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参考资料：

参考资料：

• 内核头文件：include\linux\spi\spi.h
• 内核文档：Documentation\spi\spidev
• DAC芯片手册：TLC5615.pdf

一、编写设备树

确认最大时钟频率：参看芯片手册

T = 25 + 25 = 50ns
F = 20000000 = 20MHz

修改\arch\arm\boot\dts\100ask_imx6ull-14x14.dts

&ecspi1 {pinctrl-names = "default";pinctrl-0 = <&pinctrl_ecspi1>;fsl,spi-num-chipselects = <2>;cs-gpios = <&gpio4 26 GPIO_ACTIVE_LOW>, <&gpio4 24 GPIO_ACTIVE_LOW>;status = "okay";dac:dac {compatible = "100ask,dac";reg = <0>;spi-max-frequency = <20000000>;};
};

二、 编写驱动程序

#include <linux/init.h>
#include <linux/module.h>
#include <linux/ioctl.h>
#include <linux/fs.h>
#include <linux/device.h>
#include <linux/err.h>
#include <linux/list.h>
#include <linux/errno.h>
#include <linux/mutex.h>
#include <linux/slab.h>
#include <linux/compat.h>
#include <linux/of.h>
#include <linux/of_device.h>
#include <linux/acpi.h>
#include <linux/spi/spi.h>
#include <linux/spi/spidev.h>
#include <linux/uaccess.h>#define SPI_IOC_WR 123/*-------------------------------------------------------------------------*/static struct spi_device *g_spidev_dac;
static int g_major;
static struct class *g_spidev_class;static long
spidev_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{int val;int err;unsigned char tx_buf[2];	unsigned char rx_buf[2];	struct spi_message	msg;struct spi_transfer	xfer[1];int status;/* 局部变量初始值未知，需清0 */memset(&xfer[0], 0, sizeof(xfer));/* 从用户层获取数据 */err = copy_from_user(&val, (const void __user *)arg, sizeof(int));printk("spidev_ioctl get val from user: %d\n", val);/* 发起SPI传输:     *//* 1. 把val修改为正确的格式 */val <<= 2;     /* bit0,bit1 = 0b00 */val &= 0xFFC;  /* 只保留10bit */tx_buf[1] = val & 0xff;         //低8位tx_buf[0] = (val>>8) & 0xff;	//高8位/* 2. 发起SPI传输同时写\读 *//* 2.1 构造transfer* 2.2 加入message* 2.3 调用spi_sync*/xfer[0].tx_buf = tx_buf;xfer[0].rx_buf = rx_buf;xfer[0].len = 2;spi_message_init(&msg);spi_message_add_tail(&xfer[0], &msg); status = spi_sync(g_spidev_dac, &msg);/* 3. 修改读到的数据的格式 */val = (rx_buf[0] << 8) | (rx_buf[1]);val >>= 2;/* 返回给用户层 */err = copy_to_user((void __user *)arg, &val, sizeof(int));return 0;
}/* file_operations 结构 */
static const struct file_operations spidev_fops = {.owner =	THIS_MODULE,.unlocked_ioctl = spidev_ioctl,
};/*-------------------------------------------------------------------------*/static int spidev_probe(struct spi_device *spi)
{/* 1. 记录spi_device */g_spidev_dac = spi;/* 2. 注册字符设备 */g_major = register_chrdev(0, "100ask_dac", &spidev_fops);g_spidev_class = class_create(THIS_MODULE, "100ask_dac");device_create(g_spidev_class, NULL, MKDEV(g_major, 0), NULL, "100ask_dac");	return 0;
}static int spidev_remove(struct spi_device *spi)
{/* 反注册字符设备 */device_destroy(g_spidev_class, MKDEV(g_major, 0));class_destroy(g_spidev_class);unregister_chrdev(g_major, "100ask_dac");return 0;
}static const struct of_device_id spidev_dt_ids[] = {{ .compatible = "100ask,dac" },{},
};/* 构造一个spi_driver */
static struct spi_driver spidev_spi_driver = {.driver = {.name =	"100ask_spi_dac_drv",.of_match_table = of_match_ptr(spidev_dt_ids),},.probe =	spidev_probe,.remove =	spidev_remove,
};/* 入口函数 */
static int __init spidev_init(void)
{int status;printk("%s %s %d\n", __FILE__, __FUNCTION__, __LINE__);/* 注册spi_driver */status = spi_register_driver(&spidev_spi_driver);if (status < 0) {}return status;
}/* 出口函数 */
static void __exit spidev_exit(void)
{printk("%s %s %d\n", __FILE__, __FUNCTION__, __LINE__);/* 反注册spi_driver */spi_unregister_driver(&spidev_spi_driver);
}module_init(spidev_init);
module_exit(spidev_exit);MODULE_LICENSE("GPL");

三、编写测试APP

参考: tools\spi\spidev_fdx.c

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <linux/types.h>#define SPI_IOC_WR 123/* dac_test /dev/100ask_dac <val> */int main(int argc, char **argv)
{int fd;unsigned int val;int status;if (argc != 3){printf("Usage: %s /dev/100ask_dac <val>\n", argv[0]);return 0;}fd = open(argv[1], O_RDWR);if (fd < 0) {printf("can not open %s\n", argv[1]);return 1;}val = strtoul(argv[2], NULL, 0);/* 写入数据，并读回数据 */status = ioctl(fd, SPI_IOC_WR, &val);if (status < 0) {printf("SPI_IOC_WR\n");return -1;}/* 打印 */printf("Pre val = %d\n", val);return 0;
}

四、Makefile

KERN_DIR = /home/zpz/share/imx6ullsdk/repo/100ask_imx6ull-sdk/Linux-4.9.88
all:make -C $(KERN_DIR) M=`pwd` modules $(CROSS_COMPILE)gcc -o dac_test dac_test.c
clean:make -C $(KERN_DIR) M=`pwd` modules cleanrm -rf modules.order dac_test
obj-m	+= dac_drv.o

五、上机实验

1. 编译、替换设备树

• 编译设备树

$ make dtbs 

• 拷贝到开发板

$ mount -t nfs -o nolock 192.168.124.22:/home/zpz/share/ /mnt
$ cp /mnt/imx6ullsdk/repo/100ask_imx6ull-sdk/Linux-4.9.88/arch/arm/boot/dts/100ask_imx6ull-14x14.dtb /boot/

• 重启开发板

2. 编译驱动和测试程序

3. 加载驱动

$ insmod dac_drv.ko

4. 查看设备节点

$ ls /dev/100ask_dac -l
crw------- 1 root root 240, 0 Jan  1 01:47 /dev/100ask_dac

5. 运行测试程序

//传入不用val,亮度不同
$ ./dac_test /dev/100ask_dac 100  
Pre val = 0 
$  ./dac_test /dev/100ask_dac 500
Pre val = 100
$ ./dac_test /dev/100ask_dac 1000
Pre val = 500