Bootloader 将参数放在某个约定的地方之后，再启动内核，内核启动后从这个地方获得参数。
除了约定好参数存放的地址外，还要规定参数的结构。Linux 2.4.x 以后的内核都期望以
标记列表（tagged list）的形式来传递启动参数。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，以标记ATAG_NONE 结束。
标记的数据结构为tag，它由一个tag_header 结构和一个联合（union）组成。
tag_header结构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（union），比如表示内存时使用tag_mem32，表示命令行时使用tag_cmdline。
数据结构tag 和tag_header 定义在Linux 内核源码的uboot/include/asm-arm/setup.h头文件中
参考嵌入式Linux应用开发完全手册ch15.1 p243
uboot/include/asm-arm/setup.h标记列表类型定义
/*
* linux/include/asm/setup.h
*
* Copyright (C) 1997-1999 Russell King
*
* 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.
*
* Structure passed to kernel to tell it about the
* hardware it's running on. See linux/Documentation/arm/Setup
* for more info.
*
* NOTE:
* This file contains two ways to pass information from the boot
* loader to the kernel. The old struct param_struct is deprecated,
* but it will be kept in the kernel for 5 years from now
* (2001). This will allow boot loaders to convert to the new struct
* tag way.
*/
#ifndef __ASMARM_SETUP_H
#define __ASMARM_SETUP_H
/*
* Usage:
* - do not go blindly adding fields, add them at the end
* - when adding fields, don't rely on the address until
* a patch from me has been released
* - unused fields should be zero (for future expansion)
* - this structure is relatively short-lived - only
* guaranteed to contain useful data in setup_arch()
*/
#define COMMAND_LINE_SIZE 1024
/* This is the old deprecated way to pass parameters to the kernel */
struct param_struct {
union {
struct {
unsigned long page_size; /* 0 */
unsigned long nr_pages; /* 4 */
unsigned long ramdisk_size; /* 8 */
unsigned long flags; /* 12 */
#define FLAG_READONLY 1
#define FLAG_RDLOAD 4
#define FLAG_RDPROMPT 8
unsigned long rootdev; /* 16 */
unsigned long video_num_cols; /* 20 */
unsigned long video_num_rows; /* 24 */
unsigned long video_x; /* 28 */
unsigned long video_y; /* 32 */
unsigned long memc_control_reg; /* 36 */
unsigned char sounddefault; /* 40 */
unsigned char adfsdrives; /* 41 */
unsigned char bytes_per_char_h; /* 42 */
unsigned char bytes_per_char_v; /* 43 */
unsigned long pages_in_bank[4]; /* 44 */
unsigned long pages_in_vram; /* 60 */
unsigned long initrd_start; /* 64 */
unsigned long initrd_size; /* 68 */
unsigned long rd_start; /* 72 */
unsigned long system_rev; /* 76 */
unsigned long system_serial_low; /* 80 */
unsigned long system_serial_high; /* 84 */
unsigned long mem_fclk_21285; /* 88 */
} s;
char unused[256];
} u1;
union {
char paths[8][128];
struct {
unsigned long magic;
char n[1024 - sizeof(unsigned long)];
} s;
} u2;
char commandline[COMMAND_LINE_SIZE];
};
/*
* 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;//标记起始
struct tag_mem32 mem;//内存标记，告诉内核内存的分布
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;//命令标记，一个字符串，告诉内核要做什么，即在uboot命令中设置的环境变量bootargs
//"console=ttySAC0 noinitrd root=/dev/mtdblock3 init=/linuxrc"
/*
* Acorn specific
*/
struct tag_acorn acorn;
/*
* DC21285 specific
*/
struct tag_memclk memclk;
} u;
};
struct tagtable {
u32 tag;
int (*parse)(const struct tag *);
};
#define __tag __attribute__((unused, __section__(".taglist")))
#define __tagtable(tag, fn) \
static struct tagtable __tagtable_##fn __tag = { tag, fn }
#define tag_member_present(tag,member) \
((unsigned long)(&((struct tag *)0L)->member + 1) \
<= (tag)->hdr.size * 4)
#define tag_next(t) ((struct tag *)((u32 *)(t) + (t)->hdr.size))
#define tag_size(type) ((sizeof(struct tag_header) + sizeof(struct type)) >> 2)
#define for_each_tag(t,base) \
for (t = base; t->hdr.size; t = tag_next(t))
/*
* Memory map description
*/
#define NR_BANKS 8
struct meminfo {
int nr_banks;
unsigned long end;
struct {
