问题 ：利用内核自带的spi测试测试spi驱动的时候，mosi没有直接连接miso，miso怎么也能收到数据？

问题 ：利用内核自带的spi测试测试spi驱动的时候，mosi没有直接连接miso，miso怎么也能收到数据？


#include <stdint.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/types.h>
#include <linux/spi/spidev.h>


#include <unistd.h>

#define ARRAY_SIZE(a) (sizeof(a) / sizeof((a)[0]))

static void pabort(const char *s)
{
    perror(s);
    abort();
}

static const char *device = "/dev/spidev2.0";
static uint8_t mode;
static uint8_t bits = 8;
static uint32_t speed = 1000; //500000;
static uint16_t delay;

static void transfer(int fd)
{
    int ret;
    
    //要发送的数据数组


    uint8_t tx[] = {
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0x40, 0x00, 0x00, 0x00, 0x00, 0x95,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xDE, 0xAD, 0xBE, 0xEF, 0xBA, 0xAD,
        0xF0, 0x0D,
    };



    

//    uint8_t tx[] = {
//        0x55,0xaa,0xf5};
    
    uint8_t rx[ARRAY_SIZE(tx)] = {0, };    //接收的数据数据
    //声明并初始化spi_ioc_transfer结构体
    struct spi_ioc_transfer tr = {
        .tx_buf = (unsigned long)tx,
        .rx_buf = (unsigned long)rx,
        .len = ARRAY_SIZE(tx),
        .delay_usecs = delay,
        .speed_hz = speed,
        .bits_per_word = bits,
    };

    //SPI_IOC_MESSAGE(1)的1表示spi_ioc_transfer的数量
    ret = ioctl(fd, SPI_IOC_MESSAGE(1), &tr);        //ioctl默认操作,传输数据
    if (ret == 1)
        pabort("can't send spi message");

    for (ret = 0; ret < ARRAY_SIZE(tx); ret++) {    //打印接收缓冲区
        if (!(ret % 6))                                //6个数据为一簇打印
            puts("");
        printf("%.2X ", rx[ret]);
    }
    puts("");
}

static void print_usage(const char *prog)            //参数错误则打印帮助信息
{
    printf("Usage: %s [-DsbdlHOLC3]\n", prog);
    puts("  -D --device   device to use (default /dev/spidev1.1)\n"
         "  -s --speed    max speed (Hz)\n"
         "  -d --delay    delay (usec)\n"
         "  -b --bpw      bits per word \n"
         "  -l --loop     loopback\n"
         "  -H --cpha     clock phase\n"
         "  -O --cpol     clock polarity\n"
         "  -L --lsb      least significant bit first\n"
         "  -C --cs-high  chip select active high\n"
         "  -3 --3wire    SI/SO signals shared\n");
    exit(1);
}

static void parse_opts(int argc, char *argv[])
{
    while (1) {
        static const struct option lopts[] = {
            { "device",  1, 0, 'D' },
            { "speed",   1, 0, 's' },
            { "delay",   1, 0, 'd' },
            { "bpw",     1, 0, 'b' },
            { "loop",    0, 0, 'l' },
            { "cpha",    0, 0, 'H' },
            { "cpol",    0, 0, 'O' },
            { "lsb",     0, 0, 'L' },
            { "cs-high", 0, 0, 'C' },
            { "3wire",   0, 0, '3' },
            { "no-cs",   0, 0, 'N' },
            { "ready",   0, 0, 'R' },
            { NULL, 0, 0, 0 },
        };
        int c;

        c = getopt_long(argc, argv, "D:s:d:b:lHOLC3NR", lopts, NULL);

        if (c == -1)
            break;

        switch (c) {
        case 'D':
            device = optarg;
            break;
        case 's':
            speed = atoi(optarg);
            break;
        case 'd':
            delay = atoi(optarg);
            break;
        case 'b':
            bits = atoi(optarg);
            break;
        case 'l':
            mode |= SPI_LOOP;
            break;
        case 'H':
            mode |= SPI_CPHA;
            break;
        case 'O':
            mode |= SPI_CPOL;
            break;
        case 'L':
            mode |= SPI_LSB_FIRST;
            break;
        case 'C':
            mode |= SPI_CS_HIGH;
            break;
        case '3':
            mode |= SPI_3WIRE;
            break;
        case 'N':
            mode |= SPI_NO_CS;
            break;
        case 'R':
            mode |= SPI_READY;
            break;
        default:
            print_usage(argv[0]);
            break;
        }
    }
}

int main(int argc, char *argv[])
{
    int ret = 0;
    int fd;

     int count=0;
     char buf[]={0x11,0x22,0x33,0x44,0x55};

    parse_opts(argc, argv);

    fd = open(device, O_RDWR);
    if (fd < 0)
        pabort("can't open device");

    /*
     * spi mode
     */
    ret = ioctl(fd, SPI_IOC_WR_MODE, &mode);
    if (ret == -1)
        pabort("can't set spi mode");

    ret = ioctl(fd, SPI_IOC_RD_MODE, &mode);
    if (ret == -1)
        pabort("can't get spi mode");

    /*
     * bits per word
     */
    ret = ioctl(fd, SPI_IOC_WR_BITS_PER_WORD, &bits);
    if (ret == -1)
        pabort("can't set bits per word");

    ret = ioctl(fd, SPI_IOC_RD_BITS_PER_WORD, &bits);
    if (ret == -1)
        pabort("can't get bits per word");

    /*
     * max speed hz
     */
    ret = ioctl(fd, SPI_IOC_WR_MAX_SPEED_HZ, &speed);
    if (ret == -1)
        pabort("can't set max speed hz");

    ret = ioctl(fd, SPI_IOC_RD_MAX_SPEED_HZ, &speed);
    if (ret == -1)
        pabort("can't get max speed hz");

    printf("spi mode: %d\n", mode);
    printf("bits per word: %d\n", bits);
    printf("max speed: %d Hz (%d KHz)\n", speed, speed/1000);

    while(1)
    {
        //经过测试，mosi发送数据正确，但是mosi没有直连miso脚，也能收到数据，怎么回事?
        //用示波器测量miso脚，也没有接收到数据波形，默认高电平
        transfer(fd);            //传输测试    
        sleep(1);
        transfer(fd);            //传输测试
        sleep(1);
        
        

    //count=write(fd,buf,sizeof(buf)/sizeof(buf[0]));
    //read(fd,buf,count);  
    //printf("read byte is: 0x%02X\n",buf[0]);
     //sleep(1);          
        
        
    }
    

    close(fd);

    return ret;
}

问题：   经过示波器测试，mosi发送数据正确，但是mosi没有直连miso脚，示波器测量miso脚没有数据，
测试结果：

虚拟机下编译arm-linux-gcc  spi_test.c  -o  spi_test

在超级终端下运行：./spi_test

可以见到：

       0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,

             0x40, 0x00, 0x00, 0x00, 0x00, 0x95,

             0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,

             0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,

             0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,

             0xDE, 0xAD, 0xBE, 0xEF, 0xBA, 0xAD,

             0xF0, 0x0D,


可是我没有把mosi和miso连接起来，怎么spi的接收端也会有数据呢？