參考資料：
《ARM體系結構與編程》
《嵌入式Linux應用開發完全手冊》
Linux_Memory_Address_Mapping
http://www.chinaunix.net/old_jh/4/1021226.html
更多文檔參見：http://pan.baidu.com/s/1mg3DbHQ
本文針對arm linux, 從kernel的第一條指令開始分析,一直分析到進入start_kernel()函數. 我們當前以linux-2.6.19內核版本作為范例來分析,本文中所有的代碼,前面都會加上行號以便于和源碼進行對照, 例: 在文件init/main.c中: 00478: asmlinkage void __init start_kernel(void) 前面的"00478:" 表示478行,冒號后面的內容就是源碼了. 在分析代碼的過程中,我們使用縮進來表示各個代碼的調用層次. 由于啟動部分有一些代碼是平臺特定的,雖然大部分的平臺所實現的功能都比較類似,但是為了更好的對code進行說明,對于平臺相關的代碼,我們選擇at91(ARM926EJS)平臺進行分析. 另外,本文是以uncomPRessed kernel開始講解的.對于內核解壓縮部分的code,在 arch/arm/boot/compressed中,本文不做討論.
一. 啟動條件通常從系統上電到執行到linux kenel這部分的任務是由boot loader來完成. 關于boot loader的內容,本文就不做過多介紹. 這里只討論進入到linux kernel的時候的一些限制條件,這一般是boot loader在最后跳轉到kernel之前要完成的: 1. CPU必須處于SVC(supervisor)模式,并且IRQ和FIQ中斷都是禁止的; 2. MMU(內存管理單元)必須是關閉的, 此時虛擬地址對物理地址; 3. 數據cache(Data cache)必須是關閉的 4. 指令cache(Instruction cache)可以是打開的,也可以是關閉的,這個沒有強制要求; 5. CPU 通用寄存器0 (r0)必須是 0; 6. CPU 通用寄存器1 (r1)必須是 ARM Linux machine type (關于machine type, 我們后面會有講解) 7. CPU 通用寄存器2 (r2) 必須是 kernel parameter list 的物理地址(parameter list 是由boot loader傳遞給kernel,用來描述設備信息屬性的列表,詳細內容可參考"Booting ARM Linux"文檔).
二. starting kernel首先，我們先對幾個重要的宏進行說明(我們針對有MMU的情況)：
宏 位置 默認值 說明 KERNEL_RAM_ADDR arch/arm/kernel/head.S +26 0xc0008000 kernel在RAM中的的虛擬地址 PAGE_OFFSET include/asm-arm/memeory.h +50 0xc0000000 內核空間的起始虛擬地址 TEXT_OFFSET arch/arm/Makefile +137 0x00008000 內核相對于存儲空間的偏移 TEXTADDR arch/arm/kernel/head.S +49 0xc0008000 kernel的起始虛擬地址 PHYS_OFFSET include/asm-arm/arch-xxx/memory.h 平臺相關 RAM的起始物理地址 內核的入口是stext,這是在arch/arm/kernel/vmlinux.lds.S中定義的: 00011: ENTRY(stext) 對于vmlinux.lds.S,這是ld script文件,此文件的格式和匯編及C程序都不同,本文不對ld script作過多的介紹,只對內核中用到的內容進行講解,關于ld的詳細內容可以參考ld.info 這里的ENTRY(stext) 表示程序的入口是在符號stext. 而符號stext是在arch/arm/kernel/head.S中定義的: 下面我們將arm linux boot的主要代碼列出來進行一個概括的介紹,然后,我們會逐個的進行詳細的講解. 在arch/arm/kernel/head.S中 72 - 94 行,是arm linux boot的主代碼: 00072: ENTRY(stext) 00073: msr cpsr_c, #PSR_F_BIT | PSR_I_BIT | SVC_MODE @ ensure svc mode 00074: @ and irqs disabled 00075: mrc p15, 0, r9, c0, c0 @ get processor id 00076: bl __lookup_processor_type @ r5=procinfo r9=cpuid 00077: movs r10, r5 @ invalid processor (r5=0)? 00078: beq __error_p @ yes, error 'p' 00079: bl __lookup_machine_type @ r5=machinfo 00080: movs r8, r5 @ invalid machine (r5=0)? 00081: beq __error_a @ yes, error 'a' 00082: bl __create_page_tables 00083: 00084: /* 00085: * The following calls CPU specific code in a position independent 00086: * manner. See arch/arm/mm/proc-*.S for details. r10 = base of 00087: * xxx_proc_info structure selected by __lookup_machine_type 00088: * above. On return, the CPU will be ready for the MMU to be 00089: * turned on, and r0 will hold the CPU control register value. 00090: */ 00091: ldr r13, __switch_data @ address to jump to after 00092: @ mmu has been enabled 00093: adr lr, __enable_mmu @ return (PIC) address 00094: add pc, r10, #PROCINFO_INITFUNC 其中,73行是確保kernel運行在SVC模式下,并且IRQ和FIRQ中斷已經關閉,這樣做是很謹慎的. arm linux boot的主線可以概括為以下幾個步驟: 1. 確定 processor type (75 - 78行) 2. 確定 machine type (79 - 81行) 3. 創建頁表 (82行) 4. 調用平臺特定的__cpu_flush函數 (在struct proc_info_list中) (94 行) 5. 開啟mmu (93行) 6. 切換數據 (91行) 最終跳轉到start_kernel (在__switch_data的結束的時候,調用了 b start_kernel) 下面,我們按照這個主線,逐步的分析Code.
