嵌入式Linux2.6內(nèi)核啟動(dòng)流程)

Linux內(nèi)核構(gòu)成(國(guó)嵌)Linux/arch/arm/boot/compressed/head.s1解壓縮2初始化3啟動(dòng)應(yīng)用程序1 arch/arm/boot/compressed/Makefile arch/arm/boot/compressed/vmlinux.lds2. arch/arm/kernel/vmlinux.ldsLinux內(nèi)核啟動(dòng)流程（國(guó)嵌）arch/arm/boot/compressed/start.S（head.s負(fù)責(zé)解壓縮）Start: .type start,#function .rept 8 mov r0, r0 .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這也標(biāo)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(huì)讀取cpsr并判斷是否處理器處于supervisor模式從u-boot進(jìn)入kernel，系統(tǒng)已經(jīng)處于SVC32模式；而利用angel進(jìn)入則處于user模式，還需要額外兩條指令。之后是再次確認(rèn)中斷關(guān)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(dǎo)入r0-r6、ip、sp等寄存器，并檢查代碼是否運(yùn)行在與鏈接時(shí)相同的目標(biāo)地址，以決定是否進(jìn)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(gè)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(dòng)的能力）。arm架構(gòu)下自解壓頭一般是鏈接在0x0地址而被加載到0x30008000運(yùn)行，所以要修正這個(gè)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(jiǎn)單，這些寄存器統(tǒng)統(tǒng)被加上一個(gè)你也能猜到的偏移地址 0x30008000。該地址是s3c2410相關(guān)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(kāi)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(jìn)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，打開(kāi)cache，并為后面解壓縮設(shè)置64KB的臨時(shí)malloc空間 bl cache_on mov r1, sp malloc space above stack add r2, sp, #0x10000 64k max 接下來(lái)238行進(jìn)行檢查，確定內(nèi)核解壓縮后的Image目標(biāo)地址是否會(huì)覆蓋到zImage頭，如果是則準(zhǔn)備將zImage頭轉(zhuǎn)移到解壓出來(lái)的內(nèi)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真實(shí)情況在大多數(shù)的應(yīng)用中，內(nèi)核編譯都會(huì)把壓縮的zImage和非壓縮的Image鏈接到同樣的地址，s3c2410平臺(tái)下即是0x30008000。這樣做的好處是，人們不用關(guān)心內(nèi)核是Image還是zImage，放到這個(gè)位置執(zhí)行就OK，所以在解壓縮后zImage頭必須為真正的內(nèi)核讓路。在250行解壓完畢，內(nèi)核長(zhǎng)度返回值存放在r0寄存器里。在內(nèi)核末尾空出128字節(jié)的?？臻g用，并且使其長(zhǎng)度128字節(jié)對(duì)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(jì)算內(nèi)核末尾地址并存放于r1寄存器，需要搬移代碼原來(lái)地址放在r2，需要搬移的長(zhǎng)度放在r3。然后執(zhí)行搬移，并設(shè)置好sp指針指向新的棧（原來(lái)的棧也會(huì)被內(nèi)核覆蓋掉） 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?yàn)榇a地址變化了不能讓cache再命中被內(nèi)核覆蓋的老地址。然后跳轉(zhuǎn)到新的地址繼續(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(zhuǎn)會(huì)給gdb調(diào)試內(nèi)核帶來(lái)麻煩。因?yàn)橛脕?lái)調(diào)試的符號(hào)表是在編譯是生成的，并不知道以后會(huì)被搬移到何處去，只有在內(nèi)核解壓縮完成之后，根據(jù)計(jì)算出來(lái)的參數(shù)“告訴”調(diào)試器這個(gè)變化。以撰寫本文時(shí)使用的zImage為例，內(nèi)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(hào)表也相應(yīng)的從0x30008000后移到0x30533b00開(kāi)始，這樣gdb就可以正確的對(duì)應(yīng)源代碼和機(jī)器指令。隨著頭部代碼移動(dòng)到新的位置，不會(huì)再和內(nèi)核的目標(biāo)地址沖突，可以開(kāi)始內(nèi)核自身的搬移了。此時(shí)r0寄存器存放的是內(nèi)核長(zhǎng)度（嚴(yán)格的說(shuō)是長(zhǎng)度外加128Byte的棧），r4存放的是內(nèi)核的目的地址0x30008000，r5是目前內(nèi)核存放地址，r6是CPU ID，r7是machine ID，r8是atags地址。代碼從501行開(kāi)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 relocate kernel stmia r1!, r0, r2, r3, r10 - r14 .endr cmp r5, r9 blo 1b add sp, r1, #128 relocate the stack接下來(lái)在516行清除并關(guān)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(nèi)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 restore atags pointer mov pc, r4 call kernel內(nèi)核代碼入口在arch/arm/kernel/head.S文件的83行。