人的一生其实就是在不断见证的过程，小时候见证了长辈的人生末尾，长大后见证爱情的发展，然后又见证小孩的成长。而这所谓的见证总是受生命和时代的限制，譬如，我们不可能见证五四运动时期思想的解放，不可能见证父母的青少年成长经历。人这一生只不过是时间轴上特定段落的一个点而已。
最近花了点时间看看Linux内核在启动的时候具体是怎么配置arm的，因为不同的arm核差别还是很大的，有的还有secure extension，multiprocessor extension，virtualization，在调试的时候如果遇到问题可以作为参考。分析代码的时候要时刻记得的事情有两件：
- 当前cpu处于什么状态
- 各个寄存器当前值的含义（因为启动代码都是汇编实现的）
一、内核入口
arch/arm/kernel/head.S中ENTRY(stext)
二、不同体系结构的setup
内核中把arm不同系列的setup入口编译到了.proc.info.init段中，代码位于arch/arm/mm/proc-v7.S
.section ".proc.info.init", #alloc
/*
* Standard v7 proc info content
*/
.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
#ifndef CONFIG_ARM_LPAE
/*
* ARM Ltd. Cortex A5 processor.
*/
.type __v7_ca5mp_proc_info, #object
__v7_ca5mp_proc_info:
.long 0x410fc050
.long 0xff0ffff0
__v7_proc __v7_ca5mp_proc_info, __v7_ca5mp_setup
.size __v7_ca5mp_proc_info, . - __v7_ca5mp_proc_info
/*
* ARM Ltd. Cortex A9 processor.
*/
.type __v7_ca9mp_proc_info, #object
__v7_ca9mp_proc_info:
.long 0x410fc090
.long 0xff0ffff0
__v7_proc __v7_ca9mp_proc_info, __v7_ca9mp_setup, proc_fns = ca9mp_processor_functions
.size __v7_ca9mp_proc_info, . - __v7_ca9mp_proc_info
/*
* ARM Ltd. Cortex A8 processor.
*/
.type __v7_ca8_proc_info, #object
__v7_ca8_proc_info:
.long 0x410fc080
.long 0xff0ffff0
__v7_proc __v7_ca8_proc_info, __v7_setup, proc_fns = ca8_processor_functions
.size __v7_ca8_proc_info, . - __v7_ca8_proc_info
#endif /* CONFIG_ARM_LPAE */
/*
* Marvell PJ4B processor.
*/
#ifdef CONFIG_CPU_PJ4B
.type __v7_pj4b_proc_info, #object
__v7_pj4b_proc_info:
.long 0x560f5800
.long 0xff0fff00
__v7_proc __v7_pj4b_proc_info, __v7_pj4b_setup, proc_fns = pj4b_processor_functions
.size __v7_pj4b_proc_info, . - __v7_pj4b_proc_info
#endif
/*
* ARM Ltd. Cortex R7 processor.
*/
.type __v7_cr7mp_proc_info, #object
__v7_cr7mp_proc_info:
.long 0x410fc170
.long 0xff0ffff0
__v7_proc __v7_cr7mp_proc_info, __v7_cr7mp_setup
.size __v7_cr7mp_proc_info, . - __v7_cr7mp_proc_info
/*
* ARM Ltd. Cortex A7 processor.
*/
.type __v7_ca7mp_proc_info, #object
__v7_ca7mp_proc_info:
.long 0x410fc070
.long 0xff0ffff0
__v7_proc __v7_ca7mp_proc_info, __v7_ca7mp_setup
.size __v7_ca7mp_proc_info, . - __v7_ca7mp_proc_info
/*
* ARM Ltd. Cortex A12 processor.
*/
.type __v7_ca12mp_proc_info, #object
__v7_ca12mp_proc_info:
.long 0x410fc0d0
.long 0xff0ffff0
__v7_proc __v7_ca12mp_proc_info, __v7_ca12mp_setup
.size __v7_ca12mp_proc_info, . - __v7_ca12mp_proc_info
/*
* ARM Ltd. Cortex A15 processor.
*/
.type __v7_ca15mp_proc_info, #object
__v7_ca15mp_proc_info:
.long 0x410fc0f0
.long 0xff0ffff0
__v7_proc __v7_ca15mp_proc_info, __v7_ca15mp_setup
.size __v7_ca15mp_proc_info, . - __v7_ca15mp_proc_info
/*
* Broadcom Corporation Brahma-B15 processor.
*/
.type __v7_b15mp_proc_info, #object
__v7_b15mp_proc_info:
.long 0x420f00f0
.long 0xff0ffff0
__v7_proc __v7_b15mp_proc_info, __v7_b15mp_setup
.size __v7_b15mp_proc_info, . - __v7_b15mp_proc_info
/*
* ARM Ltd. Cortex A17 processor.
*/
.type __v7_ca17mp_proc_info, #object
__v7_ca17mp_proc_info:
.long 0x410fc0e0
.long 0xff0ffff0
__v7_proc __v7_ca17mp_proc_info, __v7_ca17mp_setup
.size __v7_ca17mp_proc_info, . - __v7_ca17mp_proc_info
/*
* Qualcomm Inc. Krait processors.
*/
.type __krait_proc_info, #object
__krait_proc_info:
.long 0x510f0400 @ Required ID value
.long 0xff0ffc00 @ Mask for ID
/*
* Some Krait processors don't indicate support for SDIV and UDIV
* instructions in the ARM instruction set, even though they actually
* do support them. They also don't indicate support for fused multiply
* instructions even though they actually do support them.
*/
__v7_proc __krait_proc_info, __v7_setup, hwcaps = HWCAP_IDIV | HWCAP_VFPv4
.size __krait_proc_info, . - __krait_proc_info
/*
* Match any ARMv7 processor core.
*/
.type __v7_proc_info, #object
__v7_proc_info:
.long 0x000f0000 @ Required ID value
.long 0x000f0000 @ Mask for ID
__v7_proc __v7_proc_info, __v7_setup
.size __v7_proc_info, . - __v7_proc_info
三、cpu配置流程
- 找到processor type相关结构体
- 处理tags
- fixup smp (由宏SMP_ON_UP控制) 和fixup p/v table
- 创建mmu页表，使能
分析代码真的是很无趣的一件事情，而且最终每个人需要调试的点其实就是一两个，譬如针对多核架构，要开启asctl.smp，MVA invalidate dcache的指令变成了clean and invalidate。