linux启动流程导读(arm为例)

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

     .arm
 
     __HEAD                          @#define __HEAD  .section ".head.text","ax"
 ENTRY(stext)
 
  THUMB(    adr    r9, BSYM(1f)    )    @ Kernel is always entered in ARM.
  THUMB(    bx    r9        )    @ If this is a Thumb-2 kernel,
  THUMB(    .thumb            )    @ switch to Thumb now.
  THUMB(1:            )
 
     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
                         @ and irqs disabled
     mrc    p15, 0, r9, c0, c0        @ get processor id         获得芯片ID
     bl    __lookup_processor_type        @ r5=procinfo r9=cpuid 获得处理器型号，r5 == id，#1
     movs    r10, r5                @ invalid processor (r5=0)?  校验正确性，0错误
  THUMB( it    eq )        @ force fixup-able long branch encoding
     beq    __error_p            @ yes, error 'p'
 
 #ifndef CONFIG_XIP_KERNEL
     adr    r3, 2f
     ldmia    r3, {r4, r8}
     sub    r4, r3, r4            @ (PHYS_OFFSET - PAGE_OFFSET)

     add    r8, r8, r4            @ PHYS_OFFSET

==========

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

==========

     __CPUINIT
 __lookup_processor_type:
     adr    r3, __lookup_processor_type_data        @adr 相对偏移读取，读取下面type_data地址
     ldmia    r3, {r4 - r6}       @将该地址存放的值 放入r4(.)，r5(begin)，r6(end)
     sub    r3, r3, r4            @ get offset between virt&phys   链接地址-实际地址=偏移量

==========

继续arch/arm/kernel/head.s

==========

     bl    __vet_atags       @#1,head-common.s
 #ifdef CONFIG_SMP_ON_UP
     bl    __fixup_smp       @略
 #endif
 #ifdef CONFIG_ARM_PATCH_PHYS_VIRT
     bl    __fixup_pv_table  @略
 #endif
     bl    __create_page_tables  @#2

==========

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

==========

 __vet_atags:
     tst    r2, #0x3            @ aligned? 不对其就返回
     bne    1f
 
     ldr    r5, [r2, #0]        @读到r5
 #ifdef CONFIG_OF_FLATTREE
     ldr    r6, =OF_DT_MAGIC        @ is it a DTB?  过滤DTB
     cmp    r5, r6
     beq    2f
 #endif
     cmp    r5, #ATAG_CORE_SIZE        @ is first tag ATAG_CORE? must be first
     cmpne    r5, #ATAG_CORE_SIZE_EMPTY
     bne    1f
     ldr    r5, [r2, #4]
     ldr    r6, =ATAG_CORE
     cmp    r5, r6
     bne    1f
 
 2:    mov    pc, lr                @ atag/dtb pointer is ok  
 
 1:    mov    r2, #0
     mov    pc, lr
 ENDPROC(__vet_atags)

============

@#2：__create_page_tables

============

 __create_page_tables:
     pgtbl    r4, r8                @ page table address

============

1 .ltorg2 .align3 __enable_mmu_loc:4 .long .5 .long __enable_mmu6 .long __enable_mmu_end==============
 

     ldr    r13, =__mmap_switched        @ address to jump to after
                         @ mmu has been enabled
     adr    lr, BSYM(1f)            @ return (PIC) address
     mov    r8, r4                @ set TTBR1 to swapper_pg_dir
  ARM(    add    pc, r10, #PROCINFO_INITFUNC    )
  THUMB(    add    r12, r10, #PROCINFO_INITFUNC    )
  THUMB(    mov    pc, r12                )
 1:    b    __enable_mmu

============

进入enable_mmu

 __enable_mmu:
 #if defined(CONFIG_ALIGNMENT_TRAP) && __LINUX_ARM_ARCH__ < 6
     orr    r0, r0, #CR_A
 #else
     bic    r0, r0, #CR_A
 #endif
 #ifdef CONFIG_CPU_DCACHE_DISABLE
     bic    r0, r0, #CR_C
 #endif
 #ifdef CONFIG_CPU_BPREDICT_DISABLE
     bic    r0, r0, #CR_Z
 #endif
 #ifdef CONFIG_CPU_ICACHE_DISABLE
     bic    r0, r0, #CR_I
 #endif
     mov    r5, #(domain_val(DOMAIN_USER, DOMAIN_MANAGER) | \
               domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | \
               domain_val(DOMAIN_TABLE, DOMAIN_MANAGER) | \
               domain_val(DOMAIN_IO, DOMAIN_CLIENT))
     mcr    p15, 0, r5, c3, c0, 0        @ load domain access register
     mcr    p15, 0, r4, c2, c0, 0        @ load page table pointer
     b    __turn_mmu_on
 ENDPROC(__enable_mmu)

==========

     .align    5
 __turn_mmu_on:
     mov    r0, r0
     mcr    p15, 0, r0, c1, c0, 0        @ write control reg
     mrc    p15, 0, r3, c0, c0, 0        @ read id reg
     mov    r3, r3
     mov    r3, r13      //跳回r13，=__mmap_switched
     mov    pc, r3
 __enable_mmu_end:
 ENDPROC(__turn_mmu_on)

==========

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

     __INIT
 __mmap_switched:
     adr    r3, __mmap_switched_data
 
     ldmia    r3!, {r4, r5, r6, r7}
     cmp    r4, r5                @ Copy data segment if needed
 1:    cmpne    r5, r6
     ldrne    fp, [r4], #4
     strne    fp, [r5], #4
     bne    1b
 
     mov    fp, #0                @ Clear BSS (and zero fp)
 1:    cmp    r6, r7
     strcc    fp, [r6],#4
     bcc    1b
 
  ARM(    ldmia    r3, {r4, r5, r6, r7, sp})
  THUMB(    ldmia    r3, {r4, r5, r6, r7}    )
  THUMB(    ldr    sp, [r3, #16]        )
     str    r9, [r4]            @ Save processor ID
     str    r1, [r5]            @ Save machine type
     str    r2, [r6]            @ Save atags pointer
     bic    r4, r0, #CR_A            @ Clear 'A' bit
     stmia    r7, {r0, r4}            @ Save control register values
     b    start_kernel
 ENDPROC(__mmap_switched)

===========

 

绝对地址操作，copy data，bss清空，跳入start_kernel，进入c环境。