i.MX 6ULL 驱动开发 二十三：UART

一、UART 协议

UART详解_sternlycore的博客-CSDN博客

二、UART 和 TTY 关系

基于Linux的tty架构及UART驱动详解 - 一口Linux - 博客园 (cnblogs.com)

三、Linux UART 驱动框架中重要对象

1、UART 驱动

struct uart_driver {
	struct module		*owner;
	const char		*driver_name;
	const char		*dev_name;
	int			 major;
	int			 minor;
	int			 nr;
	struct console		*cons;

	/*
	 * these are private; the low level driver should not
	 * touch these; they should be initialised to NULL
	 */
	struct uart_state	*state;
	struct tty_driver	*tty_driver;
};

每个串口驱动都需要定义一个 uart_driver，加载驱动时向系统注册这个 uart_driver，注销驱动时注销掉注册的 uart_driver。

2、UART 控制器

struct uart_port {
	spinlock_t		lock;			/* port lock */
	unsigned long		iobase;			/* in/out[bwl] */
	unsigned char __iomem	*membase;		/* read/write[bwl] */
	unsigned int		(*serial_in)(struct uart_port *, int);
	void			(*serial_out)(struct uart_port *, int, int);
	void			(*set_termios)(struct uart_port *,
				               struct ktermios *new,
				               struct ktermios *old);
	void			(*set_mctrl)(struct uart_port *, unsigned int);
	int			(*startup)(struct uart_port *port);
	void			(*shutdown)(struct uart_port *port);
	void			(*throttle)(struct uart_port *port);
	void			(*unthrottle)(struct uart_port *port);
	int			(*handle_irq)(struct uart_port *);
	void			(*pm)(struct uart_port *, unsigned int state,
				      unsigned int old);
	void			(*handle_break)(struct uart_port *);
	int			(*rs485_config)(struct uart_port *,
						struct serial_rs485 *rs485);
	unsigned int		irq;			/* irq number */
	unsigned long		irqflags;		/* irq flags  */
	unsigned int		uartclk;		/* base uart clock */
	unsigned int		fifosize;		/* tx fifo size */
	unsigned char		x_char;			/* xon/xoff char */
	unsigned char		regshift;		/* reg offset shift */
	unsigned char		iotype;			/* io access style */
	unsigned char		unused1;

#define UPIO_PORT		(SERIAL_IO_PORT)	/* 8b I/O port access */
#define UPIO_HUB6		(SERIAL_IO_HUB6)	/* Hub6 ISA card */
#define UPIO_MEM		(SERIAL_IO_MEM)		/* 8b MMIO access */
#define UPIO_MEM32		(SERIAL_IO_MEM32)	/* 32b little endian */
#define UPIO_AU			(SERIAL_IO_AU)		/* Au1x00 and RT288x type IO */
#define UPIO_TSI		(SERIAL_IO_TSI)		/* Tsi108/109 type IO */
#define UPIO_MEM32BE		(SERIAL_IO_MEM32BE)	/* 32b big endian */

	unsigned int		read_status_mask;	/* driver specific */
	unsigned int		ignore_status_mask;	/* driver specific */
	struct uart_state	*state;			/* pointer to parent state */
	struct uart_icount	icount;			/* statistics */

	struct console		*cons;			/* struct console, if any */
#if defined(CONFIG_SERIAL_CORE_CONSOLE) || defined(SUPPORT_SYSRQ)
	unsigned long		sysrq;			/* sysrq timeout */
#endif

	/* flags must be updated while holding port mutex */
	upf_t			flags;