1. 確定 processor typearch/arm/kernel/head.S中: 00075: mrc p15, 0, r9, c0, c0 @ get processor id 00076: bl __lookup_processor_type @ r5=procinfo r9=cpuid 00077: movs r10, r5 @ invalid processor (r5=0)? 00078: beq __error_p @ yes, error 'p' 75行: 通過cp15協處理器的c0寄存器來獲得processor id的指令. 關于cp15的詳細內容可參考相關的arm手冊 76行: 跳轉到__lookup_processor_type.在__lookup_processor_type中,會把processor type 存儲在r5中 77,78行: 判斷r5中的processor type是否是0,如果是0,說明是無效的processor type,跳轉到__error_p(出錯) __lookup_processor_type 函數主要是根據從cpu中獲得的processor id和系統中的proc_info進行匹配,將匹配到的proc_info_list的基地址存到r5中, 0表示沒有找到對應的processor type. 下面我們分析__lookup_processor_type函數 arch/arm/kernel/head-common.S中: 00145: .type __lookup_processor_type, %function 00146: __lookup_processor_type: 00147: adr r3, 3f 00148: ldmda r3, {r5 - r7} 00149: sub r3, r3, r7 @ get offset between virt&phys 00150: add r5, r5, r3 @ convert virt addresses to 00151: add r6, r6, r3 @ physical address space 00152: 1: ldmia r5, {r3, r4} @ value, mask 00153: and r4, r4, r9 @ mask wanted bits 00154: teq r3, r4 00155: beq 2f 00156: add r5, r5, #PROC_INFO_SZ @ sizeof(proc_info_list) 00157: cmp r5, r6 00158: blo 1b 00159: mov r5, #0 @ unknown processor 00160: 2: mov pc, lr 00161: 00162: /* 00163: * This provides a C-API version of the above function. 00164: */ 00165: ENTRY(lookup_processor_type) 00166: stmfd sp!, {r4 - r7, r9, lr} 00167: mov r9, r0 00168: bl __lookup_processor_type 00169: mov r0, r5 00170: ldmfd sp!, {r4 - r7, r9, pc} 00171: 00172: /* 00173: * Look in include/asm-arm/procinfo.h and arch/arm/kernel/arch.[ch] for 00174: * more information about the __proc_info and __arch_info structures. 00175: */ 00176: .long __proc_info_begin 00177: .long __proc_info_end 00178: 3: .long . 00179: .long __arch_info_begin 00180: .long __arch_info_end 145, 146行是函數定義 147行: 取地址指令,這里的3f是向前symbol名稱是3的位置,即第178行,將該地址存入r3. 這里需要注意的是,adr指令取址,獲得的是基于pc的一個地址,要格外注意,這個地址是3f處的"運行時地址", 由于此時MMU還沒有打開,也可以理解成物理地址(實地址).(詳細內容可參考arm指令手冊) 148行: 因為r3中的地址是178行的位置的地址,因而執行完后: (ldmda表示棧指針遞減，即r3遞減，內存的地址編號較大的對應寄存器編號較大的) r5存的是176行符號 __proc_info_begin的地址; r6存的是177行符號 __proc_info_end的地址; r7存的是3f處的地址. 這里需要注意鏈接地址和運行時地址的區別. r3存儲的是運行時地址(物理地址),而r7中存儲的是鏈接地址(虛擬地址). __proc_info_begin和__proc_info_end是在arch/arm/kernel/vmlinux.lds.S中: 00031: __proc_info_begin = .; 00032: *(.proc.info.init) 00033: __proc_info_end = .; 這里是聲明了兩個變量:__proc_info_begin 和 __proc_info_end,其中等號后面的"."是location counter(詳細內容請參考ld.info) 這三行的意思是: __proc_info_begin 的位置上,放置所有文件中的 ".proc.info.init" 段的內容,然后緊接著是 __proc_info_end 的位置. kernel 使用struct proc_info_list來描述processor type. 在 include/asm-arm/procinfo.h 中: 00029: struct proc_info_list { 00030: unsigned int cpu_val; 00031: unsigned int cpu_mask; 00032: unsigned long __cpu_mm_mmu_flags; /* used by head.S */ 00033: unsigned long __cpu_io_mmu_flags; /* used by head.S */ 00034: unsigned long __cpu_flush; /* used by head.S */ 00035: const char *arch_name; 00036: const char *elf_name; 00037: unsigned int elf_hwcap; 00038: const char *cpu_name; 00039: struct processor *proc; 00040: struct cpu_tlb_fns *tlb; 00041: struct cpu_user_fns *user; 00042: struct cpu_cache_fns *cache; 00043: }; 我們當前以at91為例,其processor是926的. 