项目经验分享：基于RT-Thread的可引导Linux的bootloader

● 指令，ARM作为RISC精简指令集，很多功能都需要不止一个指令才能实现，相比较x86架构，x86架构指令显得更加清爽，可以通过寻址、比较指令例子进行分析：

int main(void) {
    unsigned long i = 0x7c00;
    if (*(unsigned long *)i > 10) {
        i = 0;
    } else {
        i = 255;
    }
    return i;
}

main:
    cmpq    $11, 31744
    sbbq    %rax, %rax
    movzbl  %al, %eax
    ret

main:
    mov     r0, #31744
    ldr     r1, [r0]
    mov     r0, #255
    cmp     r1, #10
    movhi   r0, #0
    bx      lr

main:
    mov     x1, 31744
    mov     x0, 255
    ldr     x1, [x1]
    cmp     x1, 10
    csel    x0, x0, xzr, ls
    ret

● 寄存器，本次项目比较重要的地方，启动Linux必然涉及部分系统寄存器，ARM的通用寄存器也可能是其他特殊寄存器，具体就是编程时操作寄存器的名称不同而不同；虽然在组成上ARM和AArch64相似，但是功能基本都不一样了：

● 模式，Linux作为主机系统肯定在特权等级上启动的，ARM和AArch64在模式上区分明显：

● ARM架构下对内存对齐问题非常敏感，这也是值得注意的地方。

● 制定方案

● linux命令实现

struct arm64_kernel_image_header
{
    uint32_t code0;          /* EFI Executable code */
    uint32_t code1;          /* EFI Executable code */
    uint64_t text_offset;    /* Image load offset, little endian */
    uint64_t image_size;     /* Effective Image size, little endian */
    uint64_t flags;          /* kernel flags, little endian */
    uint64_t res2;           /* reserved */
    uint64_t res3;           /* reserved */
    uint64_t res4;           /* reserved */
    uint32_t magic;          /* Magic number, little endian, "ARM\x64" */
    uint32_t res5;           /* reserved (used for PE COFF offset) */
};

● dtb命令实现

// 设备树的版本信息
/dts-v1/;
// 如果有公共部分，#include导入即可
// #include "raspi4b.dtsi"


// 保留的内存，如果不想给内核使用的内存可以在这设置（地址，长度）
/memreserve/    0x0000000000000000 0x0000000000001000;
// 设备树的根节点，设备树的描述必须从这里开始
/ {
    memreserve = <0x3b400000 0x4c00000>;
serial-number = "1000000030297805";
// 如果板子名称相同，则用compatible 区分
compatible = "raspberrypi,4-model-b", "brcm,bcm2711";
// 该板子的名称
model = "Raspberry Pi 4 Model B Rev 1.2";
// 表示一个address用几个u32表示
#address-cells = <0x2>;
// 表示一个size用几个u32表示
#size-cells = <0x1>;
// 中断父设备标号
    interrupt-parent = <0x1>;


     // 节点名称
    soc {
        compatible = "simple-bus";
        #address-cells = <0x1>;
        #size-cells = <0x1>;
        ranges = <0x7e000000 0x0 0xfe000000 0x1800000 0x7c000000 0x0 0xfc000000 0x2000000 0x40000000 0x0 0xff800000 0x800000>;
        dma-ranges = <0xc0000000 0x0 0x0 0x40000000>;
         // 类似设备的唯一标识
        phandle = <0x41>;
         // 设备名称
        timer@7e003000 {
            compatible = "brcm,bcm2835-system-timer";
             // 设备所映射的地址，在soc节点中address-cells为1，size-cells为1，因此这里的0x7e003000 为地址，0x1000为大小
            reg = <0x7e003000 0x1000>;
             // 中断信息，因为ARMv8有多种定时器，所以此处中断是4个（中断域 中断 触发方式）
            interrupts = <0x0 0x40 0x4 0x0 0x41 0x4 0x0 0x42 0x4 0x0 0x43 0x4>;
            clock-frequency = <0xf4240>;
            phandle = <0x42>;
        };
    };


    cpus {
        #address-cells = <0x1>;
        #size-cells = <0x0>;
// 设备的启动方式
        enable-method = "brcm,bcm2836-smp";
        phandle = <0xcf>;


        cpu@0 {
             // 设备的类型
            device_type = "cpu";
            compatible = "arm,cortex-a72";
            reg = <0x0>;
            enable-method = "spin-table";
            cpu-release-addr = <0x0 0xd8>;
            phandle = <0x26>;
        };
    };
};

struct fdt_header {
    fdt32_t magic;              /* magic word FDT_MAGIC */
    fdt32_t totalsize;          /* total size of DT block */
    fdt32_t off_dt_struct;      /* offset to structure */
    fdt32_t off_dt_strings;     /* offset to strings */
    fdt32_t off_mem_rsvmap;     /* offset to memory reserve map */
    fdt32_t version;            /* format version */
    fdt32_t last_comp_version;  /* last compatible version */


    /* version 2 fields below */
    fdt32_t boot_cpuid_phys;    /* Which physical CPU id we're booting on */
    /* version 3 fields below */
    fdt32_t size_dt_strings /* size of the strings block */
    /* version 17 fields below */
    fdt32_t size_dt_struct;     /* size of the structure block */
};

struct atag_header {
    u32 size; /* legth of tag in words including this header */
    u32 tag;  /* tag value */
};


/*
 * Tag name       Value       Size Description
 * ATAG_NONE      0x00000000  2    Empty tag used to end list
 * ATAG_CORE      0x54410001  5    (2 if empty) First tag used to start list
 * ATAG_MEM       0x54410002  4    Describes a physical area of memory
 * ATAG_VIDEOTEXT 0x54410003  5    Describes a VGA text display
 * ATAG_RAMDISK   0x54410004  5    Describes how the ramdisk will be used in kernel
 * ATAG_INITRD2   0x54420005  4    Describes where the compressed ramdisk image is placed in memory
 * ATAG_SERIAL    0x54410006  4    64 bit board serial number
 * ATAG_REVISION  0x54410007  3    32 bit board revision number
 * ATAG_VIDEOLFB  0x54410008  8    Initial values for vesafb-type framebuffers
 * ATAG_CMDLINE   0x54410009  2 + ((cmdline_len + 3) / 4)  Command line to pass to kernel
 */


struct atag {
    struct atag_header hdr;
    union {
        struct atag_core         core;  /* 必须 */
        struct atag_mem          mem;  /* 必须 */
        struct atag_videotext    videotext;
        struct atag_ramdisk      ramdisk;
        struct atag_initrd2      initrd2;
        struct atag_serialnr     serialnr;  /* 必须 */
        struct atag_revision     revision;
        struct atag_videolfb     videolfb;
        struct atag_cmdline      cmdline;
    } u;
};


#define tag_next(t)     ((struct tag *)((u32 *)(t) + (t)->hdr.size))
#define tag_size(type)  ((sizeof(struct tag_header) + sizeof(struct type)) >> 2)


static void setup_mem_tag(struct atag *params, u32_t start, u32_t len)
{
    params->hdr.tag = ATAG_MEM;             /* 设置标签类型 */
    params->hdr.size = tag_size(atag_mem);  /* 标签大小 */


    /* 该标签的属性 */
    params->u.mem.start = start;            
    params->u.mem.size = len;


    params = tag_next(params);              /* 指向下一个标签 */
}

● initrd命令实现

● boot命令实现

● http、tftp、ymodem文件下载命令实现

● 调试手段及常见问题