首先進(jìn)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 processor (r5=0)? beq _error_p yes, error p接著在87行進(jìn)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(biāo)號(hào)3的實(shí)際地址加載到r3，然后將編譯時(shí)生成的_proc_info_begin虛擬地址載入到r5，_proc_info_end虛擬地址載入到r6，標(biāo)號(hào)3的虛擬地址載入到r7。由于adr偽指令和標(biāo)號(hào)3的使用，以及_proc_info_begin等符號(hào)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(qǐng)耐心閱讀這兩個(gè)文件和adr偽指令的說(shuō)明。r3和r7分別存儲(chǔ)的是同一位置標(biāo)號(hào)3的物理地址（由于沒(méi)有啟用mmu，所以當(dāng)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r5和r6中保存的虛擬地址轉(zhuǎn)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(nèi)核編譯時(shí)寫入的processor ID和之前從cpsr中讀到的processor ID對(duì)比，查看代碼和CPU硬件是否匹配（想在arm920t上運(yùn)行為cortex-a8編譯的內(nèi)核？不讓?。?。如果編譯了多種處理器支持，如versatile板，則會(huì)循環(huán)每種type依次檢驗(yàn)，如果硬件讀出的ID在內(nèi)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/arm/kernel/head-common.S文件的197行，編碼方法與檢查processor ID完全一樣，請(qǐng)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 number?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(yè)表bl_vet_atagsbl_create_page_tables 創(chuàng)建頁(yè)表的代碼在218行，首先將內(nèi)核起始地址-0x4000到內(nèi)核起始地址之間的16K存儲(chǔ)器清0_create_page_tables:pgtblr4 page table address/* * Clear the 16K level 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(nèi)核第一個(gè)1MB空間的段地址存入r6，在s3c2410平臺(tái)該值是0x300。接著根據(jù)此值存入映射標(biāo)識(shí)movr6, pc, lsr #20 start of kernel sectionorrr3, r7, r6, lsl #20 flags + kernel basestrr3, r4, r6, lsl #2 identity mapping完成頁(yè)表設(shè)置后回到102行，為打開(kāi)虛擬地址映射作準(zhǔn)備。設(shè)置sp指針，函數(shù)返回地址lr指向_enable_mmu，并跳轉(zhuǎn)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 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(yè)表，跳轉(zhuǎn)到_turn_mmu_on_enable_mmu:#ifdef CONFIG_ALIGNMENT_TRAPorrr0, r0, #CR_A#elsebicr0, r0, #CR_A#endif#ifdef CONFIG_CPU_DCACHE_DISABLEbicr0, 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_CLIENT)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(lái)保存在sp中的地址載入pc，跳轉(zhuǎn)到head-common.S的_mmap_switched，至此代碼進(jìn)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, r3movr3, r3movpc, r13在head-common.S的37行開(kāi)始清除內(nèi)核bss段，processor ID保存在r9，machine ID報(bào)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(nèi)存地址。接下來(lái)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(gè)C語(yǔ)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 if 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_early_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(&command_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 happens 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 command 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* 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_disabled()printk(KERN_CRIT start_kernel(): bug: interrupts were enabled earlyn);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_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_INITRDif (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_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();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();cred_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();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 | CLONE_SIGHAND);numa_default_policy();pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);kthreadd_tas