在arch/arm/mm/proc-arm926.S 中: 00464: .section ".proc.info.init", #alloc, #execinstr 00465: 00466: .type __arm926_proc_info,#object 00467: __arm926_proc_info: 00468: .long 0x41069260 @ ARM926EJ-S (v5TEJ) 00469: .long 0xff0ffff0 00470: .long PMD_TYPE_SECT | / 00471: PMD_SECT_BUFFERABLE | / 00472: PMD_SECT_CACHEABLE | / 00473: PMD_BIT4 | / 00474: PMD_SECT_AP_WRITE | / 00475: PMD_SECT_AP_READ 00476: .long PMD_TYPE_SECT | / 00477: PMD_BIT4 | / 00478: PMD_SECT_AP_WRITE | / 00479: PMD_SECT_AP_READ 00480: b __arm926_setup 00481: .long cpu_arch_name 00482: .long cpu_elf_name 00483: .long HWCAP_SWP|HWCAP_HALF|HWCAP_THUMB|HWCAP_FAST_MULT|HWCAP_VFP|HWCAP_EDSP|HWCAP_java 00484: .long cpu_arm926_name 00485: .long arm926_processor_functions 00486: .long v4wbi_tlb_fns 00487: .long v4wb_user_fns 00488: .long arm926_cache_fns 00489: .size __arm926_proc_info, . - __arm926_proc_info 從464行,我們可以看到 __arm926_proc_info 被放到了".proc.info.init"段中. 對照struct proc_info_list,我們可以看到 __cpu_flush的定義是在480行,即__arm926_setup.(我們將在"4. 調用平臺特定的__cpu_flush函數"一節中詳細分析這部分的內容.) 從以上的內容我們可以看出: r5中的__proc_info_begin是proc_info_list的起始地址, r6中的__proc_info_end是proc_info_list的結束地址. 149行: 從上面的分析我們可以知道r3中存儲的是3f處的物理地址,而r7存儲的是3f處的虛擬地址,這一行是計算當前程序運行的物理地址和虛擬地址的差值,將其保存到r3中. 150行: 將r5存儲的虛擬地址(__proc_info_begin)轉換成物理地址 151行: 將r6存儲的虛擬地址(__proc_info_end)轉換成物理地址 152行: 對照struct proc_info_list,可以得知,這句是將當前proc_info的cpu_val和cpu_mask分別存r3, r4中 153行: r9中存儲了processor id(arch/arm/kernel/head.S中的75行),與r4的cpu_mask進行邏輯與操作,得到我們需要的值 154行: 將153行中得到的值與r3中的cpu_val進行比較 155行: 如果相等,說明我們找到了對應的processor type,跳到160行,返回 156行: (如果不相等) , 將r5指向下一個proc_info, 157行: 和r6比較,檢查是否到了__proc_info_end. 158行: 如果沒有到__proc_info_end,表明還有proc_info配置,返回152行繼續查找 159行: 執行到這里,說明所有的proc_info都匹配過了,但是沒有找到匹配的,將r5設置成0(unknown processor) 160行: 返回
2. 確定 machine type arch/arm/kernel/head.S中: 00079: bl __lookup_machine_type @ r5=machinfo 00080: movs r8, r5 @ invalid machine (r5=0)? 00081: beq __error_a @ yes, error 'a' 79行: 跳轉到__lookup_machine_type函數,在__lookup_machine_type中,會把struct machine_desc的基地址(machine type)存儲在r5中 80,81行: 將r5中的 machine_desc的基地址存儲到r8中,并判斷r5是否是0,如果是0,說明是無效的machine type,跳轉到__error_a(出錯) __lookup_machine_type 函數 下面我們分析__lookup_machine_type 函數: arch/arm/kernel/head-common.S中: 00176: .long __proc_info_begin 00177: .long __proc_info_end 00178: 3: .long . 00179: .long __arch_info_begin 00180: .long __arch_info_end 00181: 00182: /* 00183: * Lookup machine architecture in the linker-build list of architectures. 00184: * Note that we can't use the absolute addresses for the __arch_info 00185: * lists since we aren't running with the MMU on (and therefore, we are 00186: * not in the correct address space). We have to calculate the offset. 