嵌入式内核启动作业流程.doc

Linux内核构成
(国嵌)

Linux/arch/arm/boot/compressed/head.s
1．解压缩
2．初始化
3．启动应用程序
1 arch/arm/boot/compressed/Makefile arch/arm/boot/compressed/vmlinux.lds
2. arch/arm/kernel/vmlinux.lds

Linux内核启动流程
（国嵌）
arch/arm/boot/compressed/start.S（head.s—负责解压缩）

Start:
.type start,#function
.rept 8
mov r0，r0
.endr

b 1f
.word 0x016f2818 @ Magic numbers to help the loader
.word start @ absolute load/run zImage address
.word _edata @ zImage end address
1： mov r7，r1 @ save architecture ID
mov r8，r2 @ save atags pointer
这也标志着u-boot将系统完全交给了OS，bootloader生命终结。之后裔码在133行会读取cpsr并判断与否解决器处在supervisor模式——从u-boot进入kernel，系统已经处在SVC32模式；而运用angel进入则处在user模式，还需要额外两条指令。之后是再次确认中断关闭，并完毕cpsr写入

mrs r2，cpsr @ get current mode
tst r2，#3 @ not user?
bne not_angel
mov r0，#0x17 @ angel_SWIreason_EnterSVC
swi 0x123456 @ angel_SWI_ARM
not_angel:
mrs r2，cpsr @ turn off interrupts to
orr r2，r2，#0xc0 @ prevent angel from running
msr cpsr_c，r2
然后在LC0地址处将分段信息导入r0-r6、ip、sp等寄存器，并检查代码与否运营在与链接时相似目的地址，以决定与否进行解决。由于当前很少有人不使用loader和tags，将zImage烧写到rom直接从0x0位置执行，因此这个解决是必要（但是zImage头当前也保存了不用loader也可启动能力）。arm架构下自解压头普通是链接在0x0地址而被加载到0x30008000运营，因此要修正这个变化。涉及到

r5寄存器存储zImage基地址
r6和r12（即ip寄存器）存储got（global offset table）
r2和r3存储bss段起止地址
sp栈指针地址
很简朴，这些寄存器统统被加上一种你也能猜到偏移地址 0x30008000。该地址是s3c2410有关，其她ARM解决器可以参照下表

PXA2xx是0xa0008000
IXP2x00和IXP4xx是0x00008000
Freescale i.MX31/37是0x80008000
TI davinci DM64xx是0x80008000
TI omap系列是0x80008000
AT91RM/SAM92xx系列是0x8000
Cirrus EP93xx是0x00008000
这些操作发生在代码172行开始地方，下面只粘贴一某些

add r5，r5，r0
add r6，r6，r0
add ip，ip，r0
背面在211行进行bss段清零工作

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
然后224行，打开cache，并为背面解压缩设立64KB暂时malloc空间

bl cache_on

mov r1，sp @ malloc space above stack
add r2，sp，#0x10000 @ 64k max 接下来238行进行检查，拟定内核解压缩后Image目的地址与否会覆盖到zImage头，如果是则准备将zImage头转移到解压出来内核背面

cmp r4，r2
bhs wont_overwrite
sub r3，sp，r5 @ > compressed kernel size
add r0，r4，r3，lsl #2 @ allow for 4x expansion
cmp r0，r5
bls wont_overwrite

mov r5，r2 @ decompress after malloc space
mov r0，r5
mov r3，r7
bl decompress_kernel
真实状况——在大多数应用中，内核编译都会把压缩zImage和非压缩Image链接到同样地址，s3c2410平台下即是0x30008000。这样做好处是，人们不用关怀内核是Image还是zImage，放到这个位置执行就OK，因此在解压缩后zImage头必要为真正内核让路。

