2.u-boot啟動內核：
[code]theKernel(0,bd->bi_arch_number,bd->bi_boot_params) //啟動內核
bd->bi_arch_number //機器id
bd->bi_boot_params //uboot傳入內核參數存放的地址[code]//head.S:
@ and irqs disabled
mrc p15, 0, r9, c0, c0 @ get processor id
bl __lookup_processor_type @ r5=procinfo r9=cpuid
movs r10, r5 @ invalid processor (r5=0)?
beq __error_p @ yes, error 'p'
bl __lookup_machine_type @ r5=machinfo //查找機器ID
movs r8, r5 @ invalid machine (r5=0)?
beq __error_a @ yes, error 'a'
bl __create_page_tables
//head-common.S:
3: #3b地址定義
.long . #3b的虛擬地址
.long __arch_info_begin
.long __arch_info_end
__lookup_machine_type:
adr r3, 3b #r3 = 3b的物理地址
ldmia r3, {r4, r5, r6}
sub r3, r3, r4
add r5, r5, r3
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
//地址轉換過程
adr r3, 3b #r3 = 3b的物理地址
ldmia r3, {r4, r5, r6}
# r4= 3標號的虛擬地址
# r5=__arch_info_begin
# r6 = __arch_info_end
//__arch_info_begin,__arch_info_end在哪兒定義？
vmlinux.lds：
......
__arch_info_begin=.;
*(.arch.info.init) //這是代碼中定義的一個段
__arch_info_end=.;
再看看：arch.h
arch.h
#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 \
};
//那麼誰在使用這個宏
誰在使用這個宏：以s3c2440為例：
MACHINE_START(S3C2440, "SMDK2440")
/* Maintainer: Ben Dooks <[email protected]> */
.phys_io = S3C2410_PA_UART,
.io_pg_offst = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
.boot_params = S3C2410_SDRAM_PA + 0x100,
.init_irq = s3c24xx_init_irq,
.map_io = smdk2440_map_io,
.init_machine = smdk2440_machine_init,
.timer = &s3c24xx_timer,
MACHINE_END
將其展開：
static const struct machine_desc __mach_desc_S3C2440 = {
.nr = MACH_TYPE_S3C2440,
.name = "SMDK2440",
.phys_io = S3C2410_PA_UART,
.io_pg_offst = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
.boot_params = S3C2410_SDRAM_PA + 0x100,
.init_irq = s3c24xx_init_irq,
.map_io = smdk2440_map_io,
.init_machine = smdk2440_machine_init,
.timer = &s3c24xx_timer,
};
//machine_desc
machine_desc 打開這個結構體
描述一個單板的屬性：
struct machine_desc {
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 */ 機器id
unsigned long boot_params; /* tagged list */ 參數
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);
};
在看看head_common.S
1:
ldr r3, [r5, #MACHINFO_TYPE] @ get machine type
teq r3, r1 @ matches loader number? 比較機器id
比較成功之後：
bl __create_page_tables @創建頁表
ldr r13, __switch_data @ address to jump to after
@ mmu has been enabled
adr lr, __enable_mmu @ return (PIC) address #使能MMU
add pc, r10, #PROCINFO_INITFUNC
創建頁表，使能MMU之後跳轉到__switch_data
__switch_data:
.long __mmap_switched
.long __data_loc @ r4
.long __data_start @ r5
.long __bss_start @ r6
.long _end @ r7
.long processor_id @ r4
.long __machine_arch_type @ r5
.long cr_alignment @ r6
.long init_thread_union + THREAD_START_SP @ sp
/*
* The following fragment of code is executed with the MMU on in MMU mode,
* and uses absolute addresses; this is not position independent.
*
* r0 = cp#15 control register
* r1 = machine ID
* r9 = processor ID
*/
.type __mmap_switched, %function
__mmap_switched:
adr r3, __switch_data + 4
ldmia r3!, {r4, r5, r6, r7}
cmp r4, r5 @ Copy data segment if needed
1: cmpne r5, r6
ldrne fp, [r4], #4
strne fp, [r5], #4
bne 1b
mov fp, #0 @ Clear BSS (and zero fp)
1: cmp r6, r7
strcc fp, [r6],#4
bcc 1b
ldmia r3, {r4, r5, r6, sp}
str r9, [r4] @ Save processor ID
str r1, [r5] @ Save machine type
bic r4, r0, #CR_A @ Clear 'A' bit
stmia r6, {r0, r4} @ Save control register values
b start_kernel #啟動內核 內核的第一個C函數：總結：Head.s的作用
1、判斷內核支持不支持CPU
2、判斷內核支持不支持單板
3、建立頁表
4、使能MMU
5、跳轉到 start_kernel
