uboot命令之bootm详解

开发板:DM3730 cortex-a8
虚拟机:ubuntu 14.04
编译器:gcc-linaro-5.3-2016.02-x86_64_arm-linux-gnueabihf
开发板内核:linux 4.4.12

bootm 用于将内核镜像加载到内存的指定地址处，如果有需要还要解压镜像，然后根据操作系统和体系结构的不同给内核传递不同的启动参数，最后启动内核。
一、arm 架构处理器对 linux 内核启动之前环境的五点需求

1、cpu 寄存器设置

* R0 = 0* R1 = 板级 id* R2 = 启动参数在内存中的起始地址

2、cpu 模式

* 禁止所有中断* 必须为SVC（超级用户）模式

3、缓存、MMU

* 关闭 MMU* 指令缓存可以开启或者关闭* 数据缓存必须关闭并且不能包含任何脏数据

4、设备

* DMA 设备应当停止工作

5、boot loader 需要跳转到内核镜像的第一条指令处
这些需求都由 boot loader 实现，在常用的 uboot 中完成一系列的初始化后最后通过 bootm 命令加载 linux 内核。该命令用法介绍如下：

int do_bootm (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
{ulong       iflag;ulong       load_end = 0;int     ret;boot_os_fn  *boot_fn;
#ifndef CONFIG_RELOC_FIXUP_WORKSstatic int relocated = 0;/* 重定位启动函数表 *//* relocate boot function table */if (!relocated) {int i;for (i = 0; i < ARRAY_SIZE(boot_os); i++)if (boot_os[i] != NULL)boot_os[i] += gd->reloc_off;relocated = 1;}
#endif//判断是否有子命令/* determine if we have a sub command */if (argc > 1) {char *endp;simple_strtoul(argv[1], &endp, 16);/* endp pointing to NULL means that argv[1] was just a* valid number, pass it along to the normal bootm processing** If endp is ':' or '#' assume a FIT identifier so pass* along for normal processing.** Right now we assume the first arg should never be '-'*/if ((*endp != 0) && (*endp != ':') && (*endp != '#'))return do_bootm_subcommand(cmdtp, flag, argc, argv);}/* 获取内核相关信息 */if (bootm_start(cmdtp, flag, argc, argv))return 1;/** We have reached the point of no return: we are going to* overwrite all exception vector code, so we cannot easily* recover from any failures any more...*//* 关闭中断 */iflag = disable_interrupts();#if defined(CONFIG_CMD_USB)/** turn off USB to prevent the host controller from writing to the* SDRAM while Linux is booting. This could happen (at least for OHCI* controller), because the HCCA (Host Controller Communication Area)* lies within the SDRAM and the host controller writes continously to* this area (as busmaster!). The HccaFrameNumber is for example* updated every 1 ms within the HCCA structure in SDRAM! For more* details see the OpenHCI specification.*//* 关闭USB */usb_stop();
#endif#ifdef CONFIG_AMIGAONEG3SE/** We've possible left the caches enabled during* bios emulation, so turn them off again*//* 关闭指令cache和数据cache */icache_disable();dcache_disable();
#endif/* 加载内核 */ret = bootm_load_os(images.os, &load_end, 1);if (ret < 0) {if (ret == BOOTM_ERR_RESET)do_reset (cmdtp, flag, argc, argv);if (ret == BOOTM_ERR_OVERLAP) {if (images.legacy_hdr_valid) {if (image_get_type (&images.legacy_hdr_os_copy) == IH_TYPE_MULTI)puts ("WARNING: legacy format multi component ""image overwritten\n");} else {puts ("ERROR: new format image overwritten - ""must RESET the board to recover\n");show_boot_progress (-113);do_reset (cmdtp, flag, argc, argv);}}if (ret == BOOTM_ERR_UNIMPLEMENTED) {if (iflag)enable_interrupts();show_boot_progress (-7);return 1;}}lmb_reserve(&images.lmb, images.os.load, (load_end - images.os.load));if (images.os.type == IH_TYPE_STANDALONE) {if (iflag)enable_interrupts();/* This may return when 'autostart' is 'no' */bootm_start_standalone(iflag, argc, argv);return 0;}show_boot_progress (8);#ifdef CONFIG_SILENT_CONSOLEif (images.os.os == IH_OS_LINUX)fixup_silent_linux();
#endif//获取内核启动参数boot_fn = boot_os[images.os.os];if (boot_fn == NULL) {if (iflag)enable_interrupts();printf ("ERROR: booting os '%s' (%d) is not supported\n",genimg_get_os_name(images.os.os), images.os.os);show_boot_progress (-8);return 1;}//内核启动前的准备arch_preboot_os();/* 启动内核，不返回 */boot_fn(0, argc, argv, &images);show_boot_progress (-9);
#ifdef DEBUGputs ("\n## Control returned to monitor - resetting...\n");
#endifdo_reset (cmdtp, flag, argc, argv);return 1;
}