	/*
	 * These flags must be equivalent to the flags defined in
	 * include/uapi/linux/tty_flags.h which are the userspace definitions
	 * assigned from the serial_struct flags in uart_set_info()
	 * [for bit definitions in the UPF_CHANGE_MASK]
	 *
	 * Bits [0..UPF_LAST_USER] are userspace defined/visible/changeable
	 * except bit 15 (UPF_NO_TXEN_TEST) which is masked off.
	 * The remaining bits are serial-core specific and not modifiable by
	 * userspace.
	 */
#define UPF_FOURPORT		((__force upf_t) ASYNC_FOURPORT       /* 1  */ )
#define UPF_SAK			((__force upf_t) ASYNC_SAK            /* 2  */ )
#define UPF_SPD_HI		((__force upf_t) ASYNC_SPD_HI         /* 4  */ )
#define UPF_SPD_VHI		((__force upf_t) ASYNC_SPD_VHI        /* 5  */ )
#define UPF_SPD_CUST		((__force upf_t) ASYNC_SPD_CUST   /* 0x0030 */ )
#define UPF_SPD_WARP		((__force upf_t) ASYNC_SPD_WARP   /* 0x1010 */ )
#define UPF_SPD_MASK		((__force upf_t) ASYNC_SPD_MASK   /* 0x1030 */ )
#define UPF_SKIP_TEST		((__force upf_t) ASYNC_SKIP_TEST      /* 6  */ )
#define UPF_AUTO_IRQ		((__force upf_t) ASYNC_AUTO_IRQ       /* 7  */ )
#define UPF_HARDPPS_CD		((__force upf_t) ASYNC_HARDPPS_CD     /* 11 */ )
#define UPF_SPD_SHI		((__force upf_t) ASYNC_SPD_SHI        /* 12 */ )
#define UPF_LOW_LATENCY		((__force upf_t) ASYNC_LOW_LATENCY    /* 13 */ )
#define UPF_BUGGY_UART		((__force upf_t) ASYNC_BUGGY_UART     /* 14 */ )
#define UPF_NO_TXEN_TEST	((__force upf_t) (1 << 15))
#define UPF_MAGIC_MULTIPLIER	((__force upf_t) ASYNC_MAGIC_MULTIPLIER /* 16 */ )

/* Port has hardware-assisted h/w flow control */
#define UPF_AUTO_CTS		((__force upf_t) (1 << 20))
#define UPF_AUTO_RTS		((__force upf_t) (1 << 21))
#define UPF_HARD_FLOW		((__force upf_t) (UPF_AUTO_CTS | UPF_AUTO_RTS))
/* Port has hardware-assisted s/w flow control */
#define UPF_SOFT_FLOW		((__force upf_t) (1 << 22))
#define UPF_CONS_FLOW		((__force upf_t) (1 << 23))
#define UPF_SHARE_IRQ		((__force upf_t) (1 << 24))
#define UPF_EXAR_EFR		((__force upf_t) (1 << 25))
#define UPF_BUG_THRE		((__force upf_t) (1 << 26))
/* The exact UART type is known and should not be probed.  */
#define UPF_FIXED_TYPE		((__force upf_t) (1 << 27))
#define UPF_BOOT_AUTOCONF	((__force upf_t) (1 << 28))
#define UPF_FIXED_PORT		((__force upf_t) (1 << 29))
#define UPF_DEAD		((__force upf_t) (1 << 30))
#define UPF_IOREMAP		((__force upf_t) (1 << 31))

#define __UPF_CHANGE_MASK	0x17fff
#define UPF_CHANGE_MASK		((__force upf_t) __UPF_CHANGE_MASK)
#define UPF_USR_MASK		((__force upf_t) (UPF_SPD_MASK|UPF_LOW_LATENCY))

#if __UPF_CHANGE_MASK > ASYNC_FLAGS
#error Change mask not equivalent to userspace-visible bit defines
#endif

	/*
	 * Must hold termios_rwsem, port mutex and port lock to change;
	 * can hold any one lock to read.
	 */
	upstat_t		status;

