接前一篇文章：RS485在Linux内核（驱动）及全志T113平台上的实现（3）
本文内容参考：
RS485 Serial Communications --- The Linux Kernel documentation
Linux环境下RS-485驱动程序开发与实现-CSDN博客
【imx6ul】linux下rs485的使用_linux rs485-CSDN博客
全志R40 串口485 控制操作_485发送控制脚-CSDN博客
Allwinner T113 UART转485_t113 485配置-CSDN博客
特此致谢！
二、RS485内核（驱动）核心结构
上一回结合官方文档（RS485 Serial Communications --- The Linux Kernel documentation）讲完了核心数据结构struct rs485_serial。本回对其它相关结构进行讲解。
前文书已经讲过：
之前将重点放在了struct rs485_serial上，现在重点关注struct uart_port以及其中的rs485_config。
先来看struct uart_port。其在kernel/linux-5.10-origin/include/linux/serial_core.h中定义，如下：
cpp
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_ldisc)(struct uart_port *,
struct ktermios *);
unsigned int (*get_mctrl)(struct uart_port *);
void (*set_mctrl)(struct uart_port *, unsigned int);
unsigned int (*get_divisor)(struct uart_port *,
unsigned int baud,
unsigned int *frac);
void (*set_divisor)(struct uart_port *,
unsigned int baud,
unsigned int quot,
unsigned int quot_frac);
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);
int (*iso7816_config)(struct uart_port *,
struct serial_iso7816 *iso7816);
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 quirks; /* internal quirks */
#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) /* driver-specific */
#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 */
#define UPIO_MEM16 (SERIAL_IO_MEM16) /* 16b little endian */
/* quirks must be updated while holding port mutex */
#define UPQ_NO_TXEN_TEST BIT(0)
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 */
/* 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
* 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_MAGIC_MULTIPLIER ((__force upf_t) ASYNC_MAGIC_MULTIPLIER /* 16 */ )
#define UPF_NO_THRE_TEST ((__force upf_t) (1 << 19))
/* 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_FULL_PROBE ((__force upf_t) (1ULL << 32))
#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))
#define UPSTAT_SYNC_FIFO ((__force upstat_t) (1 << 5))
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 long sysrq; /* sysrq timeout */
unsigned int sysrq_ch; /* char for sysrq */
unsigned char has_sysrq;
unsigned char sysrq_seq; /* index in sysrq_toggle_seq */
unsigned char hub6; /* this should be in the 8250 driver */
unsigned char suspended;
unsigned char console_reinit;
const char *name; /* port name */
struct attribute_group *attr_group; /* port specific attributes */
const struct attribute_group **tty_groups; /* all attributes (serial core use only) */
struct serial_rs485 rs485;
struct gpio_desc *rs485_term_gpio; /* enable RS485 bus termination */
struct serial_iso7816 iso7816;
void *private_data; /* generic platform data pointer */
};struct uart_port是Linux内核串口子系统中对物理UART硬件的抽象，其整合了UART端口的硬件参数、寄存器地址、中断信息、底层操作函数等所有关键信息，是连接串口驱动和硬件的桥梁。所有串口驱动（如8250、ttySAC等）都会基于此结构来实现具体的硬件适配。
struct uart_port中关键成员（字段）的说明如下：
- spinlock_t lock
自旋锁，保护串口（端口）操作的原子性。
- unsigned long iobase
IO端口基地址（IO映射方式）。
- unsigned char __iomem *membase
IO内存映射基地址，经映射（如ioremap、MMIO等）后的IO内存虚拟基地址。
- unsigned int irq
串口对应的中断号。
- unsigned long irqflags
串口中断相关标志（上升沿/下降沿触发、高电平/低电平触发等）。
- unsigned int uartclk
串口时钟（频率）。用于计算计算波特率。
- unsigned int fifosize
串口发送缓冲区大小。
- unsigned char x_char
XON/XOFF字符。XON/XOFF是一种基于软件实现的异步串行通信流控制协议，用于解决数据传输速率不匹配问题。其通过发送XOFF（ASCII字符DC3/Ctrl+S）暂停发送方传输，发送XON（ASCII字符DC1/Ctrl+Q）恢复传输，动态调节数据流以防止接收方缓冲区溢出。
- unsigned char regshift
寄存器地址偏移量。
- unsigned char iotype
IO访问方式。如UPIO_PORT（IO）、UPIO_MEM（MMIO）、UPIO_MEM32等。
struct uart_port更多关键成员的说明请看下回。