arm linux 从入口到start_kernel 代码剖析 – 3

2. 确认 machine type

arch/arm/kernel/head.S中:
00079: bl__lookup_machine_type@ r5=machinfo
00080: movsr8, r5@ invalid machine (r5=0)?
00081: beq__error_a@ yes, error a

79行: 跳转到__lookup_machine_type函数,在__lookup_machine_type中,会把struct machine_desc的基地址(machine type)存储在r5中
80,81行: 将r5中的 machine_desc的基地址存储到r8中,并判别r5是否是0,假如是0,阐明是无效的machine type,跳转到__error_a(犯错)

__lookup_machine_type 函数
下面咱们剖析__lookup_machine_type 函数:

arch/arm/kernel/head-common.S中:

00176: .long__proc_info_begin
00177: .long__proc_info_end
00178: 3:.long.
00179: .long__arch_info_begin
00180: .long__arch_info_end
00181:
00182:
00193: .type__lookup_machine_type, %function
00194: __lookup_machine_type:
00195: adrr3, 3b
00196: ldmiar3, {r4, r5, r6}
00197: subr3, r3, r4@ get offset between virt&phys
00198: addr5, r5, r3@ convert virt addresses to
00199: addr6, r6, r3@ physical address space
00200: 1:ldrr3, [r5, #MACHINFO_TYPE]@ get machine type
00201: teqr3, r1@ matches loader number?
00202: beq2f@ found
00203: addr5, r5, #SIZEOF_MACHINE_DESC@ next machine_desc
00204: cmpr5, r6
00205: blo1b
00206: movr5, #0@ unknown machine
00207: 2:movpc, lr

193, 194行: 函数声明
195行: 取地址指令,这儿的3b是向后symbol名称是3的方位,即第178行,将该地址存入r3.
和上面咱们对__lookup_processor_type 函数的剖析相同,r3中寄存的是3b处物理地址.
196行: r3是3b处的地址,因此履行完后:
r4存的是 3b处的地址
r5存的是__arch_info_begin 的地址
r6存的是__arch_info_end 的地址

__arch_info_begin 和 __arch_info_end是在 arch/arm/kernel/vmlinux.lds.S中:

00034:__arch_info_begin = .;
00035:*(.arch.info.init)
00036:__arch_info_end = .;

这儿是声明晰两个变量:__arch_info_begin 和 __arch_info_end,其间等号后边的”.”是location counter(具体内容请参阅ld.info)
这三行的意思是: __arch_info_begin 的方位上,放置一切文件中的 “.arch.info.init” 段的内容,然后紧接着是 __arch_info_end 的方位.

kernel 运用struct machine_desc 来描绘 machine type.
在 include/asm-arm/mach/arch.h 中:

00017: struct machine_desc {
00018:
00022: unsigned intnr;
00023: unsigned intphys_io;
00024: unsigned intio_pg_offst;
00026:
00027: const char*name;
00028: unsigned longboot_params;
00029:
00030: unsigned intvideo_start;
00031: unsigned intvideo_end;
00032:
00033: unsigned intreserve_lp0 :1;
00034: unsigned intreserve_lp1 :1;
00035: unsigned intreserve_lp2 :1;
00036: unsigned intsoft_reboot :1;
00037: void(*fixup)(struct machine_desc *,
00038: struct tag *, char **,
00039: struct meminfo *);
00040: void(*map_io)(void);
00041: void(*init_irq)(void);
00042: struct sys_timer*timer;
00043: void(*init_machine)(void);
00044: };
00045:
00046:
00050: #define MACHINE_START(_type,_name)\
00051: static const struct machine_desc __mach_desc_##_type\
00052: __attribute_used__\
00053: __attribute__((__section__(“.arch.info.init”))) = {\
00054: .nr= MACH_TYPE_##_type,\
00055: .name= _name,
00056:
00057: #define MACHINE_END\
00058: };

内核中,一般运用宏MACHINE_START来界说machine type.
关于at91, 在 arch/arm/mach-at91rm9200/board-ek.c 中:
00137: MACHINE_START(AT91RM9200EK, “Atmel AT91RM9200-EK”)
00138:
00139: .phys_io= AT91_BASE_SYS,
00140: .io_pg_offst= (AT91_VA_BASE_SYS >> 18) & 0xfffc,
00141: .boot_params= AT91_SDRAM_BASE + 0x100,
00142: .timer= &at91rm9200_timer,
00143: .map_io= ek_map_io,
00144: .init_irq= ek_init_irq,
00145: .init_machine= ek_board_init,
00146: MACHINE_END

197行: r3中存储的是3b处的物理地址,而r4中存储的是3b处的虚拟地址,这儿核算处物理地址和虚拟地址的差值,保存到r3中
198行: 将r5存储的虚拟地址(__arch_info_begin)转换成物理地址
199行: 将r6存储的虚拟地址(__arch_info_end)转换成物理地址
200行: MACHINFO_TYPE 在 arch/arm/kernel/asm-offset.c 101行界说, 这儿是取 struct machine_desc中的nr(architecture number) 到r3中
201行: 将r3中取到的machine type 和 r1中的 machine type(见前面的”发动条件”)进行比较
202行: 假如相同,阐明找到了对应的machine type,跳转到207行的2f处,此刻r5中存储了对应的struct machine_desc的基地址
203行: (不相同), 取下一个machine_desc的地址
204行: 和r6进行比较,查看是否到了__arch_info_end.
205行: 假如不相同,阐明还有machine_desc,回来200行持续查找.
206行: 履行到这儿,阐明一切的machind_desc都查找完了,而且没有找到匹配的, 将r5设置成0(unknown machine).
207行: 回来