MIPSu-boot中codeRelocate流程分析－六四天安門英雄

u-boot的強大就在於它具有代碼relocate功能，運行時代碼在ram中跑，最明顯的一個好處就是u-boot可以自己替換自己固化在flash中的代碼。
先看一下這套代碼中和relocate有關的幾個大名鼎鼎的參數：

board/ar7100/ap83/config.mk

# ROM version

TEXT_BASE = 0xbf000000

# SDRAM version

#TEXT_BASE = 0x8020000

include/configs/ap83.h

#define CFG_MONITOR_BASE TEXT_BASE

board/ar7100/ap83/u-boot.lds

OUTPUT_FORMAT("elf32-tradbigmips", "elf32-tradbigmips", "elf32-tradbigmips")

OUTPUT_ARCH(mips)

ENTRY(_start)

SECTIONS

{

. = 0x00000000;

. = ALIGN(4);

.text :

{

*(.text)

}

. = ALIGN(4);

.rodata : { *(.rodata) }

. = ALIGN(4);

.data : { *(.data) }

. = ALIGN(4);

.sdata : { *(.sdata) }

_gp = ALIGN(16);

__got_start = .;

.got : { *(.got) }

__got_end = .;

.sdata : { *(.sdata) }

__u_boot_cmd_start = .;

.u_boot_cmd : { *(.u_boot_cmd) }

__u_boot_cmd_end = .;

uboot_end_data = .;

num_got_entries = (__got_end - __got_start) >> 2;

. = ALIGN(4);

.sbss : { *(.sbss) }

.bss : { *(.bss) }

uboot_end = .;

}

注釋：

0xbf000000 是mips 24kc Flash起始的位置，也是reset

0x80000000是ar9132平台DDR的起始位置。

鏈接腳本中的0x00000000會被這個TEXT_BASE替代，可以觀察編譯時候的輸出，你會發現有一個-DTEXT_BASE=0xbf000000的選項。

Relocate的代碼在start.S中。

啟動的大致順序如下，假設代碼固化於flash起始位置。

CPU 上電

Reset(初始化cp0寄存器)

設置GOT

lowlevel_init(初始化SDRAM)

初始化cache

Relocate代碼

下面是代碼的入口

la t9, board_init_f

j t9

nop

這個board_init_f定義在lib_mips/board.c中。

這個程序為下列元素分配合適的內存空間：

[1] u-boot代碼（.text .data .bss）

[2] malloc空間（相當於heap）

[3] Board Info

[4] Global Data

[5] Stack

這里可以看出malloc的空間位於heap中，即內存的高端，而函數中的局部變量，都在stack中的，在內存低端。

void board_init_f(ulong bootflag)

{

gd_t gd_data, *id;

bd_t *bd;

init_fnc_t **init_fnc_ptr;

ulong addr, addr_sp, len = (ulong)&uboot_end - CFG_MONITOR_BASE;

ulong *s;

/* Pointer is writable since we allocated a register for it.

gd = &gd_data;

/* compiler optimization barrier needed for GCC >= 3.4 */

__asm__ __volatile__("": : :"memory");

memset ((void *)gd, 0, sizeof (gd_t));

for (init_fnc_ptr = init_sequence; *init_fnc_ptr; ++init_fnc_ptr) {

if ((*init_fnc_ptr)() != 0) {

hang ();

}

* Now that we have DRAM mapped and working, we can

* relocate the code and continue running from DRAM.

addr = CFG_SDRAM_BASE + gd->ram_size;

/* We can reserve some RAM "on top" here.

/* round down to next 4 kB limit.

addr &= ~(4096 - 1);

debug ("Top of RAM usable for U-Boot at: %08lx\n", addr);

/* Reserve memory for U-Boot code, data & bss

* round down to next 16 kB limit

addr -= len;

addr &= ~(16 * 1024 - 1);

debug ("Reserving %ldk for U-Boot at: %08lx\n", len >> 10, addr);

/* Reserve memory for malloc() arena.

addr_sp = addr - TOTAL_MALLOC_LEN;

debug ("Reserving %dk for malloc() at: %08lx\n",

TOTAL_MALLOC_LEN >> 10, addr_sp);

* (permanently) allocate a Board Info struct

* and a permanent copy of the "global" data

addr_sp -= sizeof(bd_t);

bd = (bd_t *)addr_sp;

gd->bd = bd;

debug ("Reserving %d Bytes for Board Info at: %08lx\n",

sizeof(bd_t), addr_sp);

addr_sp -= sizeof(gd_t);

id = (gd_t *)addr_sp;

debug ("Reserving %d Bytes for Global Data at: %08lx\n",

sizeof (gd_t), addr_sp);

/* Reserve memory for boot params.

addr_sp -= CFG_BOOTPARAMS_LEN;

bd->bi_boot_params = addr_sp;

debug ("Reserving %dk for boot params() at: %08lx\n",

CFG_BOOTPARAMS_LEN >> 10, addr_sp);

* Finally, we set up a new (bigger) stack.

