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研发客服尽管一个特定的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;