说明：本文所使用的U-Boot的版本是1.1.6，平台是S3C2440。
目录
一、简介
1.1标记列表
二、设置标记存放的地址
2.1相关的结构体定义
2.2标记存放地址的设定
三、标记的设置
3.1设置标记ATAG_CORE
3.2设置内存标记ATAG_MEM
3.3设置命令行标记ATAG_CMDLINE
3.4设置ATAG_NONE
一、简介
U-Boot与Linux内核的交互是单向的，U-Boot将各类参数传递给讷河。由于他们不能同时运行，传递办法只能有一个个：U-Boot将参数放在某个约定的地方之后，在启动内核，内核启动后从这个地方获得参数。
1.1标记列表
除了约定好参数存放的地方外，还要规定参数的结构。Linux2.4.x以后的内核都以标记列表（tagged
list）的形式来传递参数。标记就是一种数据结构；标记列表就是挨着存放的多个标记。标记列表以标记ATAG_CORE开始，以ATAGE_NONE结束。
标记的数据结构为tag，它是偶一个tag_header结构和一个联合体（union）组成。tag_header结构体表示标记的类型及长度，比如是表示内存还是表示命令行参数等。对于不同类型的标记使用不同的联合体，比如表示内存=时使用tag_men32，表示命令行时使用tag_cmdline。其定定义在include/asm-arm/setup.c文件中。
/*
* The new way of passing information:
a list of tagged entries
*/
/* The list ends with an ATAG_NONE node.
*/
#define ATAG_NONE 0x00000000
struct tag_header {
u32 size;
u32 tag;
};
/* The list must start with an ATAG_CORE
node */
#define ATAG_CORE 0x54410001
struct tag_core {
u32 flags; /* bit 0 = read-only */
u32 pagesize;
u32 rootdev;
};
/* it is allowed to have multiple ATAG_MEM
nodes */
#define ATAG_MEM 0x54410002
struct tag_mem32 {
u32 size;
u32 start; /* physical start address
*/
};
/* VGA text type displays */
#define ATAG_VIDEOTEXT 0x54410003
struct tag_videotext {
u8 x;
u8 y;
u16 video_page;
u8 video_mode;
u8 video_cols;
u16 video_ega_bx;
u8 video_lines;
u8 video_isvga;
u16 video_points;
};
/* describes how the ramdisk will be
used in kernel */
#define ATAG_RAMDISK 0x54410004
struct tag_ramdisk {
u32 flags; /* bit 0 = load, bit 1
= prompt */
u32 size; /* decompressed ramdisk
size in _kilo_ bytes */
u32 start; /* starting block of floppy-based
RAM disk image */
};
/* describes where the compressed ramdisk
image lives (virtual address) */
/*
* this one accidentally used virtual
addresses - as such,
* its depreciated.
*/
#define ATAG_INITRD 0x54410005
/* describes where the compressed ramdisk
image lives (physical address) */
#define ATAG_INITRD2 0x54420005
struct tag_initrd {
u32 start; /* physical start address
*/
u32 size; /* size of compressed ramdisk
image in bytes */
};
/* board serial number. "64 bits
should be enough for everybody" */
#define ATAG_SERIAL 0x54410006
struct tag_serialnr {
u32 low;
u32 high;
};
/* board revision */
#define ATAG_REVISION 0x54410007
struct tag_revision {
u32 rev;
};
/* initial values for vesafb-type framebuffers.
see struct screen_info
* in include/linux/tty.h
*/
#define ATAG_VIDEOLFB 0x54410008
struct tag_videolfb {
u16 lfb_width;
u16 lfb_height;
u16 lfb_depth;
u16 lfb_linelength;
u32 lfb_base;
u32 lfb_size;
u8 red_size;
u8 red_pos;
u8 green_size;
u8 green_pos;
u8 blue_size;
u8 blue_pos;
u8 rsvd_size;
u8 rsvd_pos;
};
/* command line: \0 terminated string
*/
#define ATAG_CMDLINE 0x54410009
struct tag_cmdline {
char cmdline[1]; /* this is the minimum
size */
};
/* acorn RiscPC specific information
*/
#define ATAG_ACORN 0x41000101
struct tag_acorn {
u32 memc_control_reg;
u32 vram_pages;
u8 sounddefault;
u8 adfsdrives;
};
/* footbridge memory clock, see arch/arm/mach-footbridge/arch.c
*/
#define ATAG_MEMCLK 0x41000402
struct tag_memclk {
u32 fmemclk;
};
struct tag {
struct tag_header hdr;
union {
struct tag_core core;
struct tag_mem32 mem;
struct tag_videotext videotext;
struct tag_ramdisk ramdisk;
struct tag_initrd initrd;
struct tag_serialnr serialnr;
struct tag_revision revision;
struct tag_videolfb videolfb;
struct tag_cmdline cmdline;
/*
* Acorn specific
*/
struct tag_acorn acorn;
/*
* DC21285 specific
*/
struct tag_memclk memclk;
} u;
};
#define tag_next(t)<span style="white-space:pre">
</span>((struct tag *)((u32 *)(t) + (t)->hdr.size))
#define tag_size(type)<span style="white-space:pre">
</span>((sizeof(struct tag_header) + sizeof(struct
type))>> 2) //???
二、设置标记存放的地址
2.1相关的结构体定义
结构体bd中保存了标记存放的地址。bd结构体是gd结构体的一项，我们先看gd结构体，其定义在include/asm-arm/global_data.h文件中：
typedef struct global_data {
bd_t *bd;//开发板相关参数 ,结构体变量，参考u-boot.h
unsigned long flags;//指示标志，如设备已经初始化标志等
unsigned long baudrate;//串行口通讯速率
unsigned long have_console;
/* serial_init() was called 如果执行了该函数，则设置为1
*/
unsigned long reloc_off;
/*
*Relocation Offset 重定位偏移，就是实际定向的位置与编译连接时指定的位置之差，一般为0
*/
unsigned long env_addr; /* 环境参数地址*/
unsigned long env_valid; /* 环境参数CRC检验有效标志*/
unsigned long fb_base; /*帧缓冲区基地址*/
#ifdef CONFIG_VFD
unsigned char vfd_type; /* 显示类型*/
#endif
#if 0
unsigned long cpu_clk; /*cpu时钟*/
unsigned long bus_clk; //总线时钟
unsigned long ram_size; /* RAM size
*/
unsigned long reset_status; /* reset
status register at boot */
#endif
void **jt; /* jump table 跳转表，用来登记"函数调用地址"*/
} gd_t;
接来下我们来看一下bd结构体，这个结构体定义在include/asm-arm/u-boot.h文件中：
typedef struct bd_info {
int bi_baudrate; /* 串口波特率*/
unsigned long bi_ip_addr; /* IP 地址*/
unsigned char bi_enetaddr[6]; /* MAC地址*/
struct environment_s *bi_env;
ulong bi_arch_number; /* 板子的id*/
ulong bi_boot_params; /* 启动参数*/
struct /* RAM 配置*/
{
ulong start;
ulong size;
}bi_dram[CONFIG_NR_DRAM_BANKS];
#ifdef CONFIG_HAS_ETH1
/* second onboard ethernet port */
unsigned char bi_enet1addr[6];
#endif
} bd_t;
2.2标记存放地址的设定
在board/smdk2410/smdk2410.c的board_init
函数设置了bi_boot_params 参数：
int board_init (void)
{
S3C24X0_CLOCK_POWER * const clk_power
= S3C24X0_GetBase_CLOCK_POWER();//获取时钟和电源配置寄存器的第一个寄存器的地址，寄存器的地上是连续的
S3C24X0_GPIO * const gpio = S3C24X0_GetBase_GPIO();//获取GPIO配置寄存器的第一个寄存器的地址
/* to reduce PLL lock time, adjust
the LOCKTIME register */
clk_power->LOCKTIME = 0xFFFFFF;
/* configure MPLL */
clk_power->MPLLCON = ((M_MDIV <<
12) + (M_PDIV << 4) + M_SDIV);
/* some delay between MPLL and UPLL
*/
delay (4000);
/* configure UPLL */
clk_power->UPLLCON = ((U_M_MDIV
<< 12) + (U_M_PDIV << 4) + U_M_SDIV);
/* some delay between MPLL and UPLL
*/
delay (8000);
/* set up the I/O ports */
gpio->GPACON = 0x007FFFFF;
gpio->GPBCON = 0x00044555;
gpio->GPBUP = 0x000007FF;
gpio->GPCCON = 0xAAAAAAAA;
gpio->GPCUP = 0x0000FFFF;
gpio->GPDCON = 0xAAAAAAAA;
gpio->GPDUP = 0x0000FFFF;
gpio->GPECON = 0xAAAAAAAA;
gpio->GPEUP = 0x0000FFFF;
gpio->GPFCON = 0x000055AA;
gpio->GPFUP = 0x000000FF;
gpio->GPGCON = 0xFF95FFBA;
gpio->GPGUP = 0x0000FFFF;
gpio->GPHCON = 0x002AFAAA;
gpio->GPHUP = 0x000007FF;
/* arch number of SMDK2410-Board */
gd->bd->bi_arch_number = MACH_TYPE_SMDK2410;
/* adress of boot parameters */
gd->bd->bi_boot_params = 0x30000100;
icache_enable(); //调用cpu/arm920t/cpu.c中的函数
dcache_enable();
return 0;
}
三、标记的设置
U-Boot通过bootm命令引导Linux内核，bootm命令对吼调用do_bootm_linux函数来引导内核。在do_bootm_linux函数就设置了标记，该函数的定义在lib_arm/armlinux.c中：
void do_bootm_linux (cmd_tbl_t *cmdtp,
int flag, int argc, char *argv[],
ulong addr, ulong *len_ptr, int verify)
{
ulong len = 0, checksum;
ulong initrd_start, initrd_end;
ulong data;
void (*theKernel)(int zero, int arch,
uint params);
image_header_t *hdr = &header;
bd_t *bd = gd->bd;
#ifdef CONFIG_CMDLINE_TAG
char *commandline = getenv ("bootargs");
#endif
theKernel = (void (*)(int, int, uint))ntohl(hdr->ih_ep);
设置kernal加载地址
/*
* Check if there is an initrd image
*/
用户自定义了initrd之后需要加载进来，整个过程需要进行头部以及整个数据内部校，类似于内核的加载校验，这里省略了。
initial RAM disk Linux初始 RAM磁盘（initrd）是在系统引导过程中挂载的一个临时根文件系统，用来支持两阶段的引导过程。initrd文件中包含了各种可执行程序和驱动程序，它们可以用来挂载实际的根文件系统，然后再将这个
initrd RAM 磁盘卸载，并释放内存。在很多嵌入式Linux 系统中，initrd 就是最终的根文件系统。
if (argc >= 3) {
SHOW_BOOT_PROGRESS (9);
addr = simple_strtoul (argv[2], NULL,
16);
printf ("## Loading Ramdisk Image
at %08lx ...\n", addr);
/* Copy header so we can blank CRC
field for re-calculation */
#ifdef CONFIG_HAS_DATAFLASH
if (addr_dataflash (addr)) {
read_dataflash (addr, sizeof (image_header_t),
(char *) &header);
} else
#endif
memcpy (&header, (char *) addr,
sizeof (image_header_t));
if (ntohl (hdr->ih_magic) != IH_MAGIC)
{
printf ("Bad Magic Number\n");
SHOW_BOOT_PROGRESS (-10);
do_reset (cmdtp, flag, argc, argv);
}
data = (ulong) & header;
len = sizeof (image_header_t);
checksum = ntohl (hdr->ih_hcrc);
hdr->ih_hcrc = 0;
if (crc32 (0, (unsigned char *) data,
len) != checksum) {
printf ("Bad Header Checksum\n");
SHOW_BOOT_PROGRESS (-11);
do_reset (cmdtp, flag, argc, argv);
}
SHOW_BOOT_PROGRESS (10);
print_image_hdr (hdr);
data = addr + sizeof (image_header_t);
len = ntohl (hdr->ih_size);
#ifdef CONFIG_HAS_DATAFLASH
if (addr_dataflash (addr)) {
read_dataflash (data, len, (char *)
CFG_LOAD_ADDR);
data = CFG_LOAD_ADDR;
}
#endif
if (verify) {
ulong csum = 0;
printf (" Verifying Checksum
... ");
csum = crc32 (0, (unsigned char *)
data, len);
if (csum != ntohl (hdr->ih_dcrc))
{
printf ("Bad Data CRC\n");
SHOW_BOOT_PROGRESS (-12);
do_reset (cmdtp, flag, argc, argv);
}
printf ("OK\n");
}
SHOW_BOOT_PROGRESS (11);
if ((hdr->ih_os != IH_OS_LINUX)
||
(hdr->ih_arch != IH_CPU_ARM) ||
(hdr->ih_type != IH_TYPE_RAMDISK))
{
printf ("No Linux ARM Ramdisk
Image\n");
SHOW_BOOT_PROGRESS (-13);
do_reset (cmdtp, flag, argc, argv);
}
#if defined(CONFIG_B2) || defined(CONFIG_EVB4510)
|| defined(CONFIG_ARMADILLO)
/*
*we need to copy the ramdisk to SRAM
to let Linux boot
*/
memmove ((void *) ntohl(hdr->ih_load),
(uchar *)data, len);
data = ntohl(hdr->ih_load);
#endif /* CONFIG_B2 || CONFIG_EVB4510
*/
/*
* Now check if we have a multifile
image
*/
} else if ((hdr->ih_type == IH_TYPE_MULTI)
&& (len_ptr[1])) {
ulong tail = ntohl (len_ptr[0]) %
4;
int i;
SHOW_BOOT_PROGRESS (13);
/* skip kernel length and terminator
*/
data = (ulong) (&len_ptr[2]);
/* skip any additional image length
fields */
for (i = 1; len_ptr[i]; ++i)
data += 4;
/* add kernel length, and align */
data += ntohl (len_ptr[0]);
if (tail) {
data += 4 - tail;
}
len = ntohl (len_ptr[1]);
} else {
/*
* no initrd image
*/
SHOW_BOOT_PROGRESS (14);
len = data = 0;
}
#ifdef DEBUG
if (!data) {
printf ("No initrd\n");
}
#endif
if (data) {
initrd_start = data;
initrd_end = initrd_start + len;
} else {
initrd_start = 0;
initrd_end = 0;
}
SHOW_BOOT_PROGRESS (15);
debug ("## Transferring control
to Linux (at address %08lx) ...\n",
(ulong) theKernel);
#if defined (CONFIG_SETUP_MEMORY_TAGS)
|| \
defined (CONFIG_CMDLINE_TAG) || \
defined (CONFIG_INITRD_TAG) || \
defined (CONFIG_SERIAL_TAG) || \
defined (CONFIG_REVISION_TAG) || \
defined (CONFIG_LCD) || \
defined (CONFIG_VFD)
setup_start_tag (bd);设置各种tag，用于传递参数给Linux
#ifdef CONFIG_SERIAL_TAG
setup_serial_tag (?ms);
#endif
#ifdef CONFIG_REVISION_TAG
setup_revision_tag (?ms);
#endif
#ifdef CONFIG_SETUP_MEMORY_TAGS
setup_memory_tags (bd);
#endif
#ifdef CONFIG_CMDLINE_TAG
setup_commandline_tag (bd, commandline);
#endif
#ifdef CONFIG_INITRD_TAG
if (initrd_start && initrd_end)
setup_initrd_tag (bd, initrd_start,
initrd_end);
#endif
#if defined (CONFIG_VFD) || defined
(CONFIG_LCD)
setup_videolfb_tag ((gd_t *) gd);
#endif
setup_end_tag (bd);
#endif
/* we assume that the kernel is in
place */
printf ("\nStarting kernel ...\n\n");打印信息
#ifdef CONFIG_USB_DEVICE
{
extern void udc_disconnect (void);
udc_disconnect ();
}
#endif
cleanup_before_linux ();启动之前先做一些清理工作cpu/arm920t/cpu.c
调用内核需要传递的参数如下：
R0：必须为0
R1：机器类型ID，本机为ARM（bd->bi_arch_number）
R2：启动参数列表在内存中的位置（bd->bi_boot_params）
theKernel (0, bd->bi_arch_number,
bd->bi_boot_params);
}
3.1设置标记ATAG_CORE
标记列表以标记ATAG_CORE开始
static void setup_start_tag (bd_t *bd)
{
params = (struct tag *) bd->bi_boot_params;
params->hdr.tag = ATAG_CORE;
params->hdr.size = tag_size (tag_core);
params->u.core.flags = 0;
params->u.core.pagesize = 0;
params->u.core.rootdev = 0;
params = tag_next (params);//指向当前标记的末尾
}
3.2设置内存标记ATAG_MEM
在board/smdk2410/smdk2410.c的dram_init函数设置了bd的bi_dram结构体：
int dram_init (void
{
gd->bd->bi_dram[0].start = PHYS_SDRAM_1;
gd->bd->bi_dram[0].size = PHYS_SDRAM_1_SIZE;
return 0;
}
下面是这边内存标记的结构体：
static void setup_memory_tags (bd_t
*bd)
{
int i;
for (i = 0; i < CONFIG_NR_DRAM_BANKS;
i++) {
params->hdr.tag = ATAG_MEM;
params->hdr.size = tag_size (tag_mem32);
params->u.mem.start = bd->bi_dram[i].start;
params->u.mem.size = bd->bi_dram[i].size;
params = tag_next (params);
}
}
3.3设置命令行标记ATAG_CMDLINE
命令行就是一个字符串，用来控制内核的一些行为。比如“root=/dev/mtdblock2
init=/linuxrc console=ttySAC0 ”表示根文件系统在MTD2分区上系统启动后执行的第一个程序为/linuxrc，控制台是ttySAC0
。
static void setup_commandline_tag (bd_t
*bd, char *commandline)
{
char *p;
if (!commandline)
return;
/* eat leading white space */
for (p = commandline; *p == ' '; p++);
/* skip non-existent command lines
so the kernel will still
* use its default command line.
*/
if (*p == '\0')
return;
params->hdr.tag = ATAG_CMDLINE;
params->hdr.size =
(sizeof (struct tag_header) + strlen
(p) + 1 + 4) >> 2;
strcpy (params->u.cmdline.cmdline,
p);
params = tag_next (params);
}
3.4设置ATAG_NONE
标记列表以标记ATAG_NONE介绍。
static void setup_end_tag (bd_t *bd)
{
params->hdr.tag = ATAG_NONE;
params->hdr.size = 0;
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