该函数主要的工作流程是，通过bootm_start来获取内核镜像文件的信息，然后通过bootm_load_os函数来加载内核，最后通过boot_fn来启动内核。

首先看一下bootm_start，该函数主要进行镜像的有效性判定、校验、计算入口地址等操作，大部分工作通过 boot_get_kernel -> image_get_kernel 完成。

static int bootm_start(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
{void        *os_hdr;int        ret;memset ((void *)&images, 0, sizeof (images));//读取环境变量，从环境变量中检查是否要对镜像的数据（不是镜像头）进行校验images.verify = getenv_yesno ("verify");//不做任何有意义的工作，除了定义# define lmb_reserve(lmb, base, size)  bootm_start_lmb();//获取镜像头，加载地址，长度，返回指向内存中镜像头的指针/* get kernel image header, start address and length */os_hdr = boot_get_kernel (cmdtp, flag, argc, argv,&images, &images.os.image_start, &images.os.image_len);if (images.os.image_len == 0) {puts ("ERROR: can't get kernel image!\n");return 1;}//根据镜像魔数获取镜像类型  /* get image parameters */switch (genimg_get_format (os_hdr)) {case IMAGE_FORMAT_LEGACY:images.os.type = image_get_type (os_hdr);//镜像类型  images.os.comp = image_get_comp (os_hdr);//压缩类型  images.os.os = image_get_os (os_hdr);//操作系统类型 images.os.end = image_get_image_end (os_hdr);//当前镜像的尾地址images.os.load = image_get_load (os_hdr);//镜像数据的载入地址 break;
#if defined(CONFIG_FIT)case IMAGE_FORMAT_FIT:if (fit_image_get_type (images.fit_hdr_os,images.fit_noffset_os, &images.os.type)) {puts ("Can't get image type!\n");show_boot_progress (-109);return 1;}if (fit_image_get_comp (images.fit_hdr_os,images.fit_noffset_os, &images.os.comp)) {puts ("Can't get image compression!\n");show_boot_progress (-110);return 1;}if (fit_image_get_os (images.fit_hdr_os,images.fit_noffset_os, &images.os.os)) {puts ("Can't get image OS!\n");show_boot_progress (-111);return 1;}images.os.end = fit_get_end (images.fit_hdr_os);if (fit_image_get_load (images.fit_hdr_os, images.fit_noffset_os,&images.os.load)) {puts ("Can't get image load address!\n");show_boot_progress (-112);return 1;}break;
#endifdefault:puts ("ERROR: unknown image format type!\n");return 1;}//获取内核入口地址/* find kernel entry point */if (images.legacy_hdr_valid) {images.ep = image_get_ep (&images.legacy_hdr_os_copy);
#if defined(CONFIG_FIT)} else if (images.fit_uname_os) {ret = fit_image_get_entry (images.fit_hdr_os,images.fit_noffset_os, &images.ep);if (ret) {puts ("Can't get entry point property!\n");return 1;}
#endif} else {puts ("Could not find kernel entry point!\n");return 1;}if (((images.os.type == IH_TYPE_KERNEL) ||(images.os.type == IH_TYPE_MULTI)) &&(images.os.os == IH_OS_LINUX)) {//获取虚拟磁盘/* find ramdisk */ret = boot_get_ramdisk (argc, argv, &images, IH_INITRD_ARCH,&images.rd_start, &images.rd_end);if (ret) {puts ("Ramdisk image is corrupt or invalid\n");return 1;}#if defined(CONFIG_OF_LIBFDT)//获取设备树，设备树是linux 3.XX版本特有的/* find flattened device tree */ret = boot_get_fdt (flag, argc, argv, &images,&images.ft_addr, &images.ft_len);if (ret) {puts ("Could not find a valid device tree\n");return 1;}set_working_fdt_addr(images.ft_addr);
#endif}//将内核加载地址赋值给images.os.startimages.os.start = (ulong)os_hdr;//更新镜像状态images.state = BOOTM_STATE_START;return 0;
}