00187: * 00188: * r1 = machine architecture number 00189: * Returns: 00190: * r3, r4, r6 corrupted 00191: * r5 = mach_info pointer in physical address space 00192: */ 00193: .type __lookup_machine_type, %function 00194: __lookup_machine_type: 00195: adr r3, 3b 00196: ldmia r3, {r4, r5, r6} 00197: sub r3, r3, r4 @ get offset between virt&phys 00198: add r5, r5, r3 @ convert virt addresses to 00199: add r6, r6, r3 @ physical address space 00200: 1: ldr r3, [r5, #MACHINFO_TYPE] @ get machine type 00201: teq r3, r1 @ matches loader number? 00202: beq 2f @ found 00203: add r5, r5, #SIZEOF_MACHINE_DESC @ next machine_desc 00204: cmp r5, r6 00205: blo 1b 00206: mov r5, #0 @ unknown machine 00207: 2: mov pc, lr 193, 194行: 函數聲明 195行: 取地址指令,這里的3b是向后symbol名稱是3的位置,即第178行,將該地址存入r3. 和上面我們對__lookup_processor_type 函數的分析相同,r3中存放的是3b處物理地址. 196行: r3是3b處的地址,因而執行完后:（ldmia 表示棧是遞增的，即r3遞增，低內存地址對應小號寄存器） r4存的是 3b處的地址 r5存的是__arch_info_begin 的地址 r6存的是__arch_info_end 的地址 __arch_info_begin 和 __arch_info_end是在 arch/arm/kernel/vmlinux.lds.S中: 00034: __arch_info_begin = .; 00035: *(.arch.info.init) 00036: __arch_info_end = .; 這里是聲明了兩個變量:__arch_info_begin 和 __arch_info_end,其中等號后面的"."是location counter(詳細內容請參考ld.info) 這三行的意思是: __arch_info_begin 的位置上,放置所有文件中的 ".arch.info.init" 段的內容,然后緊接著是 __arch_info_end 的位置. kernel 使用struct machine_desc 來描述 machine type. 在 include/asm-arm/mach/arch.h 中: 00017: struct machine_desc { 00018: /* 00019: * Note! The first four elements are used 00020: * by assembler code in head-armv.S 00021: */ 00022: unsigned int nr; /* architecture number */ 00023: unsigned int phys_io; /* start of physical io */ 00024: unsigned int io_pg_offst; /* byte offset for io 00025: * page tabe entry */ 00026: 00027: const char *name; /* architecture name */ 00028: unsigned long boot_params; /* tagged list */ 00029: 00030: unsigned int video_start; /* start of video RAM */ 00031: unsigned int video_end; /* end of video RAM */ 00032: 00033: unsigned int reserve_lp0 :1; /* never has lp0 */ 00034: unsigned int reserve_lp1 :1; /* never has lp1 */ 00035: unsigned int reserve_lp2 :1; /* never has lp2 */ 00036: unsigned int soft_reboot :1; /* soft reboot */ 00037: void (*fixup)(struct machine_desc *, 00038: struct tag *, char **, 00039: struct meminfo *); 00040: void (*map_io)(void);/* IO mapping function */ 00041: void (*init_irq)(void); 00042: struct sys_timer *timer; /* system tick timer */ 00043: void (*init_machine)(void); 00044: }; 00045: 00046: /* 00047: * Set of macros to define architecture features. This is built into 00048: * a table by the linker. 00049: */ 00050: #define MACHINE_START(_type,_name) / 00051: static const struct machine_desc __mach_desc_##_type / 00052: __attribute_used__ / 00053: __attribute__((__section__(".arch.info.init"