#define UPSTAT_CTS_ENABLE	((__force upstat_t) (1 << 0))
#define UPSTAT_DCD_ENABLE	((__force upstat_t) (1 << 1))
#define UPSTAT_AUTORTS		((__force upstat_t) (1 << 2))
#define UPSTAT_AUTOCTS		((__force upstat_t) (1 << 3))
#define UPSTAT_AUTOXOFF		((__force upstat_t) (1 << 4))

	int			hw_stopped;		/* sw-assisted CTS flow state */
	unsigned int		mctrl;			/* current modem ctrl settings */
	unsigned int		timeout;		/* character-based timeout */
	unsigned int		type;			/* port type */
	const struct uart_ops	*ops;
	unsigned int		custom_divisor;
	unsigned int		line;			/* port index */
	unsigned int		minor;
	resource_size_t		mapbase;		/* for ioremap */
	resource_size_t		mapsize;
	struct device		*dev;			/* parent device */
	unsigned char		hub6;			/* this should be in the 8250 driver */
	unsigned char		suspended;
	unsigned char		irq_wake;
	unsigned char		unused[2];
	struct attribute_group	*attr_group;		/* port specific attributes */
	const struct attribute_group **tty_groups;	/* all attributes (serial core use only) */
	struct serial_rs485     rs485;
	void			*private_data;		/* generic platform data pointer */
};

Linux 中使用 struct uart_port 描述硬件信息。

3、UART 操作集

/*
 * This structure describes all the operations that can be done on the
 * physical hardware.  See Documentation/serial/driver for details.
 */
struct uart_ops {
	unsigned int	(*tx_empty)(struct uart_port *);
	void		(*set_mctrl)(struct uart_port *, unsigned int mctrl);
	unsigned int	(*get_mctrl)(struct uart_port *);
	void		(*stop_tx)(struct uart_port *);
	void		(*start_tx)(struct uart_port *);
	void		(*throttle)(struct uart_port *);
	void		(*unthrottle)(struct uart_port *);
	void		(*send_xchar)(struct uart_port *, char ch);
	void		(*stop_rx)(struct uart_port *);
	void		(*enable_ms)(struct uart_port *);
	void		(*break_ctl)(struct uart_port *, int ctl);
	int		(*startup)(struct uart_port *);
	void		(*shutdown)(struct uart_port *);
	void		(*flush_buffer)(struct uart_port *);
	void		(*set_termios)(struct uart_port *, struct ktermios *new,
				       struct ktermios *old);
	void		(*set_ldisc)(struct uart_port *, struct ktermios *);
	void		(*pm)(struct uart_port *, unsigned int state,
			      unsigned int oldstate);

	/*
	 * Return a string describing the type of the port
	 */
	const char	*(*type)(struct uart_port *);

	/*
	 * Release IO and memory resources used by the port.
	 * This includes iounmap if necessary.
	 */
	void		(*release_port)(struct uart_port *);

	/*
	 * Request IO and memory resources used by the port.
	 * This includes iomapping the port if necessary.
	 */
	int		(*request_port)(struct uart_port *);
	void		(*config_port)(struct uart_port *, int);
	int		(*verify_port)(struct uart_port *, struct serial_struct *);
	int		(*ioctl)(struct uart_port *, unsigned int, unsigned long);
#ifdef CONFIG_CONSOLE_POLL
	int		(*poll_init)(struct uart_port *);
	void		(*poll_put_char)(struct uart_port *, unsigned char);
	int		(*poll_get_char)(struct uart_port *);
#endif
};

四、UART 驱动编写思路

Linux 下 UART 一般都编写好，根据设备树找到相关驱动分析即可。

五、imx6ull 下 UART 驱动分析

文档路径：drivers\tty\serial\imx.c。

1、驱动框架

static struct uart_driver imx_reg = {
	.owner          = THIS_MODULE,
	.driver_name    = DRIVER_NAME,
	.dev_name       = DEV_NAME,
	.major          = SERIAL_IMX_MAJOR,
	.minor          = MINOR_START,
	.nr             = ARRAY_SIZE(imx_ports),	// imx_ports 封装 struct uart_port
	.cons           = IMX_CONSOLE,
};

static struct platform_driver serial_imx_driver = {
	.probe		= serial_imx_probe,
	.remove		= serial_imx_remove,