* Leave some safety gap for SP, force alignment on 16 byte boundary

* Clear initial stack frame

addr_sp -= 16;

addr_sp &= ~0xF;

s = (ulong *)addr_sp;

*s-- = 0;

addr_sp = (ulong)s;

debug ("Stack Pointer at: %08lx\n", addr_sp);

memcpy (id, (void *)gd, sizeof (gd_t));

/* On the purple board we copy the code in a special way

* in order to solve flash problems

#ifdef CONFIG_PURPLE

copy_code(addr);

#endif

relocate_code (addr_sp, id, addr);

/* NOTREACHED - relocate_code() does not return */

}

從代碼中可以看出，U-boot被relocate到內存的最高端。Relocate的實際部分在start.S中，通過relocate_code (addr_sp, id, addr)進入匯編，mips體系中函數參數被保存在a0,a1,a2中，所以看start.S中的注釋有：

* void relocate_code (addr_sp, gd, addr_moni)

* This "function" does not return, instead it continues in RAM

* after relocating the monitor code.

* a0 = addr_sp

* a1 = gd

* a2 = destination address

Relocate的核心代碼如下，分析一下能體會到mips匯編的delay branch

relocate_code:

move sp, a0 /* Set new stack pointer */

li t0, CFG_MONITOR_BASE

la t3, in_ram

lw t2, -12(t3) /* t2 <-- uboot_end_data */

move t1, a2

* t0 = source address

* t1 = target address

* t2 = source end address

/* On the purple board we copy the code earlier in a special way

* in order to solve flash problems

lw t3, 0(t0)

sw t3, 0(t1)

addu t0, 4

ble t0, t2, 1b

addu t1, 4 /* delay slot */

/* If caches were enabled, we would have to flush them here.

/* Jump to where we've relocated ourselves.

addi t0, a2, in_ram - _start

j t0

nop

上面計算u-boot end地址lw t2, -12(t3)是因為有下面的定義，

.word uboot_end_data

.word uboot_end

.word num_got_entries

in_ram:

一個word四個字節，所以是in_ram的位置-12個字節。拷貝代碼的時候，增加target指針位置的代碼放在跳轉代碼之后，就是由於mips的流水線。等拷貝完代碼之后，就跳轉到ram中執行了。也就是in_ram的地方(此時的in_ram在relocate后的內存高端)。

注意到在u-boot的連接腳本里面專門有一個.u_boot_cmd段，專門設置這樣一個段有什麼好處呢？可以看一下find_cmd這個函數，就可以理解了，通過把所有cmd結構指針放在一個統一的段里面在查找起來非常方便，要添加新的命令，也不用改變原來的結構。看看它是怎麼定義的吧：

原理：
每個命令都有一個命令結構體
struct cmd_tbl_s {
char*name; /* Command Name*/
intmaxargs; /* maximum number of arguments*/
intrepeatable; /* autorepeat allowed?*/
int (*cmd)(struct cmd_tbl_s *, int, int, char *[]); /* Implementation function*/
char*usage;/* Usage message(short)*/
char*help;/* Help message(long)*/
};
去定義它。Cmd為要調用的命令函數！name為該命令名字符串。

在u-boot里面有這樣的宏
#define Struct_Section __attribute__ ((unused,section (".u_boot_cmd")))

#define U_BOOT_CMD(name,maxargs,rep,cmd,usage,help) \
cmd_tbl_t __u_boot_cmd_##name Struct_Section = {#name, maxargs, rep, cmd, usage, help}

宏U_BOOT_CMD(name,maxargs,rep,cmd,usage,help)就是將
cmd_tbl_s{
name,
maxargs,
rep,
cmd,
usage,
help
}這樣的一個命令結構體放入內存.u_boot_cmd這個區域，.u_boot_cmd這個域在board/smdk2410/u-boot.lds中定義！在U-boot中的shell中，根據用戶輸入的命令，就會在.u_boot_cmd這個內存區域中查找，當.u_boot_cmd中某一個cmd_tbl_s命令結構體的cmd_tbl_s.name和輸入的命令字符串相符時，就調用該命令
結構體的cmd_tbl_s.cmd( ….)函數！

這里有一個要注意的是，cmd是一個函數指針，由於u-boot代碼是要relocate的，所以在代碼relocate之后，每個這樣的指針也要加上相應的偏移才能正常動作，代碼在board_init_r函數中。

最后是板子上電啟動時的輸出信息，看了很有幫助。

U-Boot 1.1.4-RAM VSC7395@MAC1 (Mar 21 2008 - 15:57:42)

AP83 (ar9100 with SPI flash)

DRAM: 16 MB

Top of RAM usable for U-Boot at: 81000000

Reserving 263k for U-Boot at: 80fbc000

Reserving 192k for malloc() at: 80f8c000

Reserving 56 Bytes for Board Info at: 80f8bfc8

Reserving 36 Bytes for Global Data at: 80f8bfa4

Reserving 128k for boot params() at: 80f6bfa4

Stack Pointer at: 80f6bf88