接上篇：linux内核启动过程分析(一)
三、内核启动过程
1.汇编阶段
同样可以从内核的编译链接文件(arch/arm/kernel/head.S)中得到内核代码的入口地址"stext"。
OUTPUT_ARCH(arm)
ENTRY(stext)
jiffies = jiffies_64;
SECTIONS
{
/DISCARD/ : {
*(.ARM.exidx.exit.text)
*(.ARM.extab.exit.text)
*(.exitcall.exit)
*(.discard)
*(.discard.*)
}
内核入口stext(archarmkernelhead.S)
.arm
__HEAD
ENTRY(stext)
ARM_BE8(setend be ) @ ensure we are in BE8 mode
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: )
#ifdef CONFIG_ARM_VIRT_EXT
bl __hyp_stub_install
#endif
@ ensure svc mode and all interrupts masked
safe_svcmode_maskall r9
mrc p15, 0, r9, c0, c0 @ get processor id
bl __lookup_processor_type @ r5=procinfo r9=cpuid
movs r10, r5 @ invalid processor (r5=0)?
THUMB( it eq ) @ force fixup-able long branch encoding
beq __error_p @ yes, error 'p'
#ifdef CONFIG_ARM_LPAE
mrc p15, 0, r3, c0, c1, 4 @ read ID_MMFR0
and r3, r3, #0xf @ extract VMSA support
cmp r3, #5 @ long-descriptor translation table format?
THUMB( it lo ) @ force fixup-able long branch encoding
blo __error_lpae @ only classic page table format
#endif
#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
#else
ldr r8, =PLAT_PHYS_OFFSET @ always constant in this case
#endif
bl __vet_atags
#ifdef CONFIG_SMP_ON_UP
bl __fixup_smp
#endif
#ifdef CONFIG_ARM_PATCH_PHYS_VIRT
bl __fixup_pv_table
#endif
bl __create_page_tables
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
ldr r12, [r10, #PROCINFO_INITFUNC]
add r12, r12, r10
ret r12
1: b __enable_mmu
ENDPROC(stext)
__lookup_processor_type分析：
为了更好的理解__lookup_processor_type，在这之前需要先了解一下__lookup_processor_type_data。代码中可以看到__lookup_processor_type的第一条指令中就用到了__lookup_processor_type_data这个地址。__lookup_processor_type_data被定义在archarmkernelhead-common.S中。代码如下：
.align 2 .type __lookup_processor_type_data, %object __lookup_processor_type_data: .long . .long __proc_info_begin .long __proc_info_end .size __lookup_processor_type_data, . - __lookup_processor_type_data
这段代码可以认为是定义了一个__lookup_processor_type_data的结构，然后这个结构中的前三个元素被赋值成，当前的地址（用于虚拟地址和物理地址的转化）、__proc_info_begin的地址、__proc_info_end的地址。其中__proc_info_begin和__proc_info_end在archarmkernelvmlinux.lds链接文件中作为(.proc.info.init)段的开始和结束地址被定义。(.proc.info.init)段中存放了所有支持的cpu。对于ARM架构的CPU，表示支持的CPU的源码定义在在arch/arm/mm/目录下。
内存结构入下图：
archarmincludeasmprocinfo.h中proc_info_list结构定义
struct proc_info_list {
unsigned int cpu_val;
unsigned int cpu_mask;
unsigned long __cpu_mm_mmu_flags;
unsigned long __cpu_io_mmu_flags;
unsigned long __cpu_flush;
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;
};
如下代码是ARMv7架构CPU的proc_info_list结构，被定义在archarmmmproc-v7.S中
.section ".proc.info.init", #alloc .macro __v7_proc name, initfunc, mm_mmuflags = 0, io_mmuflags = 0, hwcaps = 0, proc_fns = v7_processor_functions ALT_SMP(.long PMD_TYPE_SECT | PMD_SECT_AP_WRITE | PMD_SECT_AP_READ | PMD_SECT_AF | PMD_FLAGS_SMP | mm_mmuflags) ALT_UP(.long PMD_TYPE_SECT | PMD_SECT_AP_WRITE | PMD_SECT_AP_READ | PMD_SECT_AF | PMD_FLAGS_UP | mm_mmuflags) .long PMD_TYPE_SECT | PMD_SECT_AP_WRITE | PMD_SECT_AP_READ | PMD_SECT_AF | io_mmuflags initfn initfunc, name .long cpu_arch_name .long cpu_elf_name .long HWCAP_SWP | HWCAP_HALF | HWCAP_THUMB | HWCAP_FAST_MULT | HWCAP_EDSP | HWCAP_TLS | hwcaps .long cpu_v7_name .long proc_fns .long v7wbi_tlb_fns .long v6_user_fns .long v7_cache_fns .endm
__lookup_processor_type函数
archarmkernelhead-common.S
__lookup_processor_type:
adr r3, __lookup_processor_type_data
ldmia r3, {r4 - r6}
sub r3, r3, r4 @ get offset between virt&phys
add r5, r5, r3 @ convert virt addresses to
add r6, r6, r3 @ physical address space
1: ldmia r5, {r3, r4} @ value, mask
and r4, r4, r9 @ mask wanted bits
teq r3, r4
beq 2f
add r5, r5, #PROC_INFO_SZ @ sizeof(proc_info_list)
cmp r5, r6
blo 1b
mov r5, #0 @ unknown processor
2: ret lr
ENDPROC(__lookup_processor_type)
.align 2
.type __lookup_processor_type_data, %object
__lookup_processor_type_data:
.long .
.long __proc_info_begin
.long __proc_info_end
.size __lookup_processor_type_data, . - __lookup_processor_type_data
__enable_mmu函数分析
archarmkernelhead.S
__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 #ifndef CONFIG_ARM_LPAE 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 #endif b __turn_mmu_on ENDPROC(__enable_mmu) .align 5 .pushsection .idmap.text, "ax" ENTRY(__turn_mmu_on) mov r0, r0 instr_sync mcr p15, 0, r0, c1, c0, 0 @ write control reg mrc p15, 0, r3, c0, c0, 0 @ read id reg instr_sync mov r3, r3 mov r3, r13 ret r3 __turn_mmu_on_end: ENDPROC(__turn_mmu_on)
__mmap_switched分析
archarmkernelhead-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
cmp r7, #0
strne r0, [r7] @ Save control register values
b start_kernel
ENDPROC(__mmap_switched)
到这内核启动汇编阶段结束，下面将进入C代码阶段运行。
接下一篇：linux内核启动过程分析(三)
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