Linux架构/开发板相关的引导过程（head.S）

pulbieup · 发表于 2020-4-22 10:20

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分析内核顶层Makefile，以及arch/ARM/Makefile可知，从u-boot跳到内核后，第一个启动的文件是arch/arm/kernel/head.S。接下来就围绕这个文件来分析Linux架构/开发板相关的引导过程。

/******************************以下仅分析重要部分********************************/

#include "head-common.S" //下面会调用到这个文件里面的函数

......

/*
* Kernel startup entry point.
* ---------------------------
*
* This is normally called from the decompressor code.  The requirements
* are: MMU = off, D-cache = off, I-cache = dont care, r0 = 0,
* r1 = machine nr, r2 = atags pointer.
*
* 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.
* ........
*/
.section ".text.head", "ax"       //".text.head"段，在vmlinux.lds链接文件中定义
ENTRY(stext)
   msr cpsr_c, #PSR_F_BIT | PSR_I_BIT | SVC_MODE //确保进入管理模式并禁止中断
   mrc p15, 0, r9, c0, c0             // r9 = CPU ID
   bl  __lookup_processor_type  // r5=procinfo r9=cpuid
   movs r10, r5                         //如果不支持该CPU则r5=0
   beq __error_p                      //如果r5=0，则打印错误信息
   bl __lookup_machine_type // r5=machinfo
   movs r8, r5                            // 如果不支持该开发板r5=0
   beq __error_a                      //如果r5=0，则打印错误信息
   bl __vet_atags
   bl __create_page_tables

   .......
   ldr r13, __switch_data  @ address to jump to after  mmu has been enabled
   adr lr, __enable_mmu  @ return (PIC) address
   add pc, r10, #PROCINFO_INITFUNC
ENDPROC(stext)

.......

架构/开发板相关的引导过程就是由以上蓝色部分的函数来完成的，都是汇编子程序，下面逐个分析。

__lookup_processor_type子程序在arch/arm/kernel/head-common.S文件中定义

/*
* r9 = cpuid
* Returns:
* r3, r4, r6 corrupted
* r5 = proc_info pointer in physical address space
* r9 = cpuid (preserved)
*/

__lookup_processor_type:
   adr r3, 3f          //将下面3标号的物理地址存到r3寄存器

/* r5 = __proc_info_begin，r6 = __proc_info_end 的虚拟地址，r7 = 3标号的虚拟地址 */
   ldmda r3, {r5 - r7}
   sub r3, r3, r7 // r3 = r3 - r7，物理地址与虚拟地址之差
   add r5, r5, r3 // r5 = __ proc_info_begin 对应的物理地址
   add r6, r6, r3 // r6 = __proc_info_end 对应的物理地址
1: ldmia r5, {r3, r4} // r3 = cpu_val，r4 = cpu_mask 这两个成员在proc_info_list中定义
   and r4, r4, r9 // r4 = r4&r9 = cpu_mask & 最开始从CP15传入的cpu id
   teq r3, r4
   beq 2f             //相等则返回
   add r5, r5, #PROC_INFO_SZ  //下一个pro_info_list ，sizeof(proc_info_list)
   cmp r5, r6    //判断是否已经比较完所有的proc_info_list
   blo 1b          //否则继续比较
   mov r5, #0    //比较完后还是没有匹配的proc_info_list结构，r5 = 0
2: mov pc, lr       // 返回
ENDPROC(__lookup_processor_type)

.......

/*
* Look in <asm/procinfo.h> and arch/arm/kernel/arch.[ch] for
* more information about the __proc_info and __arch_info structures.
*/
.long __proc_info_begin //proc_info_list结构体的开始地址（虚拟地址）
.long __proc_info_end    //proc_info_list结构体的结束地址（虚拟地址）
3: .long .                            // 3标号在内存中的的物理地址
.long __arch_info_begin
.long __arch_info_end

proc_info_list结构体在arch/arm/include/asm中定义

struct proc_info_list {
   unsigned int  cpu_val;
   unsigned int  cpu_mask;
   unsigned long  __cpu_mm_mmu_flags; /* used by head.S */
   unsigned long  __cpu_io_mmu_flags; /* used by head.S */
   unsigned long  __cpu_flush;  /* used by head.S */
   const char  *arch_name;
   const char  *elf_name;
   unsigned int  elf_hwcap;
   const char  *cpu_name;
   struct processor *proc;
   struct cpu_tlb_fns *tlb;
   struct cpu_user_fns *user;
   struct cpu_cache_fns *cache;
};

对于S3C2440开发板的匹配，它的CPU ID是0x41129200，可查询协处理器的CP15获知。

而在arch/arm/mm/proc_arm920.S中定义的__arm920_proc_info结构如下

__arm920_proc_info:
.long 0x41009200
.long 0xff00fff0

........

