Linux UART 驱动框架比较简单，不需要设备驱动，只需要UART设备驱动即可。
一、Linux UART驱动框架
1. uart_driver结构体
Linux将 UART 驱动抽象为 uart_driver结构体，定义在include/linux/serial_core.h文件中，如下：
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;
};
- owner：模块拥有者
- driver_name：驱动名称
- dev_name：设备名称
- major：主设备号
- minor：次设备号
- nr：设备数
- cons：控制台
uart_driver结构体注册/注销API如下。
（1）向内核注册uart_driver
int uart_register_driver(struct uart_driver *uart);
（2）从内核注销uart_driver
void uart_unregister_driver(struct uart_driver *uart);
2. uart_port结构体
uart_port用于表示一个具体的串口，也定义在该文件中，如下：
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);
void (*set_termios)(struct uart_port *,
struct ktermios *new,
struct ktermios *old);
void (*set_mctrl)(struct uart_port *, unsigned int);
int (*startup)(struct uart_port *port);
void (*shutdown)(struct uart_port *port);
void (*throttle)(struct uart_port *port);
void (*unthrottle)(struct uart_port *port);
int (*handle_irq)(struct uart_port *);
void (*pm)(struct uart_port *, unsigned int state,
unsigned int old);
void (*handle_break)(struct uart_port *);
int (*rs485_config)(struct uart_port *,
struct serial_rs485 *rs485);
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 (SERIAL_IO_PORT) /* 8b I/O port access */
#define UPIO_HUB6 (SERIAL_IO_HUB6) /* Hub6 ISA card */
#define UPIO_MEM (SERIAL_IO_MEM) /* 8b MMIO access */
#define UPIO_MEM32 (SERIAL_IO_MEM32) /* 32b little endian */
#define UPIO_AU (SERIAL_IO_AU) /* Au1x00 and RT288x type IO */
#define UPIO_TSI (SERIAL_IO_TSI) /* Tsi108/109 type IO */
#define UPIO_MEM32BE (SERIAL_IO_MEM32BE) /* 32b big endian */
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
/* flags must be updated while holding port mutex */
upf_t flags;
/*
* These flags must be equivalent to the flags defined in
* include/uapi/linux/tty_flags.h which are the userspace definitions
* assigned from the serial_struct flags in uart_set_info()
* [for bit definitions in the UPF_CHANGE_MASK]
*
* Bits [0..UPF_LAST_USER] are userspace defined/visible/changeable
* except bit 15 (UPF_NO_TXEN_TEST) which is masked off.
* The remaining bits are serial-core specific and not modifiable by
* userspace.
*/
#define UPF_FOURPORT ((__force upf_t) ASYNC_FOURPORT /* 1 */ )
#define UPF_SAK ((__force upf_t) ASYNC_SAK /* 2 */ )
#define UPF_SPD_HI ((__force upf_t) ASYNC_SPD_HI /* 4 */ )
#define UPF_SPD_VHI ((__force upf_t) ASYNC_SPD_VHI /* 5 */ )
#define UPF_SPD_CUST ((__force upf_t) ASYNC_SPD_CUST /* 0x0030 */ )
#define UPF_SPD_WARP ((__force upf_t) ASYNC_SPD_WARP /* 0x1010 */ )
#define UPF_SPD_MASK ((__force upf_t) ASYNC_SPD_MASK /* 0x1030 */ )
#define UPF_SKIP_TEST ((__force upf_t) ASYNC_SKIP_TEST /* 6 */ )
#define UPF_AUTO_IRQ ((__force upf_t) ASYNC_AUTO_IRQ /* 7 */ )
#define UPF_HARDPPS_CD ((__force upf_t) ASYNC_HARDPPS_CD /* 11 */ )
#define UPF_SPD_SHI ((__force upf_t) ASYNC_SPD_SHI /* 12 */ )
#define UPF_LOW_LATENCY ((__force upf_t) ASYNC_LOW_LATENCY /* 13 */ )
#define UPF_BUGGY_UART ((__force upf_t) ASYNC_BUGGY_UART /* 14 */ )
#define UPF_NO_TXEN_TEST ((__force upf_t) (1 << 15))
#define UPF_MAGIC_MULTIPLIER ((__force upf_t) ASYNC_MAGIC_MULTIPLIER /* 16 */ )
/* Port has hardware-assisted h/w flow control */
#define UPF_AUTO_CTS ((__force upf_t) (1 << 20))
#define UPF_AUTO_RTS ((__force upf_t) (1 << 21))
#define UPF_HARD_FLOW ((__force upf_t) (UPF_AUTO_CTS | UPF_AUTO_RTS))
/* Port has hardware-assisted s/w flow control */
#define UPF_SOFT_FLOW ((__force upf_t) (1 << 22))
#define UPF_CONS_FLOW ((__force upf_t) (1 << 23))
#define UPF_SHARE_IRQ ((__force upf_t) (1 << 24))
#define UPF_EXAR_EFR ((__force upf_t) (1 << 25))
#define UPF_BUG_THRE ((__force upf_t) (1 << 26))
/* 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 0x17fff
#define UPF_CHANGE_MASK ((__force upf_t) __UPF_CHANGE_MASK)
#define UPF_USR_MASK ((__force upf_t) (UPF_SPD_MASK|UPF_LOW_LATENCY))
#if __UPF_CHANGE_MASK > ASYNC_FLAGS
#error Change mask not equivalent to userspace-visible bit defines
#endif
/*
* Must hold termios_rwsem, port mutex and port lock to change;
* can hold any one lock to read.
