Linux serial构架分析及驱动开发
前面介绍了tty核心分析及tty驱动开发的方法，tty设备包括串口、终端、伪终端三大类，其中终端和伪终端驱动内核都帮我们实现好了，很少需要改动。因此我们主要介绍串口驱动的开发及其在内核中的构架（其核心实现源码主要在/drivers/serial_core.c中），这一节中我们主要分析向内核中加入一个serial驱动用到的数据结构。
serial core是构建在tty core之上的。注册一个串口驱动即在tty core层注册一个tty驱动。下面我们看看串口驱动中用到的两个最重要的数据机构 struct uart_driver 表示一个serial驱动，struct uart_port 表示一个串口端口。
struct uart_driver {
struct module *owner;
const char *driver_name; //驱动名称
const char *dev_name; //设备名基础
int major; //主设备号
int minor; //起始次设备号
int nr; //设备个数
struct console *cons; //关联的控制台
/*
* these are private; the low level driver should not
* touch these; they should be initialised to NULL
*/
struct uart_state *state; //串口驱动操作设备数组
struct tty_driver *tty_driver; //表征串口驱动的tty驱动
};
上面结构中struct uart_state *state指向驱动操作的串口设备相关数据结构，其中struct uart_port *port就是我们下面要介绍的描述串口设备的结构，struct uart_info *info 指向相关联的struct tty_struct 结构和数据发射时环形缓冲区struct circ_buf xmit，即串口打开时的描述信息。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。具体的成员分析我们在后面介绍具体的操作时再分析。
struct uart_port {
spinlock_t lock; /* port lock */ //串口端口锁
unsigned int iobase; /* in/out[bwl] */ //io端口基地址
unsigned char __iomem *membase; /* read/write[bwl] */ //io内存基地址，虚拟地址
unsigned int irq; /* irq number */ //中断号
unsigned int uartclk; /* base uart clock */ //串口时钟
unsigned int fifosize; /* tx fifo size */ //串口fifo缓冲大小
unsigned char x_char; /* xon/xoff char */ //xon/xoff字符
unsigned char regshift; /* reg offset shift */ //j寄存器移位
unsigned char iotype; /* io access style */ //io访问方式
unsigned char unused1;
#define UPIO_PORT (0) //端口
#define UPIO_HUB6 (1)
#define UPIO_MEM (2) //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 */
unsigned int read_status_mask; /* driver specific */ //关心的rx error status
unsigned int ignore_status_mask; /* driver specific */ //忽略的rx error status
struct uart_info *info; /* pointer to parent info */
struct uart_icount icount; /* statistics */
struct console *cons; /* struct console, if any */
#ifdef CONFIG_SERIAL_CORE_CONSOLE
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_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))
#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; //具体端口的相关操作函数
unsigned int custom_divisor;
unsigned int line; /* port index */
resource_size_t mapbase; /* for ioremap */ //io内存物理地址
struct device *dev; /* parent device */
unsigned char hub6; /* this should be in the 8250 driver */
unsigned char suspended;
unsigned char unused[2]; //允许串口收发字符标志
void *private_data; /* generic platform data pointer */
};
uart_iconut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 uart_icount {
__u32 cts;
__u32 dsr;
__u32 rng;
__u32 dcd;
__u32 rx; //接收字符数
__u32 tx; //发送字符数
__u32 frame; //错误帧计数
__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;
};
对于实现一个串口驱动，主要的工作量就是实现struct uart_ops *ops中的各个操作函数。
* This structure describes all the operations that can be
* done on the physical hardware.
*/
struct uart_ops {
unsigned int (*tx_empty)(struct uart_port *); //串口tx FIFO缓存是否为空
void (*set_mctrl)(struct uart_port *, unsigned int mctrl); //设置串口modem控制
unsigned int (*get_mctrl)(struct uart_port *); //获得串口的modem控制
void (*stop_tx)(struct uart_port *); //停止串口发送
void (*start_tx)(struct uart_port *); //使能串口发送
void (*send_xchar)(struct uart_port *, char ch); //发送xchar
void (*stop_rx)(struct uart_port *); //禁止串口接收
void (*enable_ms)(struct uart_port *); //使能modem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
void (*shutdown)(struct uart_port *); //关闭串口
void (*flush_buffer)(struct uart_port *); //刷新缓存
void (*set_termios)(struct uart_port *, struct ktermios *new,
struct ktermios *old); //设置串口参数
void (*set_ldisc)(struct uart_port *); //设置线路规程
void (*pm)(struct uart_port *, unsigned int state,
unsigned int oldstate); //电源管理
int (*set_wake)(struct uart_port *, unsigned int state);
/*
* 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); //执行串口所需的自动配置
int (*verify_port)(struct uart_port *, struct serial_struct *); //核实串口信息
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
};