linux启动分析(4)---汇编部分(3)

前一篇介绍了汇编函数__lookup_processor_type，这一篇介绍__lookup_architecture_type函数

函数__lookup_architecture_type介绍：
每个机器(一般指的是某一个电路板)都有自己的特殊结构，如物理内存地址，物理I/O地址，显存起始地址等等，
这个结构为struct machine_desc，定义在asm-arm/mach/arch.h中:
struct machine_desc {
/*
* Note! The first four elements are used
* by assembler code in head-armv.S
*/
unsigned intnr;/* architecture number*/
unsigned intphys_ram;/* start of physical ram */
unsigned intphys_io;/* start of physical io*/
unsigned intio_pg_offst;/* byte offset for io page table entry*/

const char*name;/* architecture name*/
unsigned intparam_offset;/* parameter page*/

unsigned intvideo_start;/* start of video RAM*/
unsigned intvideo_end;/* end of video RAM*/

unsigned intreserve_lp0 :1;/* never has lp0*/,
unsigned intreserve_lp1 :1;/* never has lp1*/
unsigned intreserve_lp2 :1;/* never has lp2*/
unsigned intsoft_reboot :1;/* soft reboot*/
void(*fixup)(struct machine_desc *,
struct param_struct *, char **,
struct meminfo *);
void(*map_io)(void);/* IO mapping function*/
void(*init_irq)(void);
};

这个结构一般都定义在(以arm平台为例)kernel\arch\arm\mach-xxx\xxx.c中，是用宏来定义的，以mainstone的开发板为例：
定义在kernel\arch\arm\mach-pxa\mainstone.c文件中，如下所示：
MACHINE_START(MAINSTONE, "Intel DBBVA0 Development Platform")
     MAINTAINER("MontaVista Software Inc.")
     BOOT_MEM(0xa0000000, 0x40000000, io_p2v(0x40000000))
     FIXUP(fixup_mainstone)
     MAPIO(mainstone_map_io)
     INITIRQ(mainstone_init_irq)
MACHINE_END
这些宏也定义在kernel/include/asm-arm/mach/arch.h中，以MACHINE_START为例：
#define MACHINE_START(_type,_name)      \
const struct machine_desc __mach_desc_##_type   \
__attribute__((__section__(".arch.info"))) = { \
     .nr     = MACH_TYPE_##_type,    \
     .name       = _name,

展开之后结构的是：
__mach_desc_MAINSTONE = {
 .nr = MACH_TYPE_MAINSTIONE,
 .name = "Intel DBBVA0 Development Platform",

中间的1行__attribute__((__section__(".arch.info"))) = {说明将这个结构放到指定的段.arch.info中，这和前面的
.proc.info是一个意思，__attribute__((__section__的含义参考GNU手册。后面的宏都是类似的含义，这里就不再一一
介绍。下面开始说明源码：

第1行实现r4指向2b的地址，2b如__lookup_processor_type介绍的第19行，将machine_desc结构中的数据存放到r2, r3, r5, r6, r7。
读取__mach_desc_MAINSTONE结构中的nr参数到r5中，如第7行，比较r5和r1中的机器编号是否相同，如第8行，
r5中的nr值MACH_TYPE_MAINSTONE定义在kernel\include\asm-arm\mach-types.h中：
#define MACH_TYPE_MAINSTONE            303
r1中的值是由bootloader传递过来的，这在<>中有说明，
如果机器编号相同，跳到15行执行，r5=intphys_ram，r6=intphys_io，r7=intio_pg_offst，并返回。如果
不同则将地址指针增加，在跳到7行继续查找，如10--12行的代码，如果检索完所有的machine_desc仍然没
有找到则将r7清零并返回。

/*     
 * Lookup machine architecture in the linker-build list of architectures.
 * Note that we can't use the absolute addresses for the __arch_info
 * lists since we aren't running with the MMU on (and therefore, we are
 * not in the correct address space).  We have to calculate the offset.
 *     
 *  r1 = machine architecture number
 * Returns:
 *  r2, r3, r4 corrupted           
 *  r5 = physical start address of RAM
 *  r6 = physical address of IO
 *  r7 = byte offset into page tables for IO
 */    
1  __lookup_architecture_type:
2          adr r4, 2b
3          ldmia   r4, {r2, r3, r5, r6, r7}    @ throw away r2, r3
4          sub r5, r4, r5          @ convert addresses
5          add r4, r6, r5          @ to our address space
6          add r7, r7, r5
7  1:      ldr r5, [r4]            @ get machine type
8          teq r5, r1
9          beq 2f 
10         add r4, r4, #SIZEOF_MACHINE_DESC
11         cmp r4, r7
12         blt 1b
13         mov r7, #0              @ unknown architecture
14         mov pc, lr
15 2:      ldmib   r4, {r5, r6, r7}        @ found, get results
16         mov pc, lr