尽管一个特定的UART设备驱动完全可以按照tty驱动的设计方法来设计，即定义tty_driver并实现tty_operations其中的成员函数，但是Linux已经在文件serial_core.c中实现了UART设备的通用tty驱动层，称为串口核心层，这样，UART驱动的主要任务变成了实现serial_core.c中定义的一组uart_xxx接口而非tty_xxx接口。 

     uart设备是继tty_driver的又一层封装.实际上uart_driver就是对应tty_driver.在它的操作函数中,将操作转入uart_port.在写操作的时候,先将数据放入一个叫做circ_buf的环形缓存区.然后uart_port从缓存区中取数据,将其写入到串口设备中.当uart_port从serial设备接收到数据时,会将设备放入对应line discipline的缓存区中.这样.用户在编写串口驱动的时候,只先要注册一个uart_driver.它的主要作用是定义设备节点号.然后将对设备的各项操作封装在uart_port.驱动工程师没必要关心上层的流程,只需按硬件规范将uart_port中的接口函数完成就可以了.

      

1.下图描述了串行系统间的层次结构关系,可以概括为：用户应用层 --> 线路规划层 --> TTY层 --> 底层驱动层 --> 物理硬件层

 

2.下图是串口核心层在整个tty源文件关系及数据流向中的位置：

　　其中的xxx_uart.c在此处就是drivers/serial/samsung.c和s5pv210.c

 

3.接口关系：

从接口关系图可以看出，用户对uart设备操作的调用关系非常简单，

file_operations => [tty_ldisc_ops] => tty_operations => uart ops

其中tty_ldisc_ops线路规程并不是必要的，依赖于应用层设置是否使用ldisc处理数据。

 

4.UART驱动的总图：

 

 5.uart驱动常用的数据结构表示如下：

 

     

6.Uart驱动程序主要围绕三个关键的数据结构展开(include/linux/serial_core.h中定义)：

 　　UART特定的驱动程序结构定义：struct uart_driver s3c24xx_uart_drv;

　　UART端口结构定义： struct s3c24xx_uart_port s3c24xx_serial_ports；

　　UART相关操作函数结构定义: struct uart_ops s3c24xx_serial_ops;

 

 【1】uart_driver 封装了tty_driver，使得底层的UART驱动无需关心tty_driver

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;};

 其中的uart_state是设备私有信息结构体，

 在uart_open()中:

 tty->driver_data = state;

 在其他uart_xxx()中：

 struct uart_state *state = tty->driver_data;

 就可以获取设备私有信息结构体。

static struct uart_driver s3c24xx_uart_drv= {      .owner           =THIS_MODULE,      .dev_name      = "s3c2440_serial",  //具体设备名称      .nr          =CONFIG_SERIAL_SAMSUNG_UARTS,  //定义有几个端口      .cons             = S3C24XX_SERIAL_CONSOLE,  //console接口       .driver_name  =S3C24XX_SERIAL_NAME,  //串口名：ttySAC      .major            =S3C24XX_SERIAL_MAJOR,  //主设备号      .minor            =S3C24XX_SERIAL_MINOR,   //次设备号};

 一个tty驱动必须注册/注销tty_driver，而一个UART驱动则变为注册/注销uart_driver，使用如下接口：

int uart_register_driver(struct uart_driver *drv)；

void uart_unregister_driver(struct uart_driver *drv)；

 

