Linux 中将串口驱动进行了分层，如图：
本节讲解与底层硬件密切相关的层，以S3C2440为例剖析：
实现文件有：/drivers/serial/samsung.c /drivers/serial/samsung.h /drivers/serial/s3c2440.c (kernel 2.6.28)，Serial Core层在/drivers/serial/serial_core.c主要文件中。
硬件驱动层与Serial-Core沟通数据结构如下：
1. uart_driver包含了串口设备名、串口驱动名、主次设备号、串口控制台(可选)等信息，还封装了tty_driver(底层串口驱动无需关心tty_driver)。
structuart_driver {
struct module*owner;/*拥有该uart_driver的模块,一般为THIS_MODULE */
constchar*driver_name;/*串口驱动名，串口设备文件名以驱动名为基础 */
constchar*dev_name;/*串口设备名 */
int major;/*主设备号 */
int minor;/*次设备号 */
int nr;/*该uart_driver支持的串口个数(最大) */
struct console*cons;/*其对应的console.若该uart_driver支持serial console,否则为NULL */
/*
* 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;
};
2. uart_port用于描述串口端口的I/O端口或I/O内存地址、FIFO大小、端口类型、串口时钟等信息。实际上，一个uart_port实例对应一个串口设备。
structuart_port {
spinlock_t lock;/*串口端口锁 */
unsignedint iobase;/* IO端口基地址 */
unsignedchar __iomem*membase;/* IO内存基地址,经映射(如ioremap)后的IO内存虚拟基地址 */
unsignedint irq;/*中断号 */
unsignedint uartclk;/*串口时钟 */
unsignedint fifosize;/*串口FIFO缓冲大小 */
unsignedchar x_char;/* xon/xoff字符 */
unsignedchar regshift;/*寄存器位移 */
unsignedchar iotype;/* IO访问方式 */
unsignedchar unused1;
#define UPIO_PORT(0)/* IO端口 */
#define UPIO_HUB6(1)
#define UPIO_MEM(2)/* IO内存 */
#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 */
unsignedint read_status_mask;/*关心的Rx error status */
unsignedint ignore_status_mask;/*忽略的Rx error status */
struct uart_info*info; //重要，见下面
struct uart_icount icount; /* 计数器 uart_icount为串口信息计数器，包含了发送字符计数、接收字符计数等。在串口的发送中断处理函数和接收中断处理函数中，我们需要管理这些计数。*/
struct console *cons;/* console结构体 */
#ifdefCONFIG_SERIAL_CORE_CONSOLE
unsignedlong sysrq;/* sysrq timeout */
#endif
upf_t flags;
#define UPF_FOURPORT((__forceupf_t)(1<< 1))
#define UPF_SAK((__forceupf_t)(1<< 2))
#define UPF_SPD_MASK((__forceupf_t)(0x1030))
#define UPF_SPD_HI((__forceupf_t)(0x0010))
#define UPF_SPD_VHI((__forceupf_t)(0x0020))
#define UPF_SPD_CUST((__forceupf_t)(0x0030))
#define UPF_SPD_SHI((__forceupf_t)(0x1000))
#define UPF_SPD_WARP((__forceupf_t)(0x1010))
#define UPF_SKIP_TEST((__forceupf_t)(1<< 6))
#define UPF_AUTO_IRQ((__forceupf_t)(1<< 7))
#define UPF_HARDPPS_CD((__forceupf_t)(1<< 11))
#define UPF_LOW_LATENCY((__forceupf_t)(1<< 13))
#define UPF_BUGGY_UART((__forceupf_t)(1<< 14))
#define UPF_MAGIC_MULTIPLIER((__force upf_t)(1<< 16))
#define UPF_CONS_FLOW((__forceupf_t)(1<< 23))
#define UPF_SHARE_IRQ((__forceupf_t)(1<< 24))
#define UPF_BOOT_AUTOCONF((__forceupf_t)(1<< 28))
#define UPF_FIXED_PORT((__forceupf_t)(1<< 29))
#define UPF_DEAD((__forceupf_t)(1<< 30))
#define UPF_IOREMAP((__forceupf_t)(1<< 31))
#define UPF_CHANGE_MASK((__forceupf_t)(0x17fff))
#define UPF_USR_MASK((__forceupf_t)(UPF_SPD_MASK|UPF_LOW_LATENCY))
unsigned int mctrl;/*当前的moden设置 */
unsigned int timeout;/* character-based timeout */
unsigned int type;/*端口类型 */
const structuart_ops *ops;/*串口端口操作函数集 */
unsigned int custom_divisor;
unsigned int line;/*端口索引 */
resource_size_t mapbase;/*IO内存物理基地址，可用于ioremap */
struct device *dev;/*父设备 */
unsigned char hub6;/* this should be in the 8250 driver */
unsigned char suspended;
unsigned char unused[2];
void*private_data;/*端口私有数据,一般为platform数据指针 */
};
(1) Uart_structuart_icount {
__u32 cts;
__u32 dsr;
__u32 rng;
__u32 dcd;
__u32 rx;/*发送字符计数 */
__u32 tx;/*接受字符计数 */
__u32 frame;/*帧错误计数 */
__u32 overrun;/* Rx FIFO溢出计数 */
__u32 parity;/* 帧校验错误计数 */
__u32 brk;/* break计数 */
__u32 buf_overrun;
};
uart_info有两个成员在底层串口驱动会用到：xmit和tty。用户空间程序通过串口发送数据时，上层驱动将用户数据保存在xmit；而串口发送中断处理函数就是通过xmit获取到用户数据并将它们发送出去。串口接收中断处理函数需要通过tty将接收到的数据传递给行规则层。
structuart_info {
structtty_struct *tty; //接受
structcirc_buf xmit; //发送
uif_t flags;
/*
* Definitions for info->flags. These are _private_ to serial_core,and
* are specific to this structure. They may be queried by low leveldrivers.
