MACHINE_START and MACHINE_END Macro define

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***************************************************************************************************************************
作者：EasyWave 时间：2012.02.11

类别：linux驱动开发声明：转载，请保留链接

***************************************************************************************************************************

在linux-2.6.38中，/arch/arm/mach-w90x900/mach-nuc950evb.c文件的最后部分，有如下宏定义:

MACHINE_START(W90P950EVB, "W90P950EVB")
.phys_io = W90X900_PA_UART,
.io_pg_offst = (((u32)W90X900_VA_UART) >> 18) & 0xfffc,
.boot_params =  0x100,
.map_io  = nuc950evb_map_io,
.init_irq = nuc900_init_irq,
.init_machine = nuc950evb_init,
.timer  = &nuc900_timer,
MACHINE_END

而MACHINE_START和MACHINE_END宏定义在arch/arm/include/asm/mach/arch.h，如下：

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

将前面定义的MACHINE_START和MACHINE_END展开后得到:
static const struct machine_desc __mach_desc_W90P950EVB __used __attribute__((__section__(".arch.info.init"))) = {
.nr = MACH_TYPE_W90P950EVB, /* architecture number */
.name = "W90P950EVB"", /* architecture name */
.phys_io = W90X900_PA_UART, /* start of physical io */
.io_pg_offst = (((u32)W90X900_VA_UART) >> 18) & 0xfffc,
.boot_params = 0x100, /* tagged list */
.map_io = nuc950evb_map_io, /* IO mapping function */
.init_irq = nuc900_init_irq,
.init_machine = nuc950evb_init,
.timer = &nuc900_timer,
}

MACH_TYPE_W90P950EVB定义在arch/arm/include/asm/mach-types.h内，值为1860 十六进制为：0x744.
#define MACH_TYPE_W90P950EVB 1860

这个值是机器的类型值，编译时由arch/arm/tool/mach- types里面定义的数据生成的。
/* arch/arm/tool/mach-types */
w90x900 MACH_W90X900 W90X900 1860

而在arch/arm/kernel/head-common.S文件中的__lookup_machine_type函数中有如下的定义：

/*
* linux/arch/arm/kernel/head-common.S
*
* Copyright (C) 1994-2002 Russell King
* Copyright (c) 2003 ARM Limited
* All Rights Reserved
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*
*/

#define ATAG_CORE 0x54410001
#define ATAG_CORE_SIZE ((2*4 + 3*4) >> 2)
#define ATAG_CORE_SIZE_EMPTY ((2*4) >> 2)

.align 2
.type __switch_data, %object
__switch_data:
...................中间省略

.long processor_id                   @ r4
.long __machine_arch_type   @ r5
.long __atags_pointer            @ r6
.long cr_alignment                  @ r7
.long init_thread_union + THREAD_START_SP @ sp

...................中间省略

/*
* Look in <asm/procinfo.h> and arch/arm/kernel/arch.[ch] for
* more information about the __proc_info and __arch_info structures.
*/
.align 2
3: .long __proc_info_begin
.long __proc_info_end
4: .long .
.long __arch_info_begin
.long __arch_info_end

/*
* 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:
* r3, r4, r6 corrupted
* r5 = mach_info pointer in physical address space
*/
__lookup_machine_type:
#ifdef CONFIG_ARCH_W90X900     //这个就是前面的1860,十六进制为:0x744
       mov      r1, #0x700
       add      r1, r1, #0x40
       add      r1, r1, #0x04
#endif
adr r3, 4b
ldmia r3, {r4, r5, r6}
sub r3, r3, r4   @ get offset between virt&phys
add r5, r5, r3   @ convert virt addresses to
add r6, r6, r3   @ physical address space
1: ldr r3, [r5, #MACHINFO_TYPE] @ get machine type
teq r3, r1    @ matches loader number?
beq 2f    @ found
add r5, r5, #SIZEOF_MACHINE_DESC @ next machine_desc
cmp r5, r6
blo 1b
mov r5, #0    @ unknown machine
2: mov pc, lr
ENDPROC(__lookup_machine_type)