在250行解压完毕，内核长度返回值存储在r0寄存器里。在内核末尾空出128字节栈空间用，并且使其长度128字节对齐。

add r0，r0，#127 + 128 @ alignment + stack
bic r0，r0，#127 @ align the kernel length
算出搬移代码参数：计算内核末尾地址并存储于r1寄存器，需要搬移代码本来地址放在r2，需要搬移长度放在r3。然后执行搬移，并设立好sp指针指向新栈（本来栈也会被内核覆盖掉）

add r1，r5，r0 @ end of decompressed kernel
adr r2，reloc_start
ldr r3，LC1
add r3，r2，r3
1： ldmia r2!，{r9 - r14} @ copy relocation code
stmia r1!，{r9 - r14}
ldmia r2!，{r9 - r14}
stmia r1!，{r9 - r14}
cmp r2，r3
blo 1b
add sp，r1，#128 @ relocate the stack
搬移完毕后刷新cache，由于代码地址变化了不能让cache再命中被内核覆盖老地址。然后跳转到新地址继续执行

bl cache_clean_flush
add pc，r5，r0 @ call relocation code
注意——zImage在解压后搬移和跳转会给gdb调试内核带来麻烦。由于用来调试符号表是在编译是生成，并不懂得后来会被搬移到何处去，只有在内核解压缩完毕之后，依照计算出来参数“告诉”调试器这个变化。以撰写本文时使用zImage为例，内核自解压头重定向后，reloc_start地址由0x30008360变为0x30533e60。故咱们要把vmlinux符号表也相应从0x30008000后移到0x30533b00开始，这样gdb就可以对的相应源代码和机器指令。

随着头部代码移动到新位置，不会再和内核目的地址冲突，可以开始内核自身搬移了。此时r0寄存器存储是内核长度（严格说是长度外加128Byte栈），r4存储是内核目地址0x30008000，r5是当前内核存储地址，r6是CPU ID，r7是machine ID，r8是atags地址。代码从501行开始

reloc_start： add r9，r5，r0
sub r9，r9，#128 @ do not copy the stack
debug_reloc_start
mov r1，r4
1:
.rept 4
ldmia r5!，{r0，r2，r3，r10 - r14} @ relocate kernel
stmia r1!，{r0，r2，r3，r10 - r14}
.endr

cmp r5，r9
blo 1b
add sp，r1，#128 @ relocate the stack
接下来在516行清除并关闭cache，清零r0，将machine ID存入r1，atags指针存入r2，再跳入0x30008000执行真正内核Image

call_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
mov pc，r4 @ call kernel

内核代码入口在arch/arm/kernel/head.S文献83行。一方面进入SVC32模式，并查询CPU ID，检查合法性

msr cpsr_c，#PSR_F_BIT | PSR_I_BIT | SVC_MODE @ ensure svc mode
@ 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'
接着在87行进一步查询machine ID并检查合法性

bl __lookup_machine_type @ r5=machinfo
movs r8，r5 @ invalid machine (r5=0)?
beq __error_a @ yes，error 'a'
其中__lookup_processor_type在linux-2.6.24-moko-linuxbj/arch/arm/kernel/head-common.S文献149行，该函数首将标号3实际地址加载到r3，然后将编译时生成__proc_info_begin虚拟地址载入到r5，__proc_info_end虚拟地址载入到r6，标号3虚拟地址载入到r7。由于adr伪指令和标号3使用，以及__proc_info_begin等符号在linux-2.6.24-moko-linuxbj/arch/arm/kernel/vmlinux.lds而不是代码中被定义，此处代码不是非常直观，想弄清晰代码缘由读者请耐心阅读这两个文献和adr伪指令阐明。

r3和r7分别存储是同一位置标号3物理地址（由于没有启用mmu，因此当前必定是物理地址）和虚拟地址，因此儿者相减即得到虚拟地址和物理地址之间offset。运用此offset，将r5和r6中保存虚拟地址转变为物理地址

__lookup_processor_type:
adr r3，3f
ldmda r3，{r5 - r7}
sub r3，r3，r7 @ get offset between virt&phys
add r5，r5，r3 @ convert virt addresses to
add r6，r6，r3 @ physical address space
然后从proc_info中读出内核编译时写入processor ID和之前从cpsr中读到processor ID对比，查看代码和CPU硬件与否匹配（想在arm920t上运营为cortex-a8编译内核？不让！）。如果编译了各种解决器支持，如versatile板，则会循环每种type依次检查，如果硬件读出ID在内核中找不到匹配，则r5置0返回