【2】uart_port用于描述一个UART端口（直接对应于一个串口）的I/O端口或者IO内存地址等信息。

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);    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        (0)#define UPIO_HUB6        (1)#define UPIO_MEM        (2)#define UPIO_MEM32        (3)#define UPIO_AU            (4)            /* Au1x00 type IO */#define UPIO_TSI        (5)            /* Tsi108/109 type IO */#define UPIO_DWAPB        (6)            /* DesignWare APB UART */#define UPIO_RM9000        (7)            /* RM9000 type IO */    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    upf_t            flags;#define UPF_FOURPORT        ((__force upf_t) (1 << 1))#define UPF_SAK            ((__force upf_t) (1 << 2))#define UPF_SPD_MASK        ((__force upf_t) (0x1030))#define UPF_SPD_HI        ((__force upf_t) (0x0010))#define UPF_SPD_VHI        ((__force upf_t) (0x0020))#define UPF_SPD_CUST        ((__force upf_t) (0x0030))#define UPF_SPD_SHI        ((__force upf_t) (0x1000))#define UPF_SPD_WARP        ((__force upf_t) (0x1010))#define UPF_SKIP_TEST        ((__force upf_t) (1 << 6))#define UPF_AUTO_IRQ        ((__force upf_t) (1 << 7))#define UPF_HARDPPS_CD        ((__force upf_t) (1 << 11))#define UPF_LOW_LATENCY        ((__force upf_t) (1 << 13))#define UPF_BUGGY_UART        ((__force upf_t) (1 << 14))#define UPF_NO_TXEN_TEST    ((__force upf_t) (1 << 15))#define UPF_MAGIC_MULTIPLIER    ((__force upf_t) (1 << 16))#define UPF_CONS_FLOW        ((__force upf_t) (1 << 23))#define UPF_SHARE_IRQ        ((__force upf_t) (1 << 24))/* 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        ((__force upf_t) (0x17fff))#define UPF_USR_MASK        ((__force upf_t) (UPF_SPD_MASK|UPF_LOW_LATENCY))    unsigned int        mctrl;            /* current modem ctrl settings */    unsigned int        timeout;        /* character-based timeout */    unsigned int        type;            /* port type */    const struct uart_ops    *ops;//UART操作集------->         unsigned int        custom_divisor;    unsigned int        line;            /* port index */    resource_size_t        mapbase;        /* for ioremap */    struct device        *dev;            /* parent device */    unsigned char        hub6;            /* this should be in the 8250 driver */    unsigned char        suspended;    unsigned char        unused[2];    void            *private_data;        /* generic platform data pointer */};

 

s3c24xx_uart_port 封装了uart_port：

struct s3c24xx_uart_port {    unsigned char            rx_claimed;    unsigned char            tx_claimed;    unsigned int            pm_level;    unsigned long            baudclk_rate;    unsigned int            rx_irq;    unsigned int            tx_irq;    struct s3c24xx_uart_info    *info;    struct s3c24xx_uart_clksrc    *clksrc;    struct clk            *clk;    struct clk            *baudclk;    struct uart_port        port;#ifdef CONFIG_CPU_FREQ    struct notifier_block        freq_transition;#endif    int                channelnum;};

 

 

static struct s3c24xx_uart_port    s3c24xx_serial_ports[CONFIG_SERIAL_SAMSUNG_UARTS] = {    [0] = {//串口0；        .port = {            .lock        = __SPIN_LOCK_UNLOCKED(s3c24xx_serial_ports[0].port.lock),            .iotype        = UPIO_MEM,            .irq        = IRQ_S3CUART_RX0,            .uartclk    = 0,            .fifosize    = 16,//定义FIFO缓存区大小             .ops        = &s3c24xx_serial_ops,//串口相关操作函数            .flags        = UPF_BOOT_AUTOCONF,            .line        = 0,//线路        }    },    [1] = {//串口1；        .port = {            .lock        = __SPIN_LOCK_UNLOCKED(s3c24xx_serial_ports[1].port.lock),            .iotype        = UPIO_MEM,            .irq        = IRQ_S3CUART_RX1,            .uartclk    = 0,            .fifosize    = 16,            .ops        = &s3c24xx_serial_ops,            .flags        = UPF_BOOT_AUTOCONF,            .line        = 1,        }    },#if CONFIG_SERIAL_SAMSUNG_UARTS > 2    [2] = {        .port = {            .lock        = __SPIN_LOCK_UNLOCKED(s3c24xx_serial_ports[2].port.lock),            .iotype        = UPIO_MEM,            .irq        = IRQ_S3CUART_RX2,            .uartclk    = 0,            .fifosize    = 16,            .ops        = &s3c24xx_serial_ops,            .flags        = UPF_BOOT_AUTOCONF,            .line        = 2,        }    },#endif#if CONFIG_SERIAL_SAMSUNG_UARTS > 3    [3] = {        .port = {            .lock        = __SPIN_LOCK_UNLOCKED(s3c24xx_serial_ports[3].port.lock),            .iotype        = UPIO_MEM,            .irq        = IRQ_S3CUART_RX3,            .uartclk    = 0,            .fifosize    = 16,            .ops        = &s3c24xx_serial_ops,            .flags        = UPF_BOOT_AUTOCONF,            .line        = 3,        }    }#endif};

 

在xxx_probe()中：

struct s3c24xx_uart_port *ourport;//s3c24xx_uart_port封装了uart_portourport = &s3c24xx_serial_ports[dev->id];//s3c24xx_serial_ports是s3c24xx_uart_port结构体类型的

