linux启动流程导读(arm为例)<一>

本文以ARM为例，深入探讨Linux内核3.2版本的启动过程。从arch/arm/kernel/head.s开始，讲解了enable_mmu、page tables的创建以及如何进入C环境的详细步骤。

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以arm为例，分析一下kernel的启动过程;

内核版本：linux-3.2.tar.gz

一、arch/arm/kernel/head.s

 1 

19     .arm

20 

21     __HEAD                          @#define __HEAD  .section ".head.text","ax"

22 ENTRY(stext)

23 

24  THUMB(    adr    r9, BSYM(1f)    )    @ Kernel is always entered in ARM.

25  THUMB(    bx    r9        )    @ If this is a Thumb-2 kernel,

26  THUMB(    .thumb            )    @ switch to Thumb now.

27  THUMB(1:            )

28 

29     setmode    PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode   关闭普通中断，快速中断，使能svc模式

30                         @ and irqs disabled

31     mrc    p15, 0, r9, c0, c0        @ get processor id         获得芯片ID

32     bl    __lookup_processor_type        @ r5=procinfo r9=cpuid 获得处理器型号，r5 == id，#1

33     movs    r10, r5                @ invalid processor (r5=0)?  校验正确性，0错误

34  THUMB( it    eq )        @ force fixup-able long branch encoding

35     beq    __error_p            @ yes, error 'p'

36 

37 #ifndef CONFIG_XIP_KERNEL

38     adr    r3, 2f

39     ldmia    r3, {r4, r8}

40     sub    r4, r3, r4            @ (PHYS_OFFSET - PAGE_OFFSET)

41     add    r8, r8, r4            @ PHYS_OFFSET

42 #else

43     ldr    r8, =PHYS_OFFSET        @ always constant in this case #define PHYS_OFFSET   UL(CONFIG_DRAM_BASE)

44 #endif

#1 :arch/arm/kernel/head-common.h

 1 

14     __CPUINIT

15 __lookup_processor_type:

16     adr    r3, __lookup_processor_type_data        @adr 相对偏移读取，读取下面type_data地址

17     ldmia    r3, {r4 - r6}       @将该地址存放的值 放入r4(.)，r5(begin)，r6(end)

18     sub    r3, r3, r4            @ get offset between virt&phys   链接地址-实际地址=偏移量

19     add    r5, r5, r3            @ convert virt addresses to    得到begin的虚拟起始地址

20     add    r6, r6, r3            @ physical address space       得到end的虚拟起始地址

21 1:    ldmia    r5, {r3, r4}            @ value, mask            得到r5的两个值到r3，r4

22     and    r4, r4, r9            @ mask wanted bits             确定id值

23     teq    r3, r4

24     beq    2f                                                  @相等返回，不等读取下一组

25     add    r5, r5, #PROC_INFO_SZ        @ sizeof(proc_info_list)

26     cmp    r5, r6

27     blo    1b

28     mov    r5, #0                @ unknown processor           @没有找到返回r5==0

29 2:    mov    pc, lr

30 ENDPROC(__lookup_processor_type)

31 

32 

35     .align    2

36     .type    __lookup_processor_type_data, %object

37 __lookup_processor_type_data:

38     .long    .

39     .long    __proc_info_begin

40     .long    __proc_info_end

41     .size    __lookup_processor_type_data, . - __lookup_processor_type_data

42 

43 __error_p:

44 #ifdef CONFIG_DEBUG_LL

45     adr    r0, str_p1

46     bl    printascii

47     mov    r0, r9

48     bl    printhex8

49     adr    r0, str_p2

50     bl    printascii

51     b    __error

52 str_p1:    .asciz    "\nError: unrecognized/unsupported processor variant (0x"

53 str_p2:    .asciz    ").\n"

54     .align

55 #endif

56 ENDPROC(__error_p)

57 

58 __error:

59 #ifdef CONFIG_ARCH_RPC

60 

63     mov    r0, #0x02000000

64     mov    r3, #0x11

65     orr    r3, r3, r3, lsl #8

66     orr    r3, r3, r3, lsl #16

67     str    r3, [r0], #4

68     str    r3, [r0], #4

69     str    r3, [r0], #4

70     str    r3, [r0], #4

71 #endif

继续arch/arm/kernel/head.s

 1 

 5     bl    __vet_atags       @#1,head-common.s

 6 #ifdef CONFIG_SMP_ON_UP

 7     bl    __fixup_smp       @略

 8 #endif

 9 #ifdef CONFIG_ARM_PATCH_PHYS_VIRT

10     bl    __fixup_pv_table  @略

11 #endif

12     bl    __create_page_tables  @#2

#1 :arch/arm/kernel/head-common.h

 1 

12 __vet_atags:

13     tst    r2, #0x3            @ aligned? 不对其就返回

14     bne    1f

15 

16     ldr    r5, [r2, #0]        @读到r5

17 #ifdef CONFIG_OF_FLATTREE

18     ldr    r6, =OF_DT_MAGIC        @ is it a DTB?  过滤DTB

19     cmp    r5, r6

20     beq    2f

21 #endif

22     cmp    r5, #ATAG_CORE_SIZE        @ is first tag ATAG_CORE? must be first

23     cmpne    r5, #ATAG_CORE_SIZE_EMPTY

24     bne    1f

25     ldr    r5, [r2, #4]

26     ldr    r6, =ATAG_CORE

27     cmp    r5, r6

28     bne    1f

29 

30 2:    mov    pc, lr                @ atag/dtb pointer is ok  

31 

32 1:    mov    r2, #0

33     mov    pc, lr

34 ENDPROC(__vet_atags)

@#2：__create_page_tables

 1 

12 __create_page_tables:

13     pgtbl    r4, r8                @ page table address

       @#define PG_DIR_SIZE 0x4000

        #define PMD_ORDER 2

       @vmlinux.lds.s,#define LOAD_OFFSET TEXT_OFFSET,0x30008000?

       @.macro pgtbl, rd, phys

       @ add \rd, \phys, #TEXT_OFFSET - PG_DIR_SIZE   r4= offset+ 0x30008000-0x4000,page_dir_size = 0x4000 = 16k

       @.endm

14 

15     

18     mov    r0, r4                @r0==r4

19     mov    r3, #0                @r3==0

20     add    r6, r0, #PG_DIR_SIZE  @r6==page_dir_end

21 1:    str    r3, [r0], #4        @clear 0,四个一组

22     str    r3, [r0], #4

23     str    r3, [r0], #4

24     str    r3, [r0], #4

25     teq    r0, r6                   

26     bne    1b                   @不到结尾返回继续

27 

28     ldr    r7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags   @load mmuflags

29 

30     

34     adr    r0, __enable_mmu_loc

35     ldmia    r0, {r3, r5, r6}

36     sub    r0, r0, r3            @ virt->phys offset      重新计算得到偏移地址，应当是负值，r0位置无关地址，r3链接地址

37     add    r5, r5, r0            @  __enable_mmu          重新得到实际物理地址

38     add    r6, r6, r0            @ __enable_mmu_end

39     mov    r5, r5, lsr #SECTION_SHIFT                 @1M对齐

40     mov    r6, r6, lsr #SECTION_SHIFT

41 

42 1:    orr    r3, r7, r5, lsl #SECTION_SHIFT    @ flags + kernel base  从enable_mmu地址开始映射，1M为单位，直接映射

                                                  @r5为enable_mmu物理地址

43     str    r3, [r4, r5, lsl #PMD_ORDER]        @ identity mapping     括号内可得物理地址表项，物理地址对应表项映射回相应地址

44     cmp    r5, r6                              @ 直到 mmu_end

45     addlo    r5, r5, #1            @ next section

46     blo    1b

47 

48     

52     mov    r3, pc                              @当前pc存入r3

53     mov    r3, r3, lsr #SECTION_SHIFT          @1M对齐

54     orr    r3, r7, r3, lsl #SECTION_SHIFT      @kernel映射，0xc...虚拟地址映射到当前pc物理地址

55     add    r0, r4,  #(KERNEL_START & 0xff000000) >> (SECTION_SHIFT - PMD_ORDER)

56     str    r3, [r0, #((KERNEL_START & 0x00f00000) >> SECTION_SHIFT) << PMD_ORDER]!

57     ldr    r6, =(KERNEL_END - 1)

58     add    r0, r0, #1 << PMD_ORDER

59     add    r6, r4, r6, lsr #(SECTION_SHIFT - PMD_ORDER)

60 1:    cmp    r0, r6

61     add    r3, r3, #1 << SECTION_SHIFT

62     strls    r3, [r0], #1 << PMD_ORDER

1     .ltorg

2     .align

3 __enable_mmu_loc:

4     .long    .