接着看一下bootm_load_os函数，它的主要工作是解压内核镜像文件，并且将它移动到内核加载地址。

首先看一下两个重要的结构体

//include/image.h
typedef struct image_header {uint32_t        ih_magic;       /* Image Header Magic Number    */uint32_t        ih_hcrc;        /* Image Header CRC Checksum    */uint32_t        ih_time;        /* Image Creation Timestamp     */uint32_t        ih_size;        /* Image Data Size              */uint32_t        ih_load;        /* Data  Load  Address          */uint32_t        ih_ep;          /* Entry Point Address          */uint32_t        ih_dcrc;        /* Image Data CRC Checksum      */uint8_t         ih_os;          /* Operating System             */uint8_t         ih_arch;        /* CPU architecture             */uint8_t         ih_type;        /* Image Type                   */uint8_t         ih_comp;        /* Compression Type             */uint8_t         ih_name[IH_NMLEN];      /* Image Name           */
} image_header_t;
typedef struct image_info {ulong           start, end;             /* start/end of blob */ulong           image_start, image_len; /* start of image within blob, len of image */ulong           load;                   /* load addr for the image */uint8_t         comp, type, os;         /* compression, type of image, os type */
} image_info_t;

static int bootm_start(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
{void        *os_hdr;int        ret;memset ((void *)&images, 0, sizeof (images));//读取环境变量，从环境变量中检查是否要对镜像的数据（不是镜像头）进行校验images.verify = getenv_yesno ("verify");//不做任何有意义的工作，除了定义# define lmb_reserve(lmb, base, size)  bootm_start_lmb();//获取镜像头，加载地址，长度，返回指向内存中镜像头的指针/* get kernel image header, start address and length */os_hdr = boot_get_kernel (cmdtp, flag, argc, argv,&images, &images.os.image_start, &images.os.image_len);if (images.os.image_len == 0) {puts ("ERROR: can't get kernel image!\n");return 1;}//根据镜像魔数获取镜像类型  /* get image parameters */switch (genimg_get_format (os_hdr)) {case IMAGE_FORMAT_LEGACY:images.os.type = image_get_type (os_hdr);//镜像类型  images.os.comp = image_get_comp (os_hdr);//压缩类型  images.os.os = image_get_os (os_hdr);//操作系统类型 images.os.end = image_get_image_end (os_hdr);//当前镜像的尾地址images.os.load = image_get_load (os_hdr);//镜像数据的载入地址 break;
#if defined(CONFIG_FIT)case IMAGE_FORMAT_FIT:if (fit_image_get_type (images.fit_hdr_os,images.fit_noffset_os, &images.os.type)) {puts ("Can't get image type!\n");show_boot_progress (-109);return 1;}if (fit_image_get_comp (images.fit_hdr_os,images.fit_noffset_os, &images.os.comp)) {puts ("Can't get image compression!\n");show_boot_progress (-110);return 1;}if (fit_image_get_os (images.fit_hdr_os,images.fit_noffset_os, &images.os.os)) {puts ("Can't get image OS!\n");show_boot_progress (-111);return 1;}images.os.end = fit_get_end (images.fit_hdr_os);if (fit_image_get_load (images.fit_hdr_os, images.fit_noffset_os,&images.os.load)) {puts ("Can't get image load address!\n");show_boot_progress (-112);return 1;}break;