即r3 = cpu_val = 0x41009200，r4 = cpu_mask = 0xff00ff00，刚好匹配。

__lookup_machine_type子程序也在arch/arm/kernel/head-common.S文件中定义

/*

* ....

*  r1 = machine architecture number
* Returns:
*  r3, r4, r6 corrupted
*  r5 = mach_info pointer in physical address space
*/
__lookup_machine_type:
   adr r3, 3b                //获得3标号的物理地址

/* r4 = __arch_info_begin，r6 = __arch_info_end 的虚拟地址，r7 = 3标号的虚拟地址 */

   ldmia r3, {r4, r5, r6}
   sub r3, r3, r4 // r3 = r3 - r4  物理地址与虚拟地址之差
   add r5, r5, r3 // r5 = __arch_info_begin 的物理地址
   add r6, r6, r3 // r6 = __arch_info_end 的物理地址
1: ldr r3, [r5, #MACHINFO_TYPE] // r5 是machine_desc结构体的地址
   teq r3, r1    //判断从r1传入的机器ID是否匹配
   beq 2f          //匹配则返回
   add r5, r5, #SIZEOF_MACHINE_DESC  //否则，指向下一个 machine_desc
   cmp r5, r6 //是否已经比较完所有的machine_desc结构
   blo 1b       //没有则继续比较
   mov r5, #0 //比较完毕之后都没有找到匹配的，r5 = 0
2: mov pc, lr    //返回
ENDPROC(__lookup_machine_type)

machine_desc结构在 arch/arm/include/asm/mach/arch.h中定义

struct machine_desc {
/*
  * Note! The first four elements are used
  * by assembler code in head.S, head-common.S
  */
   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 */
   unsigned long  boot_params; /* tagged list  */

   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);
};

#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 \
};

MACHINE_START(S3C2440, "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_END

由上面二个定义得到S3C2440 的machine_desc结构体

static const struct machine_desc __mach_desc_S3C2440
__used
__attribute__((__section__(".arch.info.init"))) =

{
.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,

};

到此，__lookup_processor_type、__lookup_machine_type函数如果都匹配成功，则将继续执行

__vet_atags、（处理u-boot传入的ATAGS）

__create_page_tables创建一级页表，在arch/arm/kernel/head.S原文定义

__switch_data在arch/arm/kernel/head_common.S 定义

__switch_data:
.long __mmap_switched
.long __data_loc       @ r4
.long _data             @ r5
.long __bss_start    @ r6
.long _end                @ r7
.long processor_id    @ r4
.long __machine_arch_type  @ r5
.long __atags_pointer          @ r6
.long cr_alignment                @ r7
.long init_thread_union + THREAD_START_SP @ sp

......

__mmap_switched:
adr r3, __switch_data + 4

ldmia r3!, {r4, r5, r6, r7}
cmp r4, r5    //复制数据段
1: cmpne r5, r6
ldrne fp, [r4], #4
strne fp, [r5], #4
bne 1b

mov fp, #0 //清除BSS段
1: cmp r6, r7
strcc fp, [r6],#4
bcc 1b

ldmia r3, {r4, r5, r6, r7, sp}
str r9, [r4] //保存CPU ID到 r9 寄存器
str r1, [r5] //保存机器ID到 r1 寄存器
str r2, [r6] // 保存ATAGS指针到 r2 寄存器
bic r4, r0, #CR_A @ Clear 'A' bit
stmia r7, {r0, r4} @ Save control register values
b start_kernel       //跳到第二阶段的C函数去执行，在init/main.c中定义
ENDPROC(__mmap_switched)

__enable_mmu  使能MMU

__enable_mmu:
#ifdef CONFIG_ALIGNMENT_TRAP
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, r3
mov pc, r13
ENDPROC(__turn_mmu_on)

yin123 · 发表于 2020-4-22 13:27

Linux架构/开发板相关的引导过程

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