*/
#define UIF_CHECK_CD((__force uif_t)(1<< 25))
#define UIF_CTS_FLOW((__force uif_t)(1<< 26))
#define UIF_NORMAL_ACTIVE((__force uif_t)(1<< 29))
#define UIF_INITIALIZED((__force uif_t)(1<< 31))
#define UIF_SUSPENDED((__force uif_t)(1<< 30))
int blocked_open;
structtasklet_struct tlet; //上层驱动任务等待队列的
wait_queue_head_t open_wait;
wait_queue_head_t delta_msr_wait;
};
3. Uart_port中有一个重要的uart_ops，底层硬件需要实现：
structuart_ops {
unsignedint(*tx_empty)(struct uart_port *); /*串口的Tx FIFO缓存是否为空 */
void(*set_mctrl)(struct uart_port *,unsignedint mctrl);/*设置串口modem控制 */
unsignedint(*get_mctrl)(struct uart_port *);/*获取串口modem控制 */
void(*stop_tx)(struct uart_port*);/* 禁止串口发送数据 */
void(*start_tx)(struct uart_port*);/* 使能串口发送数据 */
void(*send_xchar)(struct uart_port*,char ch);/*发送xChar */
void(*stop_rx)(struct uart_port*);/* 禁止串口接收数据 */
void(*enable_ms)(struct uart_port*);/* 使能modem的状态信号 */
void(*break_ctl)(struct uart_port*,int ctl);/*设置break信号 */
int(*startup)(struct uart_port*);/* 启动串口,应用程序打开串口设备文件时,该函数会被调用 */
void(*shutdown)(struct uart_port*);/* 关闭串口,应用程序关闭串口设备文件时,该函数会被调用 */
void(*set_termios)(struct uart_port*,struct ktermios*new,struct ktermios*old);/*设置串口参数 */
void(*pm)(struct uart_port*,unsignedint state,
unsignedint oldstate);/*串口电源管理 */
int(*set_wake)(struct uart_port*,unsignedint state);/* */
constchar*(*type)(struct uart_port *);/*返回一描述串口类型的字符串 */
void(*release_port)(struct uart_port*);/* 释放串口已申请的IO端口/IO内存资源,必要时还需iounmap */
int(*request_port)(struct uart_port*);/* 申请必要的IO端口/IO内存资源,必要时还可以重新映射串口端口 */
void(*config_port)(struct uart_port*,int);/*执行串口所需的自动配置 */
int(*verify_port)(struct uart_port*,struct serial_struct*);/* 核实新串口的信息 */
int(*ioctl)(struct uart_port*,unsignedint,unsignedlong);/* IO控制 */
};
4. uart_driver通过Serial Core层的
int uart_register_driver(struct uart_driver*drv)
向Core注册,通过int uart_add_one_port(struct uart_driver*drv,struct uart_port*port)向该驱动添加uart_port。
Serial Core层还导出如下函数共上层或下层调用：
EXPORT_SYMBOL(uart_match_port); //判断两个uart_port是否相等
EXPORT_SYMBOL(uart_write_wakeup); //该函数常在中断处理函数中调用用来唤醒上层因向串口端口写数据而阻塞的进程
EXPORT_SYMBOL(uart_register_driver); //已解释
EXPORT_SYMBOL(uart_unregister_driver);
EXPORT_SYMBOL(uart_suspend_port); //用于挂起特定的串口端口
EXPORT_SYMBOL(uart_resume_port);
EXPORT_SYMBOL(uart_add_one_port); //已解释
EXPORT_SYMBOL(uart_remove_one_port);
具体实现请参照/drivers/serial/samsung.c /drivers/serial/s3c2440.c
下一节重点介绍Serial Core层