第3阶段——内核启动分析之创建si工程和启动内核分析

theKernel = (void (*)(int, int, unsigend int))0x30008000;

// 设置theKernel地址=0x30008000,用于后面启动内核

/*设置atag参数*/

setup_start_tag (void);                      //从0X30000100地址处开始保存start_tag数据，

setup_memory_tags (void);         //保存memory_tag数据,让内核知道内存多大 setup_commandline_tag (“boottargs=noinitrd root=/dev/mtdblock3 init=/linuxrc console=ttySAC0”);  

 /*保存命令行bootargs参数,让内核知道根文件系统位置在/dev/mtdblock3,指定开机运行第一个脚本/linuxrc,指定打印串口0*/

setup_end_tag (void);                        //初始化tag结构体结束

theKernel(0,362,0x300000100);        //362:机器ID，  0x300000100: params(atag)参数地址

/*传递参数跳转执行到0x30008000启动内核,           */

/*相当于: mov r0,#0                                */

/*ldr r1,=362                                       */

/*ldr r2,= 0x300000100                             */

/*mov pc,#0x30008000                            */

OUTPUT_ARCH(arm)                    //设置输出文件的体系架构

 ENTRY(stext)                          //设置stext全局符号为入口地址

 jiffies = jiffies_64;

SECTIONS
{
. = (0xc0000000) + 0x00008000;               

/*设置内核虚拟地址=0xc0000000+0x00008000 */
.text.head : {
  _stext = .;
  _sinittext = .;
  *(.text.head)         //添加所有.text.head段
}
.init : { /* Init code and data                */
   *(.init.text)
  _einittext = .;
  __proc_info_begin = .;
   *(.proc.info.init)               //存放处理器相关的信息初始化
  __proc_info_end = .;
  __arch_info_begin = .;
   *(.arch.info.init)              //存放与架构(arch)相关的信息(info)初始化
  __arch_info_end = .;
...  ...

/*

 * Kernel startup entry point.                      //内核 启动 入口 点

 * ---------------------------

 *

 * This is normally called from the decompressor code.  The requirements

 * are: MMU = off, D-cache = off, I-cache = dont care, r0 = 0,  

/* 前置条件是：MMU, D-cache, 关闭; r0 = 0, r1 =机器ID, r2 =atag参数地址.*/

 * r1 = machine nr.

* This code is mostly position independent, so if you link the kernel at

 * 0xc0008000, you call this at __pa(0xc0008000).

* See linux/arch/arm/tools/mach-types for the complete list of machine

 * numbers for r1.

 *

 * We're trying to keep crap to a minimum; DO NOT add any machine specific

 * crap here - that's what the boot loader (or in extreme, well justified

 * circumstances, zImage) is for.

 */

 section ".text.head", "ax"                      /* 定义一个.text.head段,段的属性a是允许段,x可 执行 */ 

         .type  stext, %function                /*定义了由bootloader进入内核的入口stext */

ENTRY(stext)

... ...

section ".text.head", "ax"                          /* 定义一个.text.head段,段的属性a是允许段,x可 执行 */ 

         .type   stext, %function                     /*定义了由bootloader进入内核的入口stext */

ENTRY(stext)                                    //入口地址stext函数

      /*msr cpsr_c,0xD3   关闭irq和fiq,设置svc管理模式  */

         msr    cpsr_c, #PSR_F_BIT | PSR_I_BIT | SVC_MODE @ ensure svc mode 

                                                          @ and irqs disabled

     /*获取cpu ID */

         mrc    p15, 0, r9, c0, c0              @ get processor id

     /*查找内核是否支持r9这个cpuID，若不支持r5=0，支持r5=处理器ID*/

         bl       __lookup_processor_type             @ r5=procinfo r9=cpuid

         movs  r10, r5                                      @ invalid processor (r5=0)?

      /*不支持则跳转到__error_p,死循环*/

         beq     __error_p                         @ yes, error 'p'

     /*查找内核是否支持uboot传入的r1机器ID(362)，若不支持r5=0，支持r5=机器ID*/

         bl       __lookup_machine_type              @ r5=machinfo

         movs  r8, r5                              @ invalid machine (r5=0)?

     /*不支持则跳转到__error_a,死循环*/

         beq     __error_a                         @ yes, error 'a'

 /*跳转到__create_page_tables 创建页表，为后面的MMU做准备*/

         bl       __create_page_tables               

3:       .long  .
         .long  __arch_info_begin
         .long  __arch_info_end


__lookup_machine_type:

         /*(b:bank)r3=后面的符号3处. 虚拟地址，由于mmu未启动，所以=物理地址*/
adr     r3, 3b               
         ldmia  r3, {r4, r5, r6}/* r4=3b处的虚拟地址 ,r5=__arch_info_begin处的虚拟地址,r6=__arch_info_end处的虚拟地址   */
         sub     r3, r3, r4                         @ get offset between virt&phys //得到虚拟地址(virtual)与物理地址(physical)的偏移值
         add     r5, r5, r3                         @ convert virt addresses to  //找到arch_info_begin处的物理地址
         add     r6, r6, r3                         @ physical address space    //找到__arch_info_end处的物理地址
1:       ldr      r3, [r5, #MACHINFO_TYPE]      @ get machine type   //r3=r5+偏移地址里内容= 单板机器ID
         teq     r3, r1                                  //判断r1(365)和单板机器ID是否相等，相等说明内核支持该单板
         beq     2f                                   @ found   //相等则直接返回到stext函数继续执行
         add     r5, r5, #SIZEOF_MACHINE_DESC      @ next machine_desc
         cmp    r5, r6

         blo     1b
         mov   r5, #0                             @ unknown machine  //r5=0,不支持该单板
2:       mov   pc, lr                       //退出