unsigned long start;
unsigned long size;
int node;
} bank[NR_BANKS];
};
extern struct meminfo meminfo;
#endif
uboot/lib_arm/bootm.c 标记列表
/*
* (C) Copyright 2002
* Sysgo Real-Time Solutions, GmbH <www.elinos.com>
* Marius Groeger <mgroeger@sysgo.de>
*
* Copyright (C) 2001 Erik Mouw (J.A.K.Mouw@its.tudelft.nl)
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*
*/
#include <common.h>
#include <command.h>
#include <image.h>
#include <u-boot/zlib.h>
#include <asm/byteorder.h>
DECLARE_GLOBAL_DATA_PTR;/**获取全局结构体指针gd***gd_t gd，此宏在uboot/include/asm-arm/global_data.h定义,参考blog.csdn.net/songqqnew/article/details/6847699**/
#if defined (CONFIG_SETUP_MEMORY_TAGS) || \
defined (CONFIG_CMDLINE_TAG) || \
defined (CONFIG_INITRD_TAG) || \
defined (CONFIG_SERIAL_TAG) || \
defined (CONFIG_REVISION_TAG) || \
defined (CONFIG_VFD) || \
defined (CONFIG_LCD)
static void setup_start_tag (bd_t *bd);
# ifdef CONFIG_SETUP_MEMORY_TAGS
static void setup_memory_tags (bd_t *bd);
# endif
static void setup_commandline_tag (bd_t *bd, char *commandline);
# ifdef CONFIG_INITRD_TAG
static void setup_initrd_tag (bd_t *bd, ulong initrd_start,
ulong initrd_end);
# endif
static void setup_end_tag (bd_t *bd);
# if defined (CONFIG_VFD) || defined (CONFIG_LCD)
static void setup_videolfb_tag (gd_t *gd);
# endif
static struct tag *params;/***************定义tag***************************************/
#endif /* CONFIG_SETUP_MEMORY_TAGS || CONFIG_CMDLINE_TAG || CONFIG_INITRD_TAG */
int do_bootm_linux(int flag, int argc, char *argv[], bootm_headers_t *images)
{
bd_t *bd = gd->bd;/***********************获取bd*************************************/
char *s;
int machid = bd->bi_arch_number;
void (*theKernel)(int zero, int arch, uint params);
#ifdef CONFIG_CMDLINE_TAG
char *commandline = getenv ("bootargs");
#endif
if ((flag != 0) && (flag != BOOTM_STATE_OS_GO))
return 1;
theKernel = (void (*)(int, int, uint))images->ep;
s = getenv ("machid");
if (s) {
machid = simple_strtoul (s, NULL, 16);
printf ("Using machid 0x%x from environment\n", machid);
}
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);
#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 (images->rd_start && images->rd_end)
setup_initrd_tag (bd, images->rd_start, images->rd_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");
//#ifdef CONFIG_USB_DEVICE
#if 0
{
extern void udc_disconnect (void);
udc_disconnect ();
}
#endif
cleanup_before_linux ();
theKernel (0, machid, bd->bi_boot_params);
/* does not return */
return 1;
}
#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)
static void setup_start_tag (bd_t *bd)
{
params = (struct tag *) bd->bi_boot_params;/*bi_boot_param=30000100,在uboot/board/mini2440/mini2440.c中指定***param指向参数区首地址，存放参数**/
params->hdr.tag = ATAG_CORE;//开始标记
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);
}
#ifdef CONFIG_SETUP_MEMORY_TAGS
static void setup_memory_tags (bd_t *bd)
{
int i;
for (i = 0; i < CONFIG_NR_DRAM_BANKS; i++) {
params->hdr.tag = ATAG_MEM;//标记类型:内存标记，hdr是struct tag_header类型结构体，为tag一成员
params->hdr.size = tag_size (tag_mem32);//标记大小
params->u.mem.start = bd->bi_dram[i].start;//内存起始地址，u是union类型，为tag一成员
params->u.mem.size = bd->bi_dram[i].size;//内存结束地址
params = tag_next (params);
}
}
#endif /* CONFIG_SETUP_MEMORY_TAGS */
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;//标记类型:命令字符串
params->hdr.size =
(sizeof (struct tag_header) + strlen (p) + 1 + 4) >> 2;
strcpy (params->u.cmdline.cmdline, p);
params = tag_next (params);
}
#ifdef CONFIG_INITRD_TAG
static void setup_initrd_tag (bd_t *bd, ulong initrd_start, ulong initrd_end)
{
/* an ATAG_INITRD node tells the kernel where the compressed
* ramdisk can be found. ATAG_RDIMG is a better name, actually.