5     .long    __enable_mmu

6     .long    __enable_mmu_end

 1 

 5     mov    r0, r2, lsr #SECTION_SHIFT

 6     movs    r0, r0, lsl #SECTION_SHIFT  @清空了r0 1m

 7     moveq    r0, r8                     @如果是0，也就是没有指定boot params则放入r8，即内存其实地址0x3------

 8     sub    r3, r0, r8                   @得到r0与r8的差，如果没指定则差为0，如果指定了得出个偏移

 9     add    r3, r3, #PAGE_OFFSET         @差值+内存基地址，得到实际参数地址

10     add    r3, r4, r3, lsr #(SECTION_SHIFT - PMD_ORDER)   @做映射，直接映射，无偏移

11     orr    r6, r7, r0

12     str    r6, [r3]

3.0选择性将enablemmu和boot params双映射，并不是以前机械的前8M双映射

1 mov    pc, lr

回到

 1     

 8     ldr    r13, =__mmap_switched        @ address to jump to after

 9                         @ mmu has been enabled

10     adr    lr, BSYM(1f)            @ return (PIC) address

11     mov    r8, r4                @ set TTBR1 to swapper_pg_dir

12  ARM(    add    pc, r10, #PROCINFO_INITFUNC    )

13  THUMB(    add    r12, r10, #PROCINFO_INITFUNC    )

14  THUMB(    mov    pc, r12                )

15 1:    b    __enable_mmu

进入enable_mmu

 1 

13 __enable_mmu:

14 #if defined(CONFIG_ALIGNMENT_TRAP) && __LINUX_ARM_ARCH__ < 6

15     orr    r0, r0, #CR_A

16 #else

17     bic    r0, r0, #CR_A

18 #endif

19 #ifdef CONFIG_CPU_DCACHE_DISABLE

20     bic    r0, r0, #CR_C

21 #endif

22 #ifdef CONFIG_CPU_BPREDICT_DISABLE

23     bic    r0, r0, #CR_Z

24 #endif

25 #ifdef CONFIG_CPU_ICACHE_DISABLE

26     bic    r0, r0, #CR_I

27 #endif

28     mov    r5, #(domain_val(DOMAIN_USER, DOMAIN_MANAGER) | \

29               domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | \

30               domain_val(DOMAIN_TABLE, DOMAIN_MANAGER) | \

31               domain_val(DOMAIN_IO, DOMAIN_CLIENT))

32     mcr    p15, 0, r5, c3, c0, 0        @ load domain access register

33     mcr    p15, 0, r4, c2, c0, 0        @ load page table pointer

34     b    __turn_mmu_on

35 ENDPROC(__enable_mmu)

 1 

15     .align    5

16 __turn_mmu_on:

17     mov    r0, r0

18     mcr    p15, 0, r0, c1, c0, 0        @ write control reg

19     mrc    p15, 0, r3, c0, c0, 0        @ read id reg

20     mov    r3, r3

21     mov    r3, r13      //跳回r13，=__mmap_switched

22     mov    pc, r3

23 __enable_mmu_end:

24 ENDPROC(__turn_mmu_on)

#arch/arm/kernel/head-common.s

 1 

10     __INIT

11 __mmap_switched:

12     adr    r3, __mmap_switched_data

13 

14     ldmia    r3!, {r4, r5, r6, r7}

15     cmp    r4, r5                @ Copy data segment if needed

16 1:    cmpne    r5, r6

17     ldrne    fp, [r4], #4

18     strne    fp, [r5], #4

19     bne    1b

20 

21     mov    fp, #0                @ Clear BSS (and zero fp)

22 1:    cmp    r6, r7

23     strcc    fp, [r6],#4

24     bcc    1b

25 

26  ARM(    ldmia    r3, {r4, r5, r6, r7, sp})

27  THUMB(    ldmia    r3, {r4, r5, r6, r7}    )

28  THUMB(    ldr    sp, [r3, #16]        )

29     str    r9, [r4]            @ Save processor ID

30     str    r1, [r5]            @ Save machine type

31     str    r2, [r6]            @ Save atags pointer

32     bic    r4, r0, #CR_A            @ Clear 'A' bit

33     stmia    r7, {r0, r4}            @ Save control register values

34     b    start_kernel

35 ENDPROC(__mmap_switched)