)) = { / 00054: .nr = MACH_TYPE_##_type, / 00055: .name = _name, 00056: 00057: #define MACHINE_END / 00058: }; 內核中,一般使用宏MACHINE_START來定義machine type. 對于at91, 在 arch/arm/mach-at91rm9200/board-ek.c 中: 00137: MACHINE_START(AT91RM9200EK, "Atmel AT91RM9200-EK"![icon_wink[1]](http://s1.vevb.com/20150728/evrsx0i4bxs35.gif "icon_wink[1]"){kind=small}
00138: /* Maintainer: SAN People/Atmel */ 00139: .phys_io = AT91_BASE_SYS, 00140: .io_pg_offst = (AT91_VA_BASE_SYS >> 1
& 0xfffc, 00141: .boot_params = AT91_SDRAM_BASE + 0x100, 00142: .timer = &at91rm9200_timer, 00143: .map_io = ek_map_io, 00144: .init_irq = ek_init_irq, 00145: .init_machine = ek_board_init, 00146: MACHINE_END 197行: r3中存儲的是3b處的物理地址,而r4中存儲的是3b處的虛擬地址,這里計算處物理地址和虛擬地址的差值,保存到r3中 198行: 將r5存儲的虛擬地址(__arch_info_begin)轉換成物理地址 199行: 將r6存儲的虛擬地址(__arch_info_end)轉換成物理地址 200行: MACHINFO_TYPE 在 arch/arm/kernel/asm-offset.c 101行定義, 這里是取 struct machine_desc中的nr(architecture number) 到r3中 201行: 將r3中取到的machine type 和 r1中的 machine type(見前面的"啟動條件"![icon_wink[2]](http://s1.vevb.com/20150728/5lzcigl3hr536.gif "icon_wink[2]"){kind=small}
進行比較 202行: 如果相同,說明找到了對應的machine type,跳轉到207行的2f處,此時r5中存儲了對應的struct machine_desc的基地址 203行: (不相同), 取下一個machine_desc的地址 204行: 和r6進行比較,檢查是否到了__arch_info_end. 205行: 如果不相同,說明還有machine_desc,返回200行繼續查找. 206行: 執行到這里,說明所有的machind_desc都查找完了,并且沒有找到匹配的, 將r5設置成0(unknown machine). 207行: 返回
通過前面的兩步,我們已經確定了processor type 和 machine type. 此時,一些特定寄存器的值如下所示: r8 = machine info (struct machine_desc的基地址) r9 = cpu id (通過cp15協處理器獲得的cpu id) r10 = procinfo (struct proc_info_list的基地址) 創建頁表是通過函數 __create_page_tables 來實現的. 這里,我們使用的是arm的L1主頁表,L1主頁表也稱為段頁表(section page table) L1 主頁表將4 GB 的地址空間分成若干個1 MB的段(section),因此L1頁表包含4096個頁表項(section entry). 每個頁表項是32 bits(4 bytes) 因而L1主頁表占用 4096 *4 = 16k的內存空間. 對于ARM926,其L1 section entry的格式為
可參考arm926EJS TRM):
(一級描述符的格式 可以參考《ARM體系結構與編程》P180) 
下面我們來分析 __create_page_tables 函數: 在 arch/arm/kernel/head.S 中: 00206: .type __create_page_tables, %function 00207: __create_page_tables: 00208: pgtbl r4 @ page table address 00209: 00210: /* 00211: * Clear the 16K level 1 swapper page table 00212: */ 00213: mov r0, r4 00214: mov r3, #0 00215: add r6, r0, #0x4000 00216: 1: str r3, [r0], #4 00217: str r3, [r0], #4 00218: str r3, [r0], #4 00219: str r3, [r0], #4 00220: teq r0, r6 00221: bne 1b 00222: 00223: ldr r7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags 00224: 00225: /* 00226: * Create identity mapping for first MB of kernel to 00227: * cater for the MMU enable. This identity mapping 00228: * will be removed by paging_init(). We use our current program 00229: * counter to determine corresponding section base address. 00230: */ 00231: mov r6, pc, lsr #20 @ start of kernel section 00232: orr r3, r7, r6, lsl #20 @ flags + kernel base 00233: str r3, [r4, r6, lsl #2] @ identity mapping 00234: 00235: /* 00236: * Now setup the pagetables for our kernel direct 00237: * mapped region. 00238: */ 00239: add r0, r4, #(TEXTADDR & 0xff000000) >> 18 @ start of kernel 00240: str r3, [r0, #(TEXTADDR & 0x00f00000) >> 18]! 