在linux的板卡初始化文件中有machine的相关定义
//arch/arm/mach-at91/board-sam9261ek.c
MACHINE_START(AT91SAM9G10EK, "Atmel AT91SAM9G10-EK")
/* Maintainer: Atmel */
.phys_io = AT91_BASE_SYS,
.io_pg_offst = (AT91_VA_BASE_SYS >> 18) & 0xfffc,
.boot_params = AT91_SDRAM_BASE + 0x100,
.timer = &at91sam926x_timer,
.map_io = ek_map_io,
.init_irq = ek_init_irq,
.init_machine = ek_board_init,
MACHINE_END
其中MACHINE_START和MACHINE_END为宏定义，定义于文件
//arch/arm/include/asm/arch.h
#ifndef __ASSEMBLY__
struct tag;
struct meminfo;
struct sys_timer;
struct machine_desc {
/*
* Note! The first four elements are used
* by assembler code in head.S, head-common.S
*/
unsigned int nr; /* architecture number */
unsigned int phys_io; /* start of physical io */
unsigned int io_pg_offst; /* byte offset for io
* page tabe entry */
const char *name; /* architecture name */
unsigned long boot_params; /* tagged list */
unsigned int video_start; /* start of video RAM */
unsigned int video_end; /* end of video RAM */
unsigned int reserve_lp0 :1; /* never has lp0 */
unsigned int reserve_lp1 :1; /* never has lp1 */
unsigned int reserve_lp2 :1; /* never has lp2 */
unsigned int soft_reboot :1; /* soft reboot */
void (*fixup)(struct machine_desc *,
struct tag *, char **,
struct meminfo *);
void (*map_io)(void);/* IO mapping function */
void (*init_irq)(void);
struct sys_timer *timer; /* system tick timer */
void (*init_machine)(void);
};
/*
* Set of macros to define architecture features. This is built into
* a table by the linker.
*/
#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 \
};
#endif
MACH_TYPE_ AT91SAM9G10EK是9G10ek开发板在linux中的机器号。
“Atmel AT91SAM9G10-EK”是开发板信息，在终端输入cat /proc/cpuinfo可以查看。
MACHINE_START主要定义了“struct machine_desc”的类型，放在section（“.arch.info.init”），是初始化数据，kernel起来之后将被丢弃。
其余各个成员函数在setup_arch()中被赋值到内核结构体，在不同时期被调用：
1. .init_machine 在 arch/arm/kernel/setup.c 中被 customize_machine 调用，放在 arch_initcall() 段里面，会自动按顺序被调用。
arch_initcall(customize_machine);
customize_machine()[setup.c#692] -> init_machine();
2. .init_irq在start_kernel()(文件init/main.c)--> init_IRQ() (在文件arch/arm/kernel/Irq.c)--> init_arch_irq()中被调用
3. .map_io 在 start_kernel()(文件init/main.c)-->setup_arch(&command_line) (在文件arch/arm/kernel/setup.c)-->
paging_init()(在文件arch/arm/kernel/mm/Mmu.c)
--> devicemaps_init()(在文件arch/arm/kernel/mm/Mmu.c)中被调用
4. .timer是定义系统时钟，定义TIMER4为系统时钟，在arch/arm/plat-s3c/time.c中体现。在start_kernel() --> time_init()(在文件arch/arm/kernel/Time.c)-->system_time->init()中调用。
5. .boot_params是bootloader向内核传递的参数的位置，这要和bootloader中参数的定义要一致。
从它们在start_kernel()中被调用的顺序，可知它们执行的先后为：map_io; init_irq; timer->time_init; init_machine。
参考：
1. http://blog.sina.com.cn/s/blog_753fd0b00100t8js.html
2. http://blog.csdn.net/thl789/article/details/6699259