函数说明：

adr r3,4b : 取地址指令,这里的4b 是向后symbol名称是4的位置,即4: .long .的位置,将该地址存入r3，r3中存放的是4b处物理地址.
r5存的是__arch_info_begin 的地址
r6存的是__arch_info_end 的地址

arm linux 从入口到start_kernel 代码分析

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

          __arch_info_begin = .;
          *(.arch.info.init)
          __arch_info_end = .;

        这里是声明了两个变量:__arch_info_begin 和 __arch_info_end,其中等号后面的"."是location counter(详细内容请参考ld.info)，这三行的意思是: __arch_info_begin 的位置上,放置所有文件中的 ".arch.info.init" 段的内容,然后紧接着是 __arch_info_end 的位置.

teq r3, r1 @ matches loader number?
如果R1中存储的值和mach-types.h不一致的话，会出现下面的错误：

Machine configuration botched (nr xxxx), unable to continue.不同的版本会出现的警报不一致。

在arch/arm/kernel/setup.c中：

void __init setup_arch(char **cmdline_p)
{
struct tag *tags = (struct tag *)&init_tags;
struct machine_desc *mdesc;
char *from = default_command_line;

unwind_init();

setup_processor();
mdesc = setup_machine(machine_arch_type);
machine_name = mdesc->name;

if (mdesc->soft_reboot)
reboot_setup("s");

if (__atags_pointer)
tags = phys_to_virt(__atags_pointer);
else if (mdesc->boot_params)
tags = phys_to_virt(mdesc->boot_params);

/*
* If we have the old style parameters, convert them to
* a tag list.
*/
if (tags->hdr.tag != ATAG_CORE)
convert_to_tag_list(tags);
if (tags->hdr.tag != ATAG_CORE)
tags = (struct tag *)&init_tags;

if (mdesc->fixup)
mdesc->fixup(mdesc, tags, &from, &meminfo);

if (tags->hdr.tag == ATAG_CORE) {
  if (meminfo.nr_banks != 0)
   squash_mem_tags(tags);
  save_atags(tags);
  parse_tags(tags);
}

init_mm.start_code = (unsigned long) _text;
init_mm.end_code   = (unsigned long) _etext;
init_mm.end_data   = (unsigned long) _edata;
init_mm.brk    = (unsigned long) _end;

/* parse_early_param needs a boot_command_line */
strlcpy(boot_command_line, from, COMMAND_LINE_SIZE);

/* populate cmd_line too for later use, preserving boot_command_line */
strlcpy(cmd_line, boot_command_line, COMMAND_LINE_SIZE);
*cmdline_p = cmd_line;

parse_early_param();

paging_init(mdesc);
request_standard_resources(&meminfo, mdesc);

#ifdef CONFIG_SMP
smp_init_cpus();
#endif

cpu_init();
tcm_init();

/*
* Set up various architecture-specific pointers
*/
init_arch_irq = mdesc->init_irq;
system_timer = mdesc->timer;
init_machine = mdesc->init_machine;

#ifdef CONFIG_VT
#if defined(CONFIG_VGA_CONSOLE)
conswitchp = &vga_con;
#elif defined(CONFIG_DUMMY_CONSOLE)
conswitchp = &dummy_con;
#endif
#endif
early_trap_init();
}

而setup_machine(unsigned int nr)函数也是在arch/arm/kernel/setup.c中定义：

static struct machine_desc * __init setup_machine(unsigned int nr)
{
struct machine_desc *list;

/*
* locate machine in the list of supported machines.
*/
list = lookup_machine_type(nr); //看到了吧，这个才是重点
if (!list) {
  printk("Machine configuration botched (nr %d), unable "
         "to continue.\n", nr);
  while (1);
}

printk("Machine: %s\n", list->name);

return list;
}

因此就会出现：Machine configuration botched (nr xxxx), unable to continue.的警告信息。

各个成员函数在不同时期被调用：
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。
2. init_irq在start_kernel() --> init_IRQ() --> init_arch_irq() 被调用
3. map_io 在 setup_arch() --> paging_init() --> devicemaps_init()被调用
其他主要都在 setup_arch() 中用到。