#endifdefault:puts ("ERROR: unknown image format type!\n");return 1;}//获取内核入口地址/* find kernel entry point */if (images.legacy_hdr_valid) {images.ep = image_get_ep (&images.legacy_hdr_os_copy);
#if defined(CONFIG_FIT)} else if (images.fit_uname_os) {ret = fit_image_get_entry (images.fit_hdr_os,images.fit_noffset_os, &images.ep);if (ret) {puts ("Can't get entry point property!\n");return 1;}
#endif} else {puts ("Could not find kernel entry point!\n");return 1;}if (((images.os.type == IH_TYPE_KERNEL) ||(images.os.type == IH_TYPE_MULTI)) &&(images.os.os == IH_OS_LINUX)) {//获取虚拟磁盘/* find ramdisk */ret = boot_get_ramdisk (argc, argv, &images, IH_INITRD_ARCH,&images.rd_start, &images.rd_end);if (ret) {puts ("Ramdisk image is corrupt or invalid\n");return 1;}#if defined(CONFIG_OF_LIBFDT)//获取设备树，设备树是linux 3.XX版本特有的/* find flattened device tree */ret = boot_get_fdt (flag, argc, argv, &images,&images.ft_addr, &images.ft_len);if (ret) {puts ("Could not find a valid device tree\n");return 1;}set_working_fdt_addr(images.ft_addr);
#endif}//将内核加载地址赋值给images.os.startimages.os.start = (ulong)os_hdr;//更新镜像状态images.state = BOOTM_STATE_START;return 0;
}
#define BOOTM_ERR_RESET        -1
#define BOOTM_ERR_OVERLAP    -2
#define BOOTM_ERR_UNIMPLEMENTED    -3
static int bootm_load_os(image_info_t os, ulong *load_end, int boot_progress)
{uint8_t comp = os.comp;//压缩格式ulong load = os.load;//加载地址ulong blob_start = os.start;//系统起始地址ulong blob_end = os.end;//系统结束地址ulong image_start = os.image_start;//镜像起始地址ulong image_len = os.image_len;//镜像大小uint unc_len = CONFIG_SYS_BOOTM_LEN;//镜像最大长度
#if defined(CONFIG_LZMA) || defined(CONFIG_LZO)int ret;
#endif /* defined(CONFIG_LZMA) || defined(CONFIG_LZO) *///获取镜像类型const char *type_name = genimg_get_type_name (os.type);switch (comp) {case IH_COMP_NONE://镜像没有压缩过if (load == blob_start) {//判断是否需要移动镜像printf ("   XIP %s ... ", type_name);} else {printf ("   Loading %s ... ", type_name);memmove_wd ((void *)load, (void *)image_start,image_len, CHUNKSZ);}*load_end = load + image_len;puts("OK\n");break;
#ifdef CONFIG_GZIPcase IH_COMP_GZIP://镜像使用gzip压缩printf ("   Uncompressing %s ... ", type_name);//解压镜像文件if (gunzip ((void *)load, unc_len,(uchar *)image_start, &image_len) != 0) {puts ("GUNZIP: uncompress, out-of-mem or overwrite error ""- must RESET board to recover\n");if (boot_progress)show_boot_progress (-6);return BOOTM_ERR_RESET;}*load_end = load + image_len;break;
#endif /* CONFIG_GZIP */
......return 0;
}

最后看一下boot_fn函数，boot_fn的定义为

boot_os_fn *boot_fn;

可以看出它是一个boot_os_fn类型的函数指针。它的定义为

//  common/cmd_bootm.c
typedef int boot_os_fn (int flag, int argc, char * const argv[],bootm_headers_t *images); /* pointers to os/initrd/fdt */
#ifdef CONFIG_BOOTM_LINUX
extern boot_os_fn do_bootm_linux;
#endif
......

