U-Boot启动引导内核分析(二)

最新推荐文章于 2025-07-02 11:16:53 发布

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文章标签：

#image #os #linux #代码分析 #parameters #cache

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本文详细解析了U-Boot中bootm命令的工作原理及其如何引导内核启动的过程。通过分析do_bootm函数，展示了从获取内核镜像到加载及验证的整个流程。

标签：ARM bootloader U-Boot 代码分析抵岸科技

也许细心的你会问：我在用UBoot的时候并没有直接进入用户命令界面呀，而是在倒计时结束后自动引导kernel。这是怎么回事呢？

在 main_loop（）函数当中有如下一段代码：

#if defined(CONFIG_BOOTDELAY) && (CONFIG_BOOTDELAY >= 0)
      •
      •
      •
s = getenv ("bootcmd");
        /*获取bootcmd 的内容*/
        /*bootcmd=nand read 0x22000000 0xB0000 0x200000; bootm */
      •
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# ifndef CFG_HUSH_PARSER
        run_command (s, 0);
        /*运行s包含的命令*/
        /*运行nand read 0x22000000 0xB0000 0x200000表示将NANDFLASH
      0xB0000处数据读取放于0x22000000处，读取长度为0x200000
        */
        /*运行bootm命令，引导内核启动*/
# else
        parse_string_outer(s, FLAG_PARSE_SEMICOLON |
                    FLAG_EXIT_FROM_LOOP);
# endif
      •
      •
      •
#endif    /* CONFIG_BOOTDELAY */

bootm命令是什么？它是怎样引导内核的？

要知道想解决这个问题，就要分析common/cmd_bootm.c中的函数do_bootm，因为引导kernel就是bootm这条命令的工作，do_bootm是命令bootm的执行函数。

现在我们来分析一下common/cmd_bootm.c中的函数do_bootm，这是bootm命令的处理函数。

int do_bootm (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
{
    ulong        iflag;
    const char    *type_name;
    uint        unc_len = CFG_BOOTM_LEN;
    uint8_t        comp, type, os;

    void        *os_hdr;
    ulong        os_data, os_len;
    ulong        image_start, image_end;
    ulong        load_start, load_end;
    ulong        mem_start;
    phys_size_t    mem_size;

    struct lmb lmb;

    memset ((void *)&images, 0, sizeof (images));
    images.verify = getenv_yesno ("verify");
    images.lmb = &lmb;

    lmb_init(&lmb);

    mem_start = getenv_bootm_low();
    mem_size = getenv_bootm_size();

    lmb_add(&lmb, (phys_addr_t)mem_start, mem_size);

    board_lmb_reserve(&lmb);

    /* get kernel image header, start address and length */
    /* 获取内核镜像头信息 */
    /* 打印 “## Booting kernel from Legacy Image at 22000000 ...
           Image Name:   Linux-2.6.30
           Image Type:   ARM Linux Kernel Image (uncompressed)
           Data Size:    1507760 Bytes = 1.4 MB
           Load Address: 20008000
           Entry Point: 20008000
           Verifying Checksum ... OK”*/
    os_hdr = boot_get_kernel (cmdtp, flag, argc, argv,
            &images, &os_data, &os_len);
    if (os_len == 0) {
        puts ("ERROR: can't get kernel image!/n");
        return 1;
    }

    /* get image parameters */
    /* 获取内核镜像格式 */
    switch (genimg_get_format (os_hdr)) {
    case IMAGE_FORMAT_LEGACY:
        /* 获取内核镜像参数 */
        type = image_get_type (os_hdr);
        comp = image_get_comp (os_hdr);
        os = image_get_os (os_hdr);

        image_end = image_get_image_end (os_hdr);
        load_start = image_get_load (os_hdr);
        break;
    }

    image_start = (ulong)os_hdr;
    load_end = 0;
    type_name = genimg_get_type_name (type);

    /* 禁止所有中断 */
    iflag = disable_interrupts();

#ifdef CONFIG_AMIGAONEG3SE
    /*
     * We've possible left the caches enabled during
     * bios emulation, so turn them off again
     */
    icache_disable();
    invalidate_l1_instruction_cache();
    flush_data_cache();
    dcache_disable();
#endif

    switch (comp) {
    case IH_COMP_NONE:
        /* 加载内核镜像 */
        /* 打印“Loading Kernel Image ... OK” */
        if (load_start == (ulong)os_hdr) {
            printf ("   XIP %s ... ", type_name);
        } else {
            printf ("   Loading %s ... ", type_name);

            memmove_wd ((void *)load_start,
                   (void *)os_data, os_len, CHUNKSZ);
        }
        load_end = load_start + os_len;
        puts("OK/n");
        break;
    }
    puts ("OK/n");
    debug ("   kernel loaded at 0x%08lx, end = 0x%08lx/n", load_start, load_end);
    show_boot_progress (7);

    /* 加载错误 */
    if ((load_start < image_end) && (load_end > image_start)) {
        debug ("image_start = 0x%lX, image_end = 0x%lx/n", image_start, image_end);
        debug ("load_start = 0x%lx, load_end = 0x%lx/n", load_start, load_end);

        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);
        }
    }

    show_boot_progress (8);

    lmb_reserve(&lmb, load_start, (load_end - load_start));

    switch (os) {
    default:            /* handled by (original) Linux case */
    case IH_OS_LINUX:
#ifdef CONFIG_SILENT_CONSOLE
        fixup_silent_linux();
#endif
        /* 引导内核启动函数 */
        do_bootm_linux (cmdtp, flag, argc, argv, &images);
        break;
    }

    show_boot_progress (-9);
#ifdef DEBUG
    puts ("/n## Control returned to monitor - resetting.../n");
    do_reset (cmdtp, flag, argc, argv);
#endif
    if (iflag)
        enable_interrupts();

    return 1;
}