305  __arch_info_begin = .;           //__arch_info_begin=信息开始地址
306     *(.arch.info.init)              //存放架构相关的信息初始化
307  __arch_info_end = .;           //__arch_info_end =信息结束地址

#define MACHINE_START(_type,_name)                     //定义了一个MACHINE_START宏, _type:CPU名字,_name:开发板名字
static const struct machine_desc __mach_desc_##_type \      //##:连词符号
 __used                                                                \
 __attribute__((__section__(".arch.info.init"))) = {       \  //强制将  MACHINE_START宏里的成员组成.arch.info.init段
         .nr               = MACH_TYPE_##_type,          \ 
         .name           = _name,


#define MACHINE_END                               \       //定义宏MACHINE_END=   };

};

1 MACHINE_START(S3C2440, "SMDK2440")       
2     /* Maintainer: Ben Dooks <ben@fluff.org> */
3     .phys_io  = S3C2410_PA_UART,
4     .io_pg_offst    = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
5     .boot_params  = S3C2410_SDRAM_PA + 0x100,
6
7     .init_irq   = s3c24xx_init_irq,
8     .map_io          = smdk2440_map_io,
9     .init_machine  = smdk2440_machine_init,
10   .timer             = &s3c24xx_timer,
11  MACHINE_END

static const struct  machine_desc  __mach_desc_ S3C2440  //定义一个machine_desc型结构体,名字为__mach_desc_ S3C2440
 __used                                                                \
 __attribute__((__section__(".arch.info.init"))) = {        //强制将MACHINE_START宏里的成员组成.arch.info.init段
         .nr               = MACH_TYPE_ S3C2440,  // __mach_desc_ S3C2440.nr= MACH_TYPE_ S3C2440  机器ID
         .name           = "SMDK2440",              //__mach_desc_ S3C2440. name = "SMDK2440"  机器ID名字   

       /* Maintainer: Ben Dooks <ben@fluff.org> */
    /*.phys_io  =0X50000000,存放物理IO基地址*/
       .phys_io  = S3C2410_PA_UART,         

 /* .io_pg_offst存放物理IO偏移地址*/  
       .io_pg_offst    = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,   

        

 

/*其中S3C2410_SDRAM_PA=0X30000000, .boot_params= 0X30000100,所以我们uboot传入的atag参数地址必须是0X30000100*/
.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替换成  };

ldr   r13, __switch_data             @ address to jump to after
//MMU使能之后会跳转(jump)到__switch_data@ mmu has been enabled 

adr  lr, __enable_mmu              @ return (PIC) address//使能MMU
add pc, r10, #PROCINFO_INITFUNC  

__enable_mmu: 
      ...  .... 
      mov    r3, r3
      mov    r3, r3
      mov    pc, r13

__switch_data:
         .long  __mmap_switched                          //进入__mmap_switched函数
         .long  __data_loc                       @ r4              
         .long  __data_start                     @ r5
         .long  __bss_start                      @ r6
         .long  _end                               @ r7
         .long                                       @ r4
         .long  __machine_arch_type                  @ r5
         .long  cr_alignment                         @ r6
         .long  init_thread_union + THREAD_START_SP @ sp

__mmap_switched:
         adr     r3, __switch_data + 4           //r3=__data_loc段内容
/*其中
__data_loc 是数据存放的位置
__data_start 是数据开始的位置    
__bss_start 是bss开始的位置
_end 是bss结束的位置, 也是内核结束的位置
这几个符号都在arch/arm/kernel/vmlinux.lds中定义的变量
*/

         ldmia  r3!, {r4, r5, r6, r7}   //r4=__data_loc ,  r5=__data_start ,  r6=__bss_start ，r7=_end ,  r3= processor_id
         cmp    r4, r5                                        // __data_loc段不等于__data_start段则执行下面1处的内容
1:       cmpne  r5, r6                  // 比较r5(__data_start段)和r6(__bss_start段)
         ldrne   fp, [r4], #4                
         strne   fp, [r5], #4                //str r4,[r5] 将整个段里内容从 __data_loc段 复制到__data_start段
         bne     1b                      //r5不等于r6，则继续复制   


         mov   fp, #0                             @ Clear BSS (and zero fp)  //清除bss段
1:       cmp    r6, r7                   //比较r6(__bss_start t段)和r7(_end段)
         strcc   fp, [r6],#4               // 清除bss段
         bcc     1b                     //(cc:小于)r6<r7,继续清除bss段 


         ldmia  r3, {r4, r5, r6, sp}       //r4=r3= processor_id, r5=__machine_arch_type,r6= cr_alignment,
//设置栈sp= init_thread_union + THREAD_START_SP,方便执行C函数start_kernel
         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                   //然后执行start_kernel函数