00241: 00242: ldr r6, =(_end - PAGE_OFFSET - 1) @ r6 = number of sections 00243: mov r6, r6, lsr #20 @ needed for kernel minus 1 00244: 00245: 1: add r3, r3, #1 << 20 00246: str r3, [r0, #4]! 00247: subs r6, r6, #1 00248: bgt 1b 00249: 00250: /* 00251: * Then map first 1MB of ram in case it contains our boot params. 00252: */ 00253: add r0, r4, #PAGE_OFFSET >> 18 00254: orr r6, r7, #PHYS_OFFSET 00255: str r6, [r0] ... 00314: mov pc, lr 00315: .ltorg 206, 207行: 函數聲明 208行: 通過宏 pgtbl 將r4設置成頁表的基地址(物理地址) 宏pgtbl 在 arch/arm/kernel/head.S 中: 00042: .macro pgtbl, rd 00043: ldr /rd, =(__virt_to_phys(KERNEL_RAM_ADDR - 0x4000)) 00044: .endm 可以看到,頁表是位于 KERNEL_RAM_ADDR 下面 16k 的位置 宏 __virt_to_phys 是在incude/asm-arm/memory.h 中: 00125: #ifndef __virt_to_phys 00126: #define __virt_to_phys(x) ((x) - PAGE_OFFSET + PHYS_OFFSET) 00127: #define __phys_to_virt(x) ((x) - PHYS_OFFSET + PAGE_OFFSET) 00128: #endif 下面從213行 - 221行, 是將這16k 的頁表清0. 213行: r0 = r4, 將頁表基地址存在r0中 214行: 將 r3 置成0 215行: r6 = 頁表基地址 + 16k, 可以看到這是頁表的尾地址 216 - 221 行: 循環,從 r0 到 r6 將這16k頁表用0填充. 223行: 獲得proc_info_list的__cpu_mm_mmu_flags的值,并存儲到 r7中. (宏PROCINFO_MM_MMUFLAGS是在arch/arm/kernel/asm-offset.c中定義，值為8)（可以參考《嵌入式Linux應用完全開發手冊》P118）（r7的值就是設置這個段描述符的權限、域字段，）
在arch/arm/mm/proc-arm926.S 中:
00464: .section ".proc.info.init", #alloc, #execinstr
00465:
00466: .type __arm926_proc_info,#object
00467: __arm926_proc_info:
00468: .long 0x41069260 @ ARM926EJ-S (v5TEJ)
00469: .long 0xff0ffff0
00470: .long PMD_TYPE_SECT | /
00471: PMD_SECT_BUFFERABLE | /
00472: PMD_SECT_CACHEABLE | /
00473: PMD_BIT4 | /
00474: PMD_SECT_AP_WRITE | /
00475: PMD_SECT_AP_READ
00476: .long PMD_TYPE_SECT | /
00477: PMD_BIT4 | /
00478: PMD_SECT_AP_WRITE | /
00479: PMD_SECT_AP_READ
00480: b __arm926_setup
00481: .long cpu_arch_name
00482: .long cpu_elf_name
00483: .long HWCAP_SWP|HWCAP_HALF|HWCAP_THUMB|HWCAP_FAST_MULT|HWCAP_VFP|HWCAP_EDSP|HWCAP_JAVA
00484: .long cpu_arm926_name
00485: .long arm926_processor_functions
00486: .long v4wbi_tlb_fns
00487: .long v4wb_user_fns
00488: .long arm926_cache_fns
00489: .size __arm926_proc_info, . - __arm926_proc_info
231行: 通過pc值的高12位(右移20位),得到kernel的section,并存儲到r6中.因為當前是通過運行時地址得到的kernel的section,因而是物理地址. 232行: r3 = r7 | (r6 << 20); flags + kernel base,得到頁表中需要設置的值. 233行: 設置頁表: mem[r4 + r6 * 4] = r3 這里,因為頁表的每一項是32 bits(4 bytes),所以要乘以4(<<2). 上面這三行,設置了kernel的第一個section(物理地址所在的page entry)的頁表項 239, 240行: TEXTADDR是內核的起始虛擬地址(0xc0008000), 這兩行是設置kernel起始虛擬地址的頁表項(注意,這里設置的頁表項和上面的231 - 233行設置的頁表項是不同的 ) 執行完后,r0指向kernel的第2個section的虛擬地址所在的頁表項. /* TODO: 這兩行的code很奇怪,為什么要先取TEXTADDR的高8位(Bit[31:24])0xff000000,然后再取后面的8位 (Bit[23:20])0x00f00000*/ 242行: 這一行計算kernel鏡像的大小(bytes). _end 是在vmlinux.lds.S中162行定義的,標記kernel的結束位置(虛擬地址): 00158 .bss : { 00159 __bss_start = .; /* BSS */ 00160 *(.bss) 00161 *(COMMON) 00162 _end = .; 00163 } kernel的size = _end - PAGE_OFFSET -1, 這里 減1的原因是因為 _end 是 location counter,它的地址是kernel鏡像后面的一個byte的地址. 243行: 地址右移20位,計算出kernel有多少sections（也就是有多少兆，因為段描述符每個可以映射1MiB的虛擬地址）,并將結果存到r6中 245 - 248行: 這幾行用來填充kernel所有section虛擬地址對應的頁表項. 253行: 將r0設置為RAM第一兆虛擬地址的頁表項地址(page entry) 254行: r7中存儲的是mmu flags, 邏輯或上RAM的起始物理地址,得到RAM第一個MB頁表項的值. 255行: 設置RAM的第一個MB虛擬地址的頁表. 上面這三行是用來設置RAM中第一兆虛擬地址的頁表. 之所以要設置這個頁表項的原因是RAM的第一兆內存中可能存儲著boot params. 這樣,kernel所需要的基本的頁表我們都設置完了, 如下圖所示: 