 

【3】uart_ops定义了针对UART的一系列操作，

/* * This structure describes all the operations that can be * done on the physical hardware. */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        (*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 *);    void        (*pm)(struct uart_port *, unsigned int state,                  unsigned int oldstate);    int        (*set_wake)(struct uart_port *, unsigned int state);    void        (*wake_peer)(struct uart_port *);    /*     * 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    void    (*poll_put_char)(struct uart_port *, unsigned char);    int        (*poll_get_char)(struct uart_port *);#endif};

 

 

//一般来说，实现下面的成员函数是UART驱动的主体工作 static struct uart_ops s3c24xx_serial_ops ={      .pm         =s3c24xx_serial_pm,   //电源管理函数      .tx_empty       = s3c24xx_serial_tx_empty, //检车发送FIFO缓冲区是否空      .get_mctrl       = s3c24xx_serial_get_mctrl, //是否串口流控      .set_mctrl       = s3c24xx_serial_set_mctrl, //是否设置串口流控cts       .stop_tx   =s3c24xx_serial_stop_tx,  //停止发送       .start_tx   =s3c24xx_serial_start_tx,  //启动发送       .stop_rx   =s3c24xx_serial_stop_rx,   //停止接收      .enable_ms     = s3c24xx_serial_enable_ms, //空函数      .break_ctl       = s3c24xx_serial_break_ctl,   //发送break信号       .startup    =s3c24xx_serial_startup,   //串口发送/接收，以及中断申请初始配置函数      .shutdown       = s3c24xx_serial_shutdown,  //关闭串口      .set_termios    = s3c24xx_serial_set_termios,//串口clk,波特率，数据位等参数设置      .type             = s3c24xx_serial_type,  // CPU类型关于串口       .release_port   =s3c24xx_serial_release_port,  //释放串口       .request_port   =s3c24xx_serial_request_port, //申请串口      .config_port    = s3c24xx_serial_config_port,  //串口的一些配置信息info      .verify_port    = s3c24xx_serial_verify_port,  //串口检测      .wake_peer    = s3c24xx_serial_wake_peer,};

 

 而在serial_core.c中定义了tty_operations的实例，包含uart_open()；uart_close()；uart_send_xchar()等成员函数，这些函数借助uart_ops结构体中的成员函数来完成具体的操作。

static const struct tty_operations uart_ops = {    .open        = uart_open,    .close        = uart_close,    .write        = uart_write,    .put_char    = uart_put_char,    .flush_chars    = uart_flush_chars,    .write_room    = uart_write_room,    .chars_in_buffer= uart_chars_in_buffer,    .flush_buffer    = uart_flush_buffer,    .ioctl        = uart_ioctl,    .throttle    = uart_throttle,    .unthrottle    = uart_unthrottle,    .send_xchar    = uart_send_xchar,    .set_termios    = uart_set_termios,    .set_ldisc    = uart_set_ldisc,    .stop        = uart_stop,    .start        = uart_start,    .hangup        = uart_hangup,    .break_ctl    = uart_break_ctl,    .wait_until_sent= uart_wait_until_sent,#ifdef CONFIG_PROC_FS    .proc_fops    = &uart_proc_fops,#endif    .tiocmget    = uart_tiocmget,    .tiocmset    = uart_tiocmset,#ifdef CONFIG_CONSOLE_POLL    .poll_init    = uart_poll_init,    .poll_get_char    = uart_poll_get_char,    .poll_put_char    = uart_poll_put_char,#endif};

 

从下面的例子中可以看出串口核心层的tty_operations与uart_ops的关系：

/* * This function is used to send a high-priority XON/XOFF character to * the device */static void uart_send_xchar(struct tty_struct *tty, char ch){    struct uart_state *state = tty->driver_data;    struct uart_port *port = state->uart_port;    unsigned long flags;    if (port->ops->send_xchar)/*如果uart_ops中实现了send_xchar成员函数*/        port->ops->send_xchar(port, ch);    else {        port->x_char = ch;        if (ch) {            spin_lock_irqsave(&port->lock, flags);            port->ops->start_tx(port);            spin_unlock_irqrestore(&port->lock, flags);        }    }}

 

这个例子的调用关系如下:

send_xchar  ---->  uart_send_xchar ---->  start_tx  --->  s3c24xx_serial_start_tx