	.suspend	= serial_imx_suspend,
	.resume		= serial_imx_resume,
	.id_table	= imx_uart_devtype,
	.driver		= {
		.name	= "imx-uart",
		.of_match_table = imx_uart_dt_ids,
	},
};

static int __init imx_serial_init(void)
{
	int ret = uart_register_driver(&imx_reg);

	if (ret)
		return ret;

	ret = platform_driver_register(&serial_imx_driver);
	if (ret != 0)
		uart_unregister_driver(&imx_reg);

	return ret;
}

static void __exit imx_serial_exit(void)
{
	platform_driver_unregister(&serial_imx_driver);
	uart_unregister_driver(&imx_reg);
}

1、UART 驱动使用 platform 驱动框架。

2、在加载驱动时注册 UART 驱动。

2、初始化

见 serial_imx_probe 函数。

3、注销

见 serial_imx_remove 函数。

六、添加设备树

1、UART 设备树相关说明

见文档：Documentation\devicetree\bindings\serial\fsl-imx-uart.txt。

2、确定使用 UART

通过原理图可以确定，使用接口为 uart3。

3、添加 pinctrl 子系统相关配置

pinctrl_uart3: uart3grp {
	fsl,pins = <
		MX6UL_PAD_UART3_TX_DATA__UART3_DCE_TX		0X1b0b1
		MX6UL_PAD_UART3_RX_DATA__UART3_DCE_RX		0X1b0b1
	>;
};

4、在 uart3 下追加配置

&uart3 {
	pinctrl-names = "default";
	pinctrl-0 = <&pinctrl_uart3>;
	status = "okay";
};

5、编译设备树

onlylove@ubuntu:~/my/linux/linux-imx-4.1.15$ make dtbs
  CHK     include/config/kernel.release
  CHK     include/generated/uapi/linux/version.h
  CHK     include/generated/utsrelease.h
make[1]: 'include/generated/mach-types.h' is up to date.
  CHK     include/generated/bounds.h
  CHK     include/generated/asm-offsets.h
  CALL    scripts/checksyscalls.sh
  DTC     arch/arm/boot/dts/imx6ull-alientek-emmc.dtb
  DTC     arch/arm/boot/dts/imx6ull-alientek-nand.dtb
onlylove@ubuntu:~/my/linux/linux-imx-4.1.15$ 

6、测试

# pwd
/proc/device-tree/soc/aips-bus@02100000
# ls
#address-cells       lcdif@021c8000       serial@021f0000
#size-cells          mmdc@021b0000        serial@021f4000
adc@02198000         name                 serial@021fc000
compatible           ocotp-ctrl@021bc000  usb@02184000
csi@021c4000         pxp@021cc000         usb@02184200
csu@021c0000         qspi@021e0000        usbmisc@02184800
ethernet@02188000    ranges               usdhc@02190000
i2c@021a0000         reg                  usdhc@02194000
i2c@021a4000         romcp@021ac000       weim@021b8000
i2c@021a8000         serial@021e8000
i2c@021f8000         serial@021ec000
# cd serial@021ec000/
# ls
clock-names    dma-names      name           reg
clocks         dmas           pinctrl-0      status
compatible     interrupts     pinctrl-names
# cat compatible
fsl,imx6ul-uartfsl,imx6q-uartfsl,imx21-uart#
#

七、驱动编写

uart 相关驱动 Linux 内核已添加，不需要我们编写。

# ls /dev/ttymxc* -l
crw-------    1 root     root      207,  16 Jan  1 05:59 /dev/ttymxc0
crw-rw----    1 root     root      207,  18 Jan  1 00:00 /dev/ttymxc2
#