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: mov pc，lr
__lookup_machine_type在linux-2.6.24-moko-linuxbj/arch/arm/kernel/head-common.S文献197行，编码办法与检查processor ID完全同样，请参照前段

__lookup_machine_type:
adr r3，3b
ldmia r3，{r4，r5，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: 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
代码回到head.S第92行，检查atags合法性，然后创立初始页表

bl __vet_atags
bl __create_page_tables
创立页表代码在218行，一方面将内核起始地址-0x4000到内核起始地址之间16K存储器清0

__create_page_tables:
pgtbl r4 @ page table address

/*
* Clear the 16K level 1 swapper page table
*/
mov r0，r4
mov r3，#0
add r6，r0，#0x4000
1: str r3，[r0]，#4
str r3，[r0]，#4
str r3，[r0]，#4
str r3，[r0]，#4
teq r0，r6
bne 1b
然后在234行将proc_info中mmu_flags加载到r7

ldr r7，[r10，#PROCINFO_MM_MMUFLAGS] @ mm_mmuflags在242行将PC指针右移20位，得到内核第一种1MB空间段地址存入r6，在s3c2410平台该值是0x300。接着依照此值存入映射标记

mov r6，pc，lsr #20 @ start of kernel section
orr r3，r7，r6，lsl #20 @ flags + kernel base
str r3，[r4，r6，lsl #2] @ identity mapping
完毕页表设立后回到102行，为打开虚拟地址映射作准备。设立sp指针，函数返回地址lr指向__enable_mmu，并跳转到linux-2.6.24-moko-linuxbj/arch/arm/mm/proc-arm920.S386行，清除I-cache、D-cache、write buffer和TLB

__arm920_setup:
mov r0，#0
mcr p15，0，r0，c7，c7 @ invalidate I,D caches on v4
mcr p15，0，r0，c7，c10，4 @ drain write buffer on v4
#ifdef CONFIG_MMU
mcr p15，0，r0，c8，c7 @ invalidate I,D TLBs on v4
#endif然后返回head.S158行，加载domain和页表，跳转到__turn_mmu_on

__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在194行把mmu使能位写入mmu，激活虚拟地址。然后将本来保存在sp中地址载入pc，跳转到head-common.S__mmap_switched，至此代码进入虚拟地址世界

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
在head-common.S37行开始清除内核bss段，processor ID保存在r9，machine ID报存在r1，atags地址保存在r2，并将控制寄存器保存到r7定义内存地址。接下来跳入linux-2.6.24-moko-linuxbj/init/main.c507行，start_kernel函数。这里只粘贴某些代码（第一种C语言函数，作一系列初始化）

__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

asmlinkage void __init start_kernel(void)
{
char * command_line;
extern struct kernel_param __start___param[]，__stop___param[];

smp_setup_processor_id();

/*
* Need to run as early as possible，to initialize the
* lockdep hash:
*/
lockdep_init();
debug_objects_early_init();
cgroup_init_early();

local_irq_disable();
early_boot_irqs_off();
early_init_irq_lock_class();

/*
* Interrupts are still disabled. Do necessary setups，then
* enable them
*/
lock_kernel();
tick_init();
boot_cpu_init();
page_address_init();
printk(KERN_NOTICE);
printk(linux_banner);
setup_arch(&command_line);
mm_init_owner(&init_mm，&init_task);
setup_command_line(command_line);
setup_per_cpu_areas();
setup_nr_cpu_ids();
smp_prepare_boot_cpu(); /* arch-specific boot-cpu hooks */

