd_sta

35、rt = 0;#endifvmalloc_init();vfs_caches_init_early();cpuset_init_early();page_cgroup_init();mem_init();enable_debug_pagealloc();cpu_hotplug_init();kmem_cache_init();debug_objects_mem_init();idr_init_cache();setup_per_cpu_pageset();numa_policy_init();if (late_time_init)late_time_init();calibrate_delay

36、();pidmap_init();pgtable_cache_init();prio_tree_init();anon_vma_init();#ifdef config_x86if (efi_enabled)efi_enter_virtual_mode();#endifthread_info_cache_init();cred_init();fork_init(num_physpages);proc_caches_init();buffer_init();key_init();security_init();vfs_caches_init(num_physpages);radix_tree_i

37、nit();signals_init();/* rootfs populating might need page-writeback */page_writeback_init();#ifdef config_proc_fsproc_root_init();#endifcgroup_init();cpuset_init();taskstats_init_early();delayacct_init();check_bugs();acpi_early_init(); /* before lapic and smp init */ftrace_init();/* do the rest non-

38、_init'ed, we're now alive */rest_init();tatic noinline void _init_refok rest_init(void)_releases(kernel_lock)int pid;kernel_thread(kernel_init, null, clone_fs | clone_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_starting()