通过以上消息，uart3 驱动加载成功，uart3 在应用层映射为 ttymxc2。

八、应用编写

1、概述

在 Linux 下 UART 驱动和 tty 关系密切，在应用层使用 UART 是需特别注意，否则一些特殊字符传输可能有问题。

2、程序

/***************************************************************
 Copyright © ALIENTEK Co., Ltd. 1998-2021. All rights reserved.
 文件名 : uart_test.c
 作者 : 邓涛
 版本 : V1.0
 描述 : 串口在原始模式下进行数据传输--应用程序示例代码
 其他 : 无
 论坛 : www.openedv.com
 日志 : 初版 V1.0 2021/7/20 邓涛创建
 ***************************************************************/

#define _GNU_SOURCE     //在源文件开头定义_GNU_SOURCE宏
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

typedef struct uart_hardware_cfg {
    unsigned int baudrate;      /* 波特率 */
    unsigned char dbit;         /* 数据位 */
    char parity;                /* 奇偶校验 */
    unsigned char sbit;         /* 停止位 */
} uart_cfg_t;

static struct termios old_cfg;  //用于保存终端的配置参数
static int fd;      //串口终端对应的文件描述符

/**
 ** 串口初始化操作
 ** 参数device表示串口终端的设备节点
 **/
static int uart_init(const char *device)
{
    /* 打开串口终端 */
    fd = open(device, O_RDWR | O_NOCTTY);
    if (0 > fd) {
        fprintf(stderr, "open error: %s: %s\n", device, strerror(errno));
        return -1;
    }

    /* 获取串口当前的配置参数 */
    if (0 > tcgetattr(fd, &old_cfg)) {
        fprintf(stderr, "tcgetattr error: %s\n", strerror(errno));
        close(fd);
        return -1;
    }

    return 0;
}

/**
 ** 串口配置
 ** 参数cfg指向一个uart_cfg_t结构体对象
 **/
static int uart_cfg(const uart_cfg_t *cfg)
{
    struct termios new_cfg = {0};   //将new_cfg对象清零
    speed_t speed;

    /* 设置为原始模式 */
    cfmakeraw(&new_cfg);

    /* 使能接收 */
    new_cfg.c_cflag |= CREAD;

    /* 设置波特率 */
    switch (cfg->baudrate) {
    case 1200: speed = B1200;
        break;
    case 1800: speed = B1800;
        break;
    case 2400: speed = B2400;
        break;
    case 4800: speed = B4800;
        break;
    case 9600: speed = B9600;
        break;
    case 19200: speed = B19200;
        break;
    case 38400: speed = B38400;
        break;
    case 57600: speed = B57600;
        break;
    case 115200: speed = B115200;
        break;
    case 230400: speed = B230400;
        break;
    case 460800: speed = B460800;
        break;
    case 500000: speed = B500000;
        break;
    default:    //默认配置为115200
        speed = B115200;
        printf("default baud rate: 115200\n");
        break;
    }

    if (0 > cfsetspeed(&new_cfg, speed)) {
        fprintf(stderr, "cfsetspeed error: %s\n", strerror(errno));
        return -1;
    }

    /* 设置数据位大小 */
    new_cfg.c_cflag &= ~CSIZE;  //将数据位相关的比特位清零
    switch (cfg->dbit) {
    case 5:
        new_cfg.c_cflag |= CS5;
        break;
    case 6:
        new_cfg.c_cflag |= CS6;
        break;
    case 7:
        new_cfg.c_cflag |= CS7;
        break;
    case 8:
        new_cfg.c_cflag |= CS8;
        break;
    default:    //默认数据位大小为8
        new_cfg.c_cflag |= CS8;
        printf("default data bit size: 8\n");
        break;
    }