下面是linux-2.6.30.4中的arch/arm/kernel/head.S，代碼有一些不同，但是效果一樣：
1: /*
2: * linux/arch/arm/kernel/head.S
3: *
4: * Copyright (C) 1994-2002 Russell King
5: * Copyright (c) 2003 ARM Limited
6: * All Rights Reserved
7: *
8: * This program is free software; you can redistribute it and/or modify
9: * it under the terms of the GNU General Public License version 2 as
10: * published by the Free Software Foundation.
11: *
12: * Kernel startup code for all 32-bit CPUs
13: */
14: #include <linux/linkage.h>
15: #include <linux/init.h>
16:
17: #include <asm/assembler.h>
18: #include <asm/domain.h>
19: #include <asm/ptrace.h>
20: #include <asm/asm-offsets.h>
21: #include <asm/memory.h>
22: #include <asm/thread_info.h>
23: #include <asm/system.h>
24:
25: #if (PHYS_OFFSET & 0x001fffff)
26: #error "PHYS_OFFSET must be at an even 2MiB boundary!"
27: #endif
28:
29: #define KERNEL_RAM_VADDR (PAGE_OFFSET + TEXT_OFFSET)
30: #define KERNEL_RAM_PADDR (PHYS_OFFSET + TEXT_OFFSET)
31:
32:
33: /*
34: * swapper_pg_dir is the virtual address of the initial page table.
35: * We place the page tables 16K below KERNEL_RAM_VADDR. Therefore, we must
36: * make sure that KERNEL_RAM_VADDR is correctly set. Currently, we expect
37: * the least significant 16 bits to be 0x8000, but we could probably
38: * relax this restriction to KERNEL_RAM_VADDR >= PAGE_OFFSET + 0x4000.
39: */
40: #if (KERNEL_RAM_VADDR & 0xffff) != 0x8000
41: #error KERNEL_RAM_VADDR must start at 0xXXXX8000
42: #endif
43:
44: .globl swapper_pg_dir
45: .equ swapper_pg_dir, KERNEL_RAM_VADDR - 0x4000
46:
47: .macro pgtbl, rd
48: ldr /rd, =(KERNEL_RAM_PADDR - 0x4000)
49: .endm
50:
51: #ifdef CONFIG_XIP_KERNEL
52: #define KERNEL_START XIP_VIRT_ADDR(CONFIG_XIP_PHYS_ADDR)
53: #define KERNEL_END _edata_loc
54: #else
55: #define KERNEL_START KERNEL_RAM_VADDR
56: #define KERNEL_END _end
57: #endif
58:
59: /*
60: * Kernel startup entry point.
61: * ---------------------------
62: *
63: * This is normally called from the decompressor code. The requirements
64: * are: MMU = off, D-cache = off, I-cache = dont care, r0 = 0,
65: * r1 = machine nr, r2 = atags pointer.
66: *
67: * This code is mostly position independent, so if you link the kernel at
68: * 0xc0008000, you call this at __pa(0xc0008000).
69: *
70: * See linux/arch/arm/tools/mach-types for the complete list of machine
71: * numbers for r1.
72: *
73: * We're trying to keep crap to a minimum; DO NOT add any machine specific
74: * crap here - that's what the boot loader (or in extreme, well justified
75: * circumstances, zImage) is for.
