spidev_test.c中的错误 spi 9bits 模式 messge构造

本文链接：https://blog.youkuaiyun.com/yasin_lee/article/details/54632982

本文主要探讨在Linux环境中，使用spidev_test.c进行SPI通信时遇到的9位模式消息构造问题。通过分析源代码和调试过程，解析错误原因并提供解决方案。

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看注释吧，

/*
 * 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 "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/spidev0.0";
static uint8_t mode;
static uint8_t bits = 8;
static uint32_t speed = 500000;
static uint16_t delay;

static void transfer_test(int fd)
{
	int ret;
	uint8_t tx[] = {
		0xAE,0x00,0x10,0x40,0x81,
		0xdb,0xA1,0xA6,0xA8,0x63,
		0xC8,0xD3,0x00,0xD5,0x80,
		0xDA,0x12,0x8D,0x10,0xDB,
		0x20,0xAF,
	};
	uint8_t rx[ARRAY_SIZE(tx)] = {0, };
	struct spi_ioc_transfer tr = {
		.tx_buf = (unsigned long)tx,
		.rx_buf = (unsigned long)rx,
		.len = sizeof(tx),//源码此处用ARRAY_SIZE计算是错误的。因为spi驱动要求此处的len为发送buffer的大小，即字节个数！
		.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("");
}

/*发送位宽为9bi