    /* 设置奇偶校验 */
    switch (cfg->parity) {
    case 'N':       //无校验
        new_cfg.c_cflag &= ~PARENB;
        new_cfg.c_iflag &= ~INPCK;
        break;
    case 'O':       //奇校验
        new_cfg.c_cflag |= (PARODD | PARENB);
        new_cfg.c_iflag |= INPCK;
        break;
    case 'E':       //偶校验
        new_cfg.c_cflag |= PARENB;
        new_cfg.c_cflag &= ~PARODD; /* 清除PARODD标志，配置为偶校验 */
        new_cfg.c_iflag |= INPCK;
        break;
    default:    //默认配置为无校验
        new_cfg.c_cflag &= ~PARENB;
        new_cfg.c_iflag &= ~INPCK;
        printf("default parity: N\n");
        break;
    }

    /* 设置停止位 */
    switch (cfg->sbit) {
    case 1:     //1个停止位
        new_cfg.c_cflag &= ~CSTOPB;
        break;
    case 2:     //2个停止位
        new_cfg.c_cflag |= CSTOPB;
        break;
    default:    //默认配置为1个停止位
        new_cfg.c_cflag &= ~CSTOPB;
        printf("default stop bit size: 1\n");
        break;
    }

    /* 将MIN和TIME设置为0 */
    new_cfg.c_cc[VTIME] = 0;
    new_cfg.c_cc[VMIN] = 0;

    /* 清空缓冲区 */
    if (0 > tcflush(fd, TCIOFLUSH)) {
        fprintf(stderr, "tcflush error: %s\n", strerror(errno));
        return -1;
    }

    /* 写入配置、使配置生效 */
    if (0 > tcsetattr(fd, TCSANOW, &new_cfg)) {
        fprintf(stderr, "tcsetattr error: %s\n", strerror(errno));
        return -1;
    }

    /* 配置OK 退出 */
    return 0;
}

/***
--dev=/dev/ttymxc2
--brate=115200
--dbit=8
--parity=N
--sbit=1
--type=read
***/
/**
 ** 打印帮助信息
 **/
static void show_help(const char *app)
{
    printf("Usage: %s [选项]\n"
        "\n必选选项:\n"
        "  --dev=DEVICE     指定串口终端设备名称, 譬如--dev=/dev/ttymxc2\n"
        "  --type=TYPE      指定操作类型, 读串口还是写串口, 譬如--type=read(read表示读、write表示写、其它值无效)\n"
        "\n可选选项:\n"
        "  --brate=SPEED    指定串口波特率, 譬如--brate=115200\n"
        "  --dbit=SIZE      指定串口数据位个数, 譬如--dbit=8(可取值为: 5/6/7/8)\n"
        "  --parity=PARITY  指定串口奇偶校验方式, 譬如--parity=N(N表示无校验、O表示奇校验、E表示偶校验)\n"
        "  --sbit=SIZE      指定串口停止位个数, 譬如--sbit=1(可取值为: 1/2)\n"
        "  --help           查看本程序使用帮助信息\n\n", app);
}

/**
 ** 信号处理函数，当串口有数据可读时，会跳转到该函数执行
 **/
static void io_handler(int sig, siginfo_t *info, void *context)
{
    unsigned char buf[10] = {0};
    int ret;
    int n;

    if(SIGRTMIN != sig)
        return;

    /* 判断串口是否有数据可读 */
    if (POLL_IN == info->si_code) {
        ret = read(fd, buf, 8);     //一次最多读8个字节数据
        printf("[ ");
        for (n = 0; n < ret; n++)
            printf("0x%hhx ", buf[n]);
        printf("]\n");
    }
}

/**
 ** 异步I/O初始化函数
 **/
static void async_io_init(void)
{
    struct sigaction sigatn;
    int flag;

    /* 使能异步I/O */
    flag = fcntl(fd, F_GETFL);  //使能串口的异步I/O功能
    flag |= O_ASYNC;
    fcntl(fd, F_SETFL, flag);

    /* 设置异步I/O的所有者 */
    fcntl(fd, F_SETOWN, getpid());

    /* 指定实时信号SIGRTMIN作为异步I/O通知信号 */
    fcntl(fd, F_SETSIG, SIGRTMIN);