*/
upstat_t status;
#define UPSTAT_CTS_ENABLE ((__force upstat_t) (1 << 0))
#define UPSTAT_DCD_ENABLE ((__force upstat_t) (1 << 1))
#define UPSTAT_AUTORTS ((__force upstat_t) (1 << 2))
#define UPSTAT_AUTOCTS ((__force upstat_t) (1 << 3))
#define UPSTAT_AUTOXOFF ((__force upstat_t) (1 << 4))
int hw_stopped; /* sw-assisted CTS flow state */
unsigned int mctrl; /* current modem ctrl settings */
unsigned int timeout; /* character-based timeout */
unsigned int type; /* port type */
const struct uart_ops *ops;
unsigned int custom_divisor;
unsigned int line; /* port index */
unsigned int minor;
resource_size_t mapbase; /* for ioremap */
resource_size_t mapsize;
struct device *dev; /* parent device */
unsigned char hub6; /* this should be in the 8250 driver */
unsigned char suspended;
unsigned char irq_wake;
unsigned char unused[2];
struct attribute_group *attr_group; /* port specific attributes */
const struct attribute_group **tty_groups; /* all attributes (serial core use only) */
struct serial_rs485 rs485;
void *private_data; /* generic platform data pointer */
};
在uart_port中，ops成员中包含了具体的串口驱动函数，每个串口都有一个uart_port。
3. ops函数指针实现
uart_ops结构体是 serial_core 和具体的硬件驱动之间最主要的接口，包含了控制硬件的所有方法。
Linux内核最终收发数据调用的都是ops中的函数，定义如下：
/*
* This structure describes all the operations that can be done on the
* physical hardware. See Documentation/serial/driver for details.
*/
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 (*throttle)(struct uart_port *);
void (*unthrottle)(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 *, struct ktermios *);
void (*pm)(struct uart_port *, unsigned int state,
unsigned int oldstate);
/*
* 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
int (*poll_init)(struct uart_port *);
void (*poll_put_char)(struct uart_port *, unsigned char);
int (*poll_get_char)(struct uart_port *);
#endif
};
关于ops成员中每个函数指针的意义，可以参考文档：Documentation/serial/driver。
（1）tx_empty
该函数测试串口的发送FIFO是否为空。如果为空，返回TIOCSER_TEMT，否则返回0，如果该串口不支持此操作，则返回TIOCSER_TEMT。
（2）set_mctrl：这个函数将’port’所描述的端口的调制解调器控制线设置为mctrl所描述的状态。
（3）get_mctrl：返回调制解调器控制输入的当前状态。
（4）stop_tx：停止传输字符。
（5）start_tx：开始传输字符。
（6）send_xchar：发送一个高优先级的字符，甚至是端口已经停止的收。
（7）stop_rx：停止接收字符。
（8）stop_rx：启用modem状态中断。
（9）break_ctl：控制中断信号的传输。
（10）startup：获取任何中断资源并初始化任何底层驱动状态。
（11）shutdown：禁用端口，禁用任何可能生效的中断条件，并释放任何中断资源。
（12）flush_buffer：刷新任何写缓冲区，重置任何DMA状态，并停止任何正在进行的DMA传输。
（13）set_termios：更改端口参数，包括字长、奇偶校验、停止位。
（14）pm：在指定端口上执行电源管理相关操作。
（15）type：返回一个指向描述指定端口的字符串常量的指针，或者返回NULL，在这种情况下，字符串’unknown’被替换。
（16）release_port：释放端口当前使用的内存和IO区域资源。
（17）request_port：请求端口所需的任何内存和IO区域资源。
（18）config_port：执行端口所需的任何自动配置步骤。
（19）verify_port
（20）ioctl
（21）poll_init
（22）poll_put_char
（23）poll_get_char
4. 向uart_driver添加/移除某个uart_port
（1）向uart_driver添加一个uart_port：
int uart_add_one_port(struct uart_driver *reg, struct uart_port *port);
（2）从uart_driver中移除某个uart_port：
int uart_remove_one_port(struct uart_driver *reg, struct uart_port *port);
二、NXP对于UART驱动框架的实现
在设备树中找到uart3节点：
根据兼容性找对应的驱动文件：
grep -nR "fsl,imx21-uart" *
1. UART驱动的platform框架
（1）驱动模块的加载与卸载
可以看到，在驱动模块的加载函数中，调用 uart_register_driver 向内核注册 uart_driver，接着注册platform设备驱动。
在驱动模块的卸载中，首先注销platform设备驱动，然后从内核注销uart_driver。
（2）uart_driver驱动实现
（3）platform驱动框架实现
2. uart_port的添加/移除
在probe挂载函数中，经过一系列操作，最终调用 uart_add_one_port 函数添加了一个uart_port：
在remove卸载函数中，调用 uart_remove_one_port 函数移除对应的 uart_port：
这里添加和移除uart_port来自于imx定义的sport结构体，如下：
3. uart_pos函数指针实现
在probe函数中，对ops成员的初始化如下：
imx_pops的定义如下：
这其中的函数都是imx开头，是与底层SOC的UART外设相关操作函数，比如 imx_tx_empty 这个函数：