76: */
77: .section ".text.head", "ax"
78: ENTRY(stext)
79: msr cpsr_c, #PSR_F_BIT | PSR_I_BIT | SVC_MODE @ ensure svc mode
80: @ and irqs disabled
81: mrc p15, 0, r9, c0, c0 @ get processor id
82: bl __lookup_processor_type @ r5=procinfo r9=cpuid
83: movs r10, r5 @ invalid processor (r5=0)?
84: beq __error_p @ yes, error 'p'
85: bl __lookup_machine_type @ r5=machinfo
86: movs r8, r5 @ invalid machine (r5=0)?
87: beq __error_a @ yes, error 'a'
88: bl __vet_atags
89: bl __create_page_tables
90:
91: /*
92: * The following calls CPU specific code in a position independent
93: * manner. See arch/arm/mm/proc-*.S for details. r10 = base of
94: * xxx_proc_info structure selected by __lookup_machine_type
95: * above. On return, the CPU will be ready for the MMU to be
96: * turned on, and r0 will hold the CPU control register value.
97: */
98: ldr r13, __switch_data @ address to jump to after
99: @ mmu has been enabled
100: adr lr, __enable_mmu @ return (PIC) address
101: add pc, r10, #PROCINFO_INITFUNC
102: ENDPROC(stext)
103:
104: #if defined(CONFIG_SMP)
105: ENTRY(secondary_startup)
106: /*
107: * Common entry point for secondary CPUs.
108: *
109: * Ensure that we're in SVC mode, and IRQs are disabled. Lookup
110: * the processor type - there is no need to check the machine type
111: * as it has already been validated by the primary processor.
112: */
113: msr cpsr_c, #PSR_F_BIT | PSR_I_BIT | SVC_MODE
114: mrc p15, 0, r9, c0, c0 @ get processor id
115: bl __lookup_processor_type
116: movs r10, r5 @ invalid processor?
117: moveq r0, #'p' @ yes, error 'p'
118: beq __error
119:
120: /*
121: * Use the page tables supplied from __cpu_up.
122: */
123: adr r4, __secondary_data
124: ldmia r4, {r5, r7, r13} @ address to jump to after125: sub r4, r4, r5 @ mmu has been enabled
126: ldr r4, [r7, r4] @ get secondary_data.pgdir
127: adr lr, __enable_mmu @ return address
128: add pc, r10, #PROCINFO_INITFUNC @ initialise processor
129: @ (return control reg)
130: ENDPROC(secondary_startup)
131:
132: /*
133: * r6 = &secondary_data
134: */
135: ENTRY(__secondary_switched)
136: ldr sp, [r7, #4] @ get secondary_data.stack
137: mov fp, #0
138: b secondary_start_kernel
139: ENDPROC(__secondary_switched)
140:
141: .type __secondary_data, %object
142: __secondary_data:
143: .long .
144: .long secondary_data
145: .long __secondary_switched
146: #endif /* defined(CONFIG_SMP) */
147:
148:
149:
150: /*
151: * Setup common bits before finally enabling the MMU. Essentially
152: * this is just loading the page table pointer and domain access
153: * registers.
154: */
155: __enable_mmu:
156: #ifdef CONFIG_ALIGNMENT_TRAP
157: orr r0, r0, #CR_A
158: #else
159: bic r0, r0, #CR_A
160: #endif
161: #ifdef CONFIG_CPU_DCACHE_DISABLE
162: bic r0, r0, #CR_C
163: #endif
164: #ifdef CONFIG_CPU_BPREDICT_DISABLE
165: bic r0, r0, #CR_Z
166: #endif
167: #ifdef CONFIG_CPU_ICACHE_DISABLE
168: bic r0, r0, #CR_I
169: #endif
170: mov r5, #(domain_val(DOMAIN_USER, DOMAIN_MANAGER) | /
171: domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | /
172: domain_val(DOMAIN_TABLE, DOMAIN_MANAGER) | /
173: domain_val(DOMAIN_IO, DOMAIN_CLIENT))
174: mcr p15, 0, r5, c3, c0, 0 @ load domain access register
175: mcr p15, 0, r4, c2, c0, 0 @ load page table pointer
176: b __turn_mmu_on
177: ENDPROC(__enable_mmu)
178:
179: /*
180: * Enable the MMU. This completely changes the structure of the visible
181: * memory space. You will not be able to trace execution through this.
182: * If you have an enquiry about this, *please* check the linux-arm-kernel
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