    /* 为实时信号SIGRTMIN注册信号处理函数 */
    sigatn.sa_sigaction = io_handler;   //当串口有数据可读时，会跳转到io_handler函数
    sigatn.sa_flags = SA_SIGINFO;
    sigemptyset(&sigatn.sa_mask);
    sigaction(SIGRTMIN, &sigatn, NULL);
}

int main(int argc, char *argv[])
{
    uart_cfg_t cfg = {0};
    char *device = NULL;
    int rw_flag = -1;
    unsigned char w_buf[10] = {0x11, 0x22, 0x33, 0x44,0x55, 0x66, 0x77, 0x88};    //通过串口发送出去的数据
    int n;

    /* 解析出参数 */
    for (n = 1; n < argc; n++) {

        if (!strncmp("--dev=", argv[n], 6))
            device = &argv[n][6];
        else if (!strncmp("--brate=", argv[n], 8))
            cfg.baudrate = atoi(&argv[n][8]);
        else if (!strncmp("--dbit=", argv[n], 7))
            cfg.dbit = atoi(&argv[n][7]);
        else if (!strncmp("--parity=", argv[n], 9))
            cfg.parity = argv[n][9];
        else if (!strncmp("--sbit=", argv[n], 7))
            cfg.sbit = atoi(&argv[n][7]);
        else if (!strncmp("--type=", argv[n], 7)) {
            if (!strcmp("read", &argv[n][7]))
                rw_flag = 0;        //读
            else if (!strcmp("write", &argv[n][7]))
                rw_flag = 1;        //写
        }
        else if (!strcmp("--help", argv[n])) {
            show_help(argv[0]); //打印帮助信息
            exit(EXIT_SUCCESS);
        }
    }

    if (NULL == device || -1 == rw_flag) {
        fprintf(stderr, "Error: the device and read|write type must be set!\n");
        show_help(argv[0]);
        exit(EXIT_FAILURE);
    }

    /* 串口初始化 */
    if (uart_init(device))
        exit(EXIT_FAILURE);

    /* 串口配置 */
    if (uart_cfg(&cfg)) {
        tcsetattr(fd, TCSANOW, &old_cfg);   //恢复到之前的配置
        close(fd);
        exit(EXIT_FAILURE);
    }

    /* 读|写串口 */
    switch (rw_flag) {
    case 0:  //读串口数据
        async_io_init();	//我们使用异步I/O方式读取串口的数据，调用该函数去初始化串口的异步I/O
        for ( ; ; )
            sleep(1);   	//进入休眠、等待有数据可读，有数据可读之后就会跳转到io_handler()函数
        break;
    case 1:   //向串口写入数据
        for ( ; ; ) {   		//循环向串口写入数据
            write(fd, w_buf, 8); 	//一次向串口写入8个字节
            sleep(1);       		//间隔1秒钟
        }
        break;
    }

    /* 退出 */
    tcsetattr(fd, TCSANOW, &old_cfg);   //恢复到之前的配置
    close(fd);
    exit(EXIT_SUCCESS);
}

程序使用正点原子提供。

九、测试

1、数据接收

# ./uart_app --dev=/dev/ttymxc2 --type=read
default baud rate: 115200
default data bit size: 8
default parity: N
default stop bit size: 1

[ 0x11 0x22 0x33 0x44 0x55 0x66 0x77 0x88 ]
[ 0x11 0x22 0x33 0x44 0x55 0x66 0x77 0x88 ]
[ 0x11 0x22 0x33 0x44 0x55 0x66 0x77 0x88 ]
[ 0x11 0x22 0x33 0x44 0x55 0x66 0x77 0x88 ]
[ 0x11 0x22 0x33 0x44 0x55 0x66 0x77 0x88 ]
[ 0x11 0x22 0x33 0x44 0x55 0x66 0x77 0x88 ]
[ 0x11 0x22 0x33 0x44 0x55 0x66 0x77 0x88 ]
[ 0x11 0x22 0x33 0x44 0x55 0x66 0x77 0x88 ]

#

2、数据发送