Linux内核的启动分为压缩内核和非压缩内核两种，这里我们以压缩内核为例。压缩内核运行时，将运行一段解压缩程序，得到真正的内核镜像，然后跳转到内核镜像运行。此时，Linux进入非压缩内核入口，在非压缩内核入口中，完成各种初始化操作后跳转到C语言入口处运行。主要流程如下所示。
1.解压缩内核镜像
解压缩程序通常在arch/arm/boot/compressed/目录中
├── atags_to_fdt.c ├── big-endian.S ├── decompress.c ├── head.S ├── head-sa1100.S ├── head-shark.S ├── head-sharpsl.S ├── head-shmobile.S ├── head-vt8500.S ├── head-xscale.S ├── libfdt_env.h ├── ll_char_wr.S ├── Makefile ├── misc.c ├── mmcif-sh7372.c ├── ofw-shark.c ├── piggy.gzip.S ├── piggy.lzma.S ├── piggy.lzo.S ├── piggy.xzkern.S ├── sdhi-sh7372.c ├── sdhi-shmobile.c ├── sdhi-shmobile.h ├── string.c └── vmlinux.lds.in
它们经过编译后，生成的内容独立于真正的Linux内核，这部分内容的功能就是初始化环境，解压缩和运行真正的Linux内核。在压缩内核启动时，首先进入arch/arm/boot/compressed目录中的compressed目录中的head.S文件。
start:
.type start,#function
.rept 7
mov r0, r0
.endr
ARM( mov r0, r0 )
ARM( b 1f )
THUMB( adr r12, BSYM(1f) )
THUMB( bx r12 )
.word 0x016f2818 @ Magic numbers to help the loader
.word start @ absolute load/run zImage address
.word _edata @ zImage end address
THUMB( .thumb )
1: mov r7, r1 @ save architecture ID
mov r8, r2 @ save atags pointer
start是head.S的程序的开始，在此之前都是一些宏定义。在1标号处保存由bootloader传递过来的参数
#ifndef __ARM_ARCH_2__
/*
* Booting from Angel - need to enter SVC mode and disable
* FIQs/IRQs (numeric definitions from angel arm.h source).
* We only do this if we were in user mode on entry.
*/
@获取当前运行模式
mrs r2, cpsr @ get current mode
@测试是否为usr模式
tst r2, #3 @ not user?
bne not_angel
mov r0, #0x17 @ angel_SWIreason_EnterSVC
ARM( swi 0x123456 ) @ angel_SWI_ARM
THUMB( svc 0xab ) @ angel_SWI_THUMB
not_angel:
mrs r2, cpsr @ turn off interrupts to
orr r2, r2, #0xc0 @ prevent angel from running
msr cpsr_c, r2
#else
teqp pc, #0x0c000003 @ turn off interrupts
#endif
如果内核从angel运行进入的运行模式将是usr mode，这时需要进入svc mode ，并禁止所有FIQ和IRQ中断。这些只有在进入时处于用户模式的时候才会执行。正常情况下，将运行not_angel标号处关闭中断的代码。然后对内核代码进行重定向(telocate)--（这部分代码还没看懂-_-！！！）,重定向完成之后会跳转到not_relocated标号处运行。
not_relocated: mov r0, #0
1: str r0, [r2], #4 @ clear bss
str r0, [r2], #4
str r0, [r2], #4
str r0, [r2], #4
cmp r2, r3
blo 1b
/*
* The C runtime environment should now be setup sufficiently.
* Set up some pointers, and start decompressing.
* r4 = kernel execution address
* r7 = architecture ID
* r8 = atags pointer
*/
mov r0, r4
mov r1, sp @ malloc space above stack
add r2, sp, #0x10000 @ 64k max
mov r3, r7
bl decompress_kernel
bl cache_clean_flush
bl cache_off
mov r0, #0 @ must be zero
mov r1, r7 @ restore architecture number
mov r2, r8 @ restore atags pointer
ARM( mov pc, r4 ) @ call kernel
重定向完成之后，首先清bss段，这时所有初始化C语言运行环境都要做的，然后调用decompress_kernel解压内核，之后跳转到非压缩内核启动阶段。
2.汇编阶段启动流程
对于tiny4412而言，内核的链接脚本为arch/arm/kernel/vmlinux.lds，它是由arch/arm/kernel/vmlinux.lds.S生成的。在链接脚本中，我们可以找到内核的入口
OUTPUT_ARCH(arm)
ENTRY(stext)
jiffies = jiffies_64;
SECTIONS
{
可以看出内核的入口为stext，它在 linux/arch/arm/kernel/head.S 中被定义。
.arm
__HEAD
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: )
@设定为SVC模式，关闭IRQ、FIQ
setmode PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode
@ and irqs disabled
@检查CPU ID 是否匹配
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_p @ 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, =PHYS_OFFSET @ always constant in this case
#endif
/*
* r1 = machine no, r2 = atags or dtb,
* r8 = phys_offset, r9 = cpuid, r10 = procinfo
*/
@检查bootloader传入的参数列表atags的合法性
bl __vet_atags
#ifdef CONFIG_SMP_ON_UP
bl __fixup_smp
#endif
#ifdef CONFIG_ARM_PATCH_PHYS_VIRT
bl __fixup_pv_table
#endif
@创建初始页表
bl __create_page_tables
/*
* The following calls CPU specific code in a position independent
* manner. See arch/arm/mm/proc-*.S for details. r10 = base of
* xxx_proc_info structure selected by __lookup_processor_type
* above. On return, the CPU will be ready for the MMU to be
* turned on, and r0 will hold the CPU control register value.
*/
@建立C语言环境（代码重定位、清bss段）
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 )
@开启MMU
1: b __enable_mmu
ENDPROC(stext)
__mmap_switched定义在arch/arm/kernel/head-common.S中
__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
汇编阶段代码主要完成了以下工作
①设置处理器为SVC模式并关闭中断
②调用__lookup_processor_type查找处理器信息结构体proc_info
③调用__enable_mmu打开MMU
④调用__create_page_tables创建初始页表
⑤调用__mmap_switched初始化C语言运行环境，最红跳转到C语言阶段入口函数start_kernel
3.C语言阶段启动流程
内核启动流程，知识储备还不完善，以后更新 -_- ......
参考文章：
http://blog.csdn.net/zqixiao_09/article/details/50821995
时间： 2024-10-12 16:56:49