/*
* Set up the scheduler prior starting any interrupts (such as the
* timer interrupt). Full topology setup happens at smp_init()
* time - but meanwhile we still have a functioning scheduler.
*/
sched_init();
/*
* Disable preemption - early bootup scheduling is extremely
* fragile until we cpu_idle() for the first time.
*/
preempt_disable();
build_all_zonelists();
page_alloc_init();
printk(KERN_NOTICE "Kernel command line：%s\n"，boot_command_line);
parse_early_param();
parse_args("Booting kernel"，static_command_line，__start___param,
__stop___param - __start___param,
&unknown_bootoption);
if (!irqs_disabled()) {
printk(KERN_WARNING "start_kernel()：bug：interrupts were "
"enabled *very* early，fixing it\n");
local_irq_disable();
}
sort_main_extable();
trap_init();
rcu_init();
/* init some links before init_ISA_irqs() */
early_irq_init();
init_IRQ();
pidhash_init();
init_timers();
hrtimers_init();
softirq_init();
timekeeping_init();
time_init();
sched_clock_init();
profile_init();
if (!irqs_disabled())
printk(KERN_CRIT "start_kernel()：bug：interrupts were "
"enabled early\n");
early_boot_irqs_on();
local_irq_enable();

/*
* HACK ALERT！This is early. We're enabling the console before
* we've done PCI setups etc，and console_init() must be aware of
* this. But we do want output early，in case something goes wrong.
*/
console_init();
if (panic_later)
panic(panic_later，panic_param);

lockdep_info();

/*
* Need to run this when irqs are enabled，because it wants
* to self-test [hard/soft]-irqs on/off lock inversion bugs
* too:
*/
locking_selftest();

#ifdef CONFIG_BLK_DEV_INITRD
if (initrd_start && !initrd_below_start_ok &&
page_to_pfn(virt_to_page((void *)initrd_start)) < min_low_pfn) {
printk(KERN_CRIT "initrd overwritten (0x%08lx < 0x%08lx) - "
"disabling it.\n",
page_to_pfn(virt_to_page((void *)initrd_start)),
min_low_pfn);
initrd_start = 0;
}
#endif
vmalloc_init();
vfs_caches_init_early();
cpuset_init_early();
page_cgroup_init();
mem_init();
enable_debug_pagealloc();
cpu_hotplug_init();
kmem_cache_init();
debug_objects_mem_init();
idr_init_cache();
setup_per_cpu_pageset();
numa_policy_init();
if (late_time_init)
late_time_init();
calibrate_delay();
pidmap_init();
pgtable_cache_init();
prio_tree_init();
anon_vma_init();
#ifdef CONFIG_X86
if (efi_enabled)
efi_enter_virtual_mode();
#endif
thread_info_cache_init();
cred_init();
fork_init(num_physpages);
proc_caches_init();
buffer_init();
key_init();
security_init();
vfs_caches_init(num_physpages);
radix_tree_init();
signals_init();
/* rootfs populating might need page-writeback */
page_writeback_init();
#ifdef CONFIG_PROC_FS
proc_root_init();
#endif
cgroup_init();
cpuset_init();
taskstats_init_early();
delayacct_init();

check_bugs();

acpi_early_init()；/* before LAPIC and SMP init */

ftrace_init();

/* Do the rest non-__init'ed，we're now alive */
rest_init();
}

tatic noinline void __init_refok rest_init(void)
__releases(kernel_lock)
{
int pid;

kernel_thread(kernel_init，NULL，CLONE_FS | CLONE_SIGHAND);
numa_default_policy();
pid = kernel_thread(kthreadd，NULL，CLONE_FS | CLONE_FILES);
kthreadd_task = find_task_by_pid_ns(pid，&init_pid_ns);
unlock_kernel();

/*
* The boot idle thread must execute schedule()
* at least once to get things moving:
*/
init_idle_bootup_task(current);
rcu_scheduler_starting();
preempt_enable_no_resched();
schedule();
preempt_disable();

/* Call into cpu_idle with preempt disabled */
cpu_idle();
}

static noinline int init_post(void)
{
/* need to finish all async __init code before freeing the memory */
async_synchronize_full();
free_initmem();
unlock_kernel();
mark_rodata_ro();
system_