Android arm linux kernel启动流程二

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然后跳转到__lookup_processor_type，这个函数定义在head-common.S里面，这里的bl指令会保存当前的pc在lr里面，最后__lookup_processor_type会从这个函数返回，我们具体看看这个函数：

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__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
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
ENDPROC(__lookup_processor_type)
__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
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
ENDPROC(__lookup_processor_type)

他这里的执行过程其实比较简单就是在__proc_info_begin和__proc_info_end这个段里面里面去读取我们注册在里面的proc_info_list这个结构体，这个结构体的定义在arch/arm/include/asm/procinfo.h，具体实现根据你使用的cpu的架构在arch/arm/mm/里面找到具体的实现，这里我们使用的ARM11是proc-v6.S，我们可以看看这个结构体：

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.section ".proc.info.init", #alloc, #execinstr
/*
* Match any ARMv6 processor core.
*/
.type __v6_proc_info, #object
_proc_info:
.long 0x0007b000
.long 0x0007f000
.long PMD_TYPE_SECT | /
PMD_SECT_BUFFERABLE | /
PMD_SECT_CACHEABLE | /
PMD_SECT_AP_WRITE | /
PMD_SECT_AP_READ
.long PMD_TYPE_SECT | /
PMD_SECT_XN | /
PMD_SECT_AP_WRITE | /
PMD_SECT_AP_READ
b __v6_setup
.long cpu_arch_name
.long cpu_elf_name
.long HWCAP_SWP|HWCAP_HALF|HWCAP_THUMB|HWCAP_FAST_MULT|HWCAP_EDSP|HWCAP_JAVA
.long cpu_v6_name
.long v6_processor_functions
.long v6wbi_tlb_fns
.long v6_user_fns
.long v6_cache_fns
.size __v6_proc_info, . - __v6_proc_info
.section ".proc.info.init", #alloc, #execinstr
/*
* Match any ARMv6 processor core.
*/
.type __v6_proc_info, #object
__v6_proc_info:
.long 0x0007b000
.long 0x0007f000
.long PMD_TYPE_SECT | /
PMD_SECT_BUFFERABLE | /
PMD_SECT_CACHEABLE | /
PMD_SECT_AP_WRITE | /
PMD_SECT_AP_READ
.long PMD_TYPE_SECT | /
PMD_SECT_XN | /
PMD_SECT_AP_WRITE | /
PMD_SECT_AP_READ
b __v6_setup
.long cpu_arch_name
.long cpu_elf_name
.long HWCAP_SWP|HWCAP_HALF|HWCAP_THUMB|HWCAP_FAST_MULT|HWCAP_EDSP|HWCAP_JAVA
.long cpu_v6_name
.long v6_processor_functions
.long v6wbi_tlb_fns
.long v6_user_fns
.long v6_cache_fns
.size __v6_proc_info, . - __v6_proc_info

对着.h我们就知道各个成员变量的含义了，他这里lookup的过程实际上是先求出这个proc_info_list的实际物理地址，并将其内容读出，然后将其中的mask也就是我们这里的0x007f000与寄存器与之后与0x007b00进行比较，如果一样的话呢就校验成功了，如果不一样呢就会读下一个proc_info的信息，因为proc一般都是只有一个的，所以这里一般不会循环，如果检测正确寄存器就会将正确的proc_info_list的物理地址赋给寄存器，如果检测不到就会将寄存器值赋0，然后通过LR返回。

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bl __lookup_machine_type @ r5=machinfo
movs r8, r5 @ invalid machine (r5=0)?
beq __error_a @ yes, error a
bl __lookup_machine_type @ r5=machinfo
movs r8, r5 @ invalid machine (r5=0)?
beq __error_a @ yes, error a

检测完proc_info_list以后就开始检测machine_type了，这个函数的实现也在head－common.S里面，我们看看它具体的实现：

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__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
ENDPROC(__lookup_machine_type)
__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
ENDPROC(__lookup_machine_type)