*/
params->hdr.tag = ATAG_INITRD2;
params->hdr.size = tag_size (tag_initrd);
params->u.initrd.start = initrd_start;
params->u.initrd.size = initrd_end - initrd_start;
params = tag_next (params);
}
#endif /* CONFIG_INITRD_TAG */
#if defined (CONFIG_VFD) || defined (CONFIG_LCD)
extern ulong calc_fbsize (void);
static void setup_videolfb_tag (gd_t *gd)
{
/* An ATAG_VIDEOLFB node tells the kernel where and how large
* the framebuffer for video was allocated (among other things).
* Note that a _physical_ address is passed !
*
* We only use it to pass the address and size, the other entries
* in the tag_videolfb are not of interest.
*/
params->hdr.tag = ATAG_VIDEOLFB;
params->hdr.size = tag_size (tag_videolfb);
params->u.videolfb.lfb_base = (u32) gd->fb_base;
/* Fb size is calculated according to parameters for our panel
*/
params->u.videolfb.lfb_size = calc_fbsize();
params = tag_next (params);
}
#endif /* CONFIG_VFD || CONFIG_LCD */
#ifdef CONFIG_SERIAL_TAG
void setup_serial_tag (struct tag **tmp)
{
struct tag *params = *tmp;
struct tag_serialnr serialnr;
void get_board_serial(struct tag_serialnr *serialnr);
get_board_serial(&serialnr);
params->hdr.tag = ATAG_SERIAL;
params->hdr.size = tag_size (tag_serialnr);
params->u.serialnr.low = serialnr.low;
params->u.serialnr.high= serialnr.high;
params = tag_next (params);
*tmp = params;
}
#endif
#ifdef CONFIG_REVISION_TAG
void setup_revision_tag(struct tag **in_params)
{
u32 rev = 0;
u32 get_board_rev(void);
rev = get_board_rev();
params->hdr.tag = ATAG_REVISION;
params->hdr.size = tag_size (tag_revision);
params->u.revision.rev = rev;
params = tag_next (params);
}
#endif /* CONFIG_REVISION_TAG */
static void setup_end_tag (bd_t *bd)
{
params->hdr.tag = ATAG_NONE;
params->hdr.size = 0;
}
#endif /* CONFIG_SETUP_MEMORY_TAGS || CONFIG_CMDLINE_TAG || CONFIG_INITRD_TAG */
2012-12-22 22:19:53 添加
内核在启动的时候会从内存中取得标记列表,使用setup_arch
asmlinkage void __init start_kernel(void)
{
char * command_line;
extern const struct kernel_param __start___param[], __stop___param[];
smp_setup_processor_id();
/*
* Need to run as early as possible, to initialize the
* lockdep hash:
*/
lockdep_init();
debug_objects_early_init();
/*
* Set up the the initial canary ASAP:
*/
boot_init_stack_canary();
cgroup_init_early();
local_irq_disable();
early_boot_irqs_disabled = true;
/*
* Interrupts are still disabled. Do necessary setups, then
* enable them
*/
tick_init();
boot_cpu_init();
page_address_init();
printk(KERN_NOTICE "%s", linux_banner);
setup_arch(&command_line);
...
}
setup_arch---------->squash_mem_tags(tags);
static void __init squash_mem_tags(struct tag *tag)
{
for (; tag->hdr.size; tag = tag_next(tag))
if (tag->hdr.tag == ATAG_MEM)
tag->hdr.tag = ATAG_NONE;
}
command_line就是这个时候建立的。内核中也有一个default_command_line，是在内核 Boot options ---> 中定义的。如果在uboot没有传递command_line tag,内核就使用default_command_line。从squash_mem_tags函数可以看出来(uboot没传递的话，在tag list里面就找不到tag_cmdline)