spidev应用

Linux用户空间层提供了可以直接操作SPI控制器向丛设备通讯的能力，方便特殊设备的操作。

struct spi_ioc_transfer {
    __u64        tx_buf;
    __u64        rx_buf;

    __u32        len;
    __u32        speed_hz;

    __u16        delay_usecs;
    __u8        bits_per_word;
    __u8        cs_change;
    __u32        pad;
};

每个 spi_ioc_transfer都可以包含读和写的请求，其中读和写的长度必须相等。所以成员len不是tx_buf和rx_buf缓冲的长度之和，而是它们各自的长度。SPI控制器驱动会先将tx_buf写到SPI总线上，然后再读取len长度的内容到rx_buf。如果只想进行一个方向的传输，把另一个方向的缓冲置为0就可以了。
speed_hz可以为每次通信配置不同的通信速率（必须小于spi_device的max_speed_hz）

bits_per_word这两个成员可以为每次通信配置不同的字长，如果它们为0的话就会使用spi_device中的配置。
delay_usecs可以指定两个spi_ioc_transfer之间的延时，单位是微妙。一般不用定义。
cs_change指定这个cs_change结束之后是否需要改变片选线。一般针对同一设备的连续的几个spi_ioc_transfer，只有最后一个需要将这个成员置位。这样省去了来回改变片选线的时间，有助于提高通信速率。

以下是spidev的应用参考代码：

/* 
 * SPI testing utility (using spidev driver) 
 * 
 * Copyright (c) 2007  MontaVista Software, Inc. 
 * Copyright (c) 2007  Anton Vorontsov <avorontsov@ru.mvista.com> 
 * 
 * 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. 
 * 
 * Cross-compile with cross-gcc -I/path/to/cross-kernel/include 
 */  

#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>  

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

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

static const char *device = "/dev/spidev1.1";  
static uint8_t mode;  
static uint8_t bits = 8;  
static uint32_t speed = 500000;  
static uint16_t delay;  

static void transfer(int fd)  
{  
        int ret;  
        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 rx[ARRAY_SIZE(tx)] = {0, };  
        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,  
        };  

        ret = ioctl(fd, SPI_IOC_MESSAGE(1), &tr);  
        if (ret == 1)  
                pabort("can't send spi message");  

        for (ret = 0; ret < ARRAY_SIZE(tx); ret++) {  
                if (!(ret % 6))  
                        puts("");  
                printf("%.2X ", rx[ret]);  
        }  
        puts("");  
}  

static void print_usage(const char *prog)  
{  
        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;  

        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);  

        transfer(fd);  

        close(fd);  

        return ret;  
}