关于linux下UART串口编程的困惑

之前对struct termios结构体的几个字段一直困惑，主要不知道它的作用，内核对应的struct ktermios结构体如下

struct ktermios {tcflag_t c_iflag;       /* input mode flags */tcflag_t c_oflag;     /* output mode flags */tcflag_t c_cflag;        /* control mode flags */tcflag_t c_lflag;       /* local mode flags */cc_t c_cc[NCCS];      /* control characters */cc_t c_line;            /* line discipline (== c_cc[19]) */speed_t c_ispeed;      /* input speed */speed_t c_ospeed;      /* output speed */
};

其中的tcflag_t c_cflag字段是控制字段，包括奇偶校验、停止位...，应用层在调用tcsetattr()函数时最终调用串口驱动nuc970_serial.c的驱动函数，如下

static void
nuc970serial_set_termios(struct uart_port *port, struct ktermios *termios,struct ktermios *old)
{struct uart_nuc970_port *up = (struct uart_nuc970_port *)port;unsigned int lcr = 0;unsigned long flags;unsigned int baud, quot;switch (termios->c_cflag & CSIZE) {case CS5:lcr = 0;break;case CS6:lcr |= 1;break;case CS7:lcr |= 2;break;default:case CS8:lcr |= 3;break;}if (termios->c_cflag & CSTOPB)lcr |= NSB;if (termios->c_cflag & PARENB)lcr |= PBE;if (!(termios->c_cflag & PARODD))lcr |= EPE;if (termios->c_cflag & CMSPAR)lcr |= SPE;baud = uart_get_baud_rate(port, termios, old,port->uartclk / 16 / 0xffff,port->uartclk / 16);quot = nuc970serial_get_divisor(port, baud);/** Ok, we're now changing the port state.  Do it with* interrupts disabled.*/spin_lock_irqsave(&up->port.lock, flags);up->port.read_status_mask = RX_OVER_IF /*| UART_LSR_THRE | UART_LSR_DR*/;if (termios->c_iflag & INPCK)up->port.read_status_mask |= FEF | PEF;if (termios->c_iflag & (BRKINT | PARMRK))up->port.read_status_mask |= BIF;/** