U-Boot与Linux内核的交互

U-Boot与Linux内核的交互

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)	((struct tag *)((u32 *)(t) + (t)->hdr.size))
#define tag_size(type)	((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;
}