以tiny4412为例：
arch/arm/mach-exynos/mach-tiny4412.c
MACHINE_START(TINY4412, "TINY4412") /* Maintainer: FriendlyARM (www.arm9.net) */ .boot_params = S5P_PA_SDRAM + 0x100, .init_irq = exynos4_init_irq, .map_io = smdk4x12_map_io, .init_machine = smdk4x12_machine_init, .timer = &exynos4_timer, .reserve = &exynos4_reserve, MACHINE_END
其中：
#define MACHINE_START(_type,_name) static const struct machine_desc __mach_desc_##_type __used __attribute__((__section__(".arch.info.init"))) = { .nr = MACH_TYPE_##_type, .name = _name, #define MACHINE_END \ };
启动时：
start_kernel ----- init/main.c
----> setup_arch ---- arch/arm/kernel/setup.c
----> mdesc = setup_machine_tags(machine_arch_type); 到这里，根据machine_arch_type就找到上面这个结构体了。
----> machine_desc = mdesc;
----> paging_init(mdesc) (arch/arm/mm/mmu.c)
----> devicemaps_init(mdesc)
----> mdesc->map_io() 调用了函数 smdk4x12_map_io
原文：http://www.cnblogs.com/pengdonglin137/p/4194582.html