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_port ourport = &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