然后boot_fn在do_bootm函数中被赋值为

boot_fn = boot_os[images.os.os];

boot_os是一个函数指针数组

//  common/cmd_bootm.c
static boot_os_fn *boot_os[] = {
#ifdef CONFIG_BOOTM_LINUX[IH_OS_LINUX] = do_bootm_linux,
#endif
#ifdef CONFIG_BOOTM_NETBSD[IH_OS_NETBSD] = do_bootm_netbsd,
#endif
#ifdef CONFIG_LYNXKDI[IH_OS_LYNXOS] = do_bootm_lynxkdi,
#endif
#ifdef CONFIG_BOOTM_RTEMS[IH_OS_RTEMS] = do_bootm_rtems,
#endif
#if defined(CONFIG_BOOTM_OSE)[IH_OS_OSE] = do_bootm_ose,
#endif
#if defined(CONFIG_CMD_ELF)[IH_OS_VXWORKS] = do_bootm_vxworks,[IH_OS_QNX] = do_bootm_qnxelf,
#endif
#ifdef CONFIG_INTEGRITY[IH_OS_INTEGRITY] = do_bootm_integrity,
#endif
};

可以看出 boot_fn 函数指针最后指向的函数是位于 arch/arm/lib/bootm.c的 do_bootm_linux，这是内核启动前最后的一个函数，该函数主要完成启动参数的初始化，并将板子设定为满足内核启动的环境。

int do_bootm_linux(int flag, int argc, char *argv[], bootm_headers_t *images)
{//从全局变量结构体中获取串口参数bd_t    *bd = gd->bd;char    *s;//获取机器码int    machid = bd->bi_arch_number;//内核入口函数void    (*kernel_entry)(int zero, int arch, uint params);int    ret;//获取启动参数
#ifdef CONFIG_CMDLINE_TAGchar *commandline = getenv ("bootargs");
#endifif ((flag != 0) && (flag != BOOTM_STATE_OS_GO))return 1;//从环境变量中获取机器码s = getenv ("machid");if (s) {machid = simple_strtoul (s, NULL, 16);printf ("Using machid 0x%x from environment\n", machid);}//获取ramdiskret = boot_get_ramdisk(argc, argv, images, IH_ARCH_ARM, &(images->rd_start), &(images->rd_end));if(ret)printf("[err] boot_get_ramdisk\n");show_boot_progress (15);
#ifdef CONFIG_OF_LIBFDTif (images->ft_len)return bootm_linux_fdt(machid, images);
#endifkernel_entry = (void (*)(int, int, uint))images->ep;debug ("## Transferring control to Linux (at address %08lx) ...\n",(ulong) kernel_entry);
#if defined (CONFIG_SETUP_MEMORY_TAGS) || \defined (CONFIG_CMDLINE_TAG) || \defined (CONFIG_INITRD_TAG) || \defined (CONFIG_SERIAL_TAG) || \defined (CONFIG_REVISION_TAG)setup_start_tag (bd);
#ifdef CONFIG_SERIAL_TAGsetup_serial_tag (params);
#endif
#ifdef CONFIG_REVISION_TAGsetup_revision_tag (params);
#endif
#ifdef CONFIG_SETUP_MEMORY_TAGSsetup_memory_tags (bd);
#endif
#ifdef CONFIG_CMDLINE_TAGsetup_commandline_tag (bd, commandline);
#endif
#ifdef CONFIG_INITRD_TAGif (images->rd_start && images->rd_end)setup_initrd_tag (bd, images->rd_start, images->rd_end);
#endifsetup_end_tag(bd);
#endifannounce_and_cleanup();
#ifdef CONFIG_ENABLE_MMUtheLastJump((void *)virt_to_phys(kernel_entry), machid, bd->bi_boot_params);
#elsekernel_entry(0, machid, bd->bi_boot_params);/* does not return */
#endifreturn 1;
}

kernel_entry(0, machid, r2)

真正将控制权交给内核，启动内核；

满足arm架构linux内核启动时的寄存器设置条件:第一个参数为0 ；第二个参数为板子id需与内核中的id匹配，第三个参数为启动参数地址bi_boot_params 。

（1）首先取出环境变量bootargs，这就是要传递给内核的参数。

（2）调用setup_XXX_tag

kernel_entry(0, machid, r2) 真正将控制权交给内核， 启动内核；满足arm架构linux内核启动时的寄存器设置条件:第一个参数为0 ；第二个参数为板子id需与内核中的id匹配，第三个参数为启动参数地址bi_boot_params 。（1）首先取出环境变量bootargs，这就是要传递给内核的参数。（2）调用setup_XXX_tag

params是一个用来存储要传给kernel的参数的静态全局变量。

u-boot 是通过标记列表向内核传递参数，标记在源代码中定义为tag,是一个结构体，在 arch/arm/include/asm/setup.h 中定义。

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;

tag包括hdr和各种类型的tag_*,hdr来标志当前的tag是哪种类型的tag。setup_start_tag是初始化了第一个tag，是tag_core类型的tag。最后调用tag_next跳到第一个tag末尾，为下一个tag做准备。

tag_next是一个宏定义，被定义在arch/arm/include/asm/setup.h中

#define tag_next(t)     ((struct tag *)((u32 *)(t) + (t)->hdr.size))

struct tag_header {u32 size;u32 tag;
};

