linux内核启动head.s

linux内核启动head.s

已知条件：
1.首先通过分析Kernel的Makefile的链接知道 之后内核启动执行的第一个脚本是：../arch/arm/kernel/head.S
2.u-boot启动内核：

theKernel(0,bd->bi_arch_number,bd->bi_boot_params) //启动内核
    bd->bi_arch_number  //机器id
    bd->bi_boot_params  //uboot传入内核参数存放的地址

分析

//head.S:
                                @ 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'
    bl  __lookup_machine_type   @ r5=machinfo  //查找机器ID
    movs    r8, r5              @ invalid machine (r5=0)?
    beq __error_a               @ yes, error 'a'
    bl  __create_page_tables

//head-common.S:
3:                              #3b地址定义
    .long   .                   #3b的虚拟地址
    .long   __arch_info_begin
    .long   __arch_info_end

 __lookup_machine_type:
    adr r3, 3b                      #r3 = 3b的物理地址
    ldmia   r3, {r4, r5, r6}        
    sub r3, r3, r4   
    add r5, r5, r3          
    add r6, r6, r3                  @ physical address space
1:  ldr r3, [r5, #MACHINFO_TYPE]    @ get machine type
    teq r3, r1                      @ matches loader number?
    beq 2f                          @ found
    add r5, r5, #SIZEOF_MACHINE_DESC    @ next machine_desc
    cmp r5, r6
    blo 1b
    mov r5, #0                      @ unknown machine
2:  mov pc, lr
//地址转换过程
adr r3, 3b  #r3 = 3b的物理地址
ldmia   r3, {r4, r5, r6}
# r4= 3标号的虚拟地址
# r5=__arch_info_begin 
# r6 = __arch_info_end
//__arch_info_begin,__arch_info_end在哪儿定义？
vmlinux.lds：
  ......
  __arch_info_begin=.;
  *(.arch.info.init) //这是代码中定义的一个段
  __arch_info_end=.;

再看看：arch.h
arch.h
  #define MACHINE_START(_type,_name)            \
  static const struct machine_desc __mach_desc_##_type  \
   __used                           \
   __attribute__((__section__(".arch.info.init"))) = {  \
    .nr     = MACH_TYPE_##_type,        \
    .name       = _name,

   #define MACHINE_END              \
  };

 //那么谁在使用这个宏
 谁在使用这个宏：以s3c2440为例：
MACHINE_START(S3C2440, "SMDK2440")
    /* Maintainer: Ben Dooks  */
    .phys_io    = S3C2410_PA_UART,
    .io_pg_offst    = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
    .boot_params    = S3C2410_SDRAM_PA + 0x100,
    .init_irq   = s3c24xx_init_irq,
    .map_io     = smdk2440_map_io,
    .init_machine   = smdk2440_machine_init,
    .timer      = &s3c24xx_timer,
MACHINE_END

将其展开：
static const struct machine_desc __mach_desc_S3C2440 = {
    .nr     = MACH_TYPE_S3C2440,        
    .name       = "SMDK2440",
    .phys_io    = S3C2410_PA_UART,
    .io_pg_offst    = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
    .boot_params    = S3C2410_SDRAM_PA + 0x100,
    .init_irq   = s3c24xx_init_irq,
    .map_io     = smdk2440_map_io,
    .init_machine   = smdk2440_machine_init,
    .timer      = &s3c24xx_timer,
};
//machine_desc
machine_desc 打开这个结构体
描述一个单板的属性：
struct machine_desc {
    unsigned int        nr;     /* architecture number  */
    unsigned int        phys_io;    /* start of physical io */
    unsigned int        io_pg_offst;    /* byte offset for io * page tabe entry */

    const char      *name;      /* architecture name    */ 机器id
    unsigned long       boot_params;    /* tagged list      */ 参数

    unsigned int        video_start;    /* start of video RAM   */
    unsigned int        video_end;  /* end of video RAM */

    unsigned int        reserve_lp0 :1; /* never has lp0    */
    unsigned int        reserve_lp1 :1; /* never has lp1    */
    unsigned int        reserve_lp2 :1; /* never has lp2    */
    unsigned int        soft_reboot :1; /* soft reboot      */
    void            (*fixup)(struct machine_desc *,
                     struct tag *, char **,
                     struct meminfo *);
    void            (*map_io)(void);/* IO mapping function  */
    void            (*init_irq)(void);
    struct sys_timer    *timer;     /* system tick timer    */
    void            (*init_machine)(void);
};

在看看head_common.S
    1:  
    ldr r3, [r5, #MACHINFO_TYPE]    @ get machine type
    teq r3, r1                      @ matches loader number? 比较机器id

比较成功之后：
     bl     __create_page_tables    @创建页表
     ldr    r13, __switch_data      @ address to jump to after
     @ mmu has been enabled
     adr    lr, __enable_mmu        @ return (PIC) address  #使能MMU
     add    pc, r10, #PROCINFO_INITFUNC
创建页表，使能MMU之后跳转到__switch_data
 __switch_data:
    .long   __mmap_switched
    .long   __data_loc              @ r4
    .long   __data_start            @ r5
    .long   __bss_start             @ r6
    .long   _end                    @ r7
    .long   processor_id            @ r4
    .long   __machine_arch_type     @ r5
    .long   cr_alignment            @ r6
    .long   init_thread_union + THREAD_START_SP   @ sp

/*
 * The following fragment of code is executed with the MMU on in MMU mode,
 * and uses absolute addresses; this is not position independent.
 *
 *  r0  = cp#15 control register
 *  r1  = machine ID
 *  r9  = processor ID
 */
    .type   __mmap_switched, %function
__mmap_switched:
    adr r3, __switch_data + 4

    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

    ldmia   r3, {r4, r5, r6, sp}
    str r9, [r4]                @ Save processor ID
    str r1, [r5]                @ Save machine type
    bic r4, r0, #CR_A           @ Clear 'A' bit
    stmia   r6, {r0, r4}        @ Save control register values
    b   start_kernel    #启动内核 内核的第一个C函数：

总结：Head.s的作用
1、判断内核支持不支持CPU
2、判断内核支持不支持单板
3、建立页表
4、使能MMU
5、跳转到 start_kernel