最后调用setup_end_tag，将末尾的tag设置为ATAG_NONE，标志tag列表结束。

static void setup_end_tag (bd_t *bd)
{params->hdr.tag = ATAG_NONE;params->hdr.size = 0;
}

u-boot将参数以tag数组的形式布局在内存的某一个地址，每个tag代表一种类型的参数，首尾tag标志开始和结束，首地址传给kernel供其解析

通过上面的分析，我们可以尝试自己写一个bootm来引导内核

//atag.h
#define ATAG_CORE    0x54410001
#define ATAG_MEM    0x54410002
#define ATAG_CMDLINE    0x54410009
#define ATAG_NONE    0x00000000
struct tag_header {unsigned int size;unsigned int tag;
};
struct tag_core {unsigned int flags;        unsigned int pagesize;unsigned int rootdev;
};
struct tag_mem32 {unsigned int    size;unsigned int    start;
};
struct tag_cmdline {char    cmdline[1];
};
struct tag {struct tag_header hdr;union {struct tag_core        core;struct tag_mem32    mem;struct tag_cmdline    cmdline;} u;
};
#define tag_size(type)    ((sizeof(struct tag_header) + sizeof(struct type)) >> 2)
#define tag_next(t)    ((struct tag *)((unsigned int *)(t) + (t)->hdr.size))

//boot.c
#include "atag.h"
#include "string.h"
void (*theKernel)(int , int , unsigned int );
#define SDRAM_KERNEL_START 0x51000000
#define SDRAM_TAGS_START   0x50000100
#define SDRAM_ADDR_START   0x50000000
#define SDRAM_TOTAL_SIZE   0x16000000
struct tag *pCurTag;
const char *cmdline = "console=ttySAC0,115200 init=/init";
void setup_core_tag()
{pCurTag = (struct tag *)SDRAM_TAGS_START;pCurTag->hdr.tag = ATAG_CORE;pCurTag->hdr.size = tag_size(tag_core); pCurTag->u.core.flags = 0;pCurTag->u.core.pagesize = 4096;pCurTag->u.core.rootdev = 0;pCurTag = tag_next(pCurTag);
}
void setup_mem_tag()
{pCurTag->hdr.tag = ATAG_MEM;pCurTag->hdr.size = tag_size(tag_mem32); pCurTag->u.mem.start = SDRAM_ADDR_START;pCurTag->u.mem.size = SDRAM_TOTAL_SIZE;pCurTag = tag_next(pCurTag);
}
void setup_cmdline_tag()
{int linelen = strlen(cmdline);pCurTag->hdr.tag = ATAG_CMDLINE;pCurTag->hdr.size = (sizeof(struct tag_header)+linelen+1+4)>>2;strcpy(pCurTag->u.cmdline.cmdline,cmdline);pCurTag = tag_next(pCurTag);
}
void setup_end_tag()
{pCurTag->hdr.tag = ATAG_NONE;pCurTag->hdr.size = 0;
}
void boot_linux(){//1.获取Linux启动地址theKernel = (void (*)(int , int , unsigned int ))SDRAM_KERNEL_START;printf("huo qu linux qi dong di zhi");//2.设置启动参数//2.1.设置核心启动参数setup_core_tag();//2.2.设置内存参数setup_mem_tag();//2.3.设置命令行参数setup_cmdline_tag();//2.4.设置结束标志setup_end_tag();//4.启动Linux内核theKernel(0,1626,SDRAM_TAGS_START);printf("qi dong linux nei he");}

转自http://www.cnblogs.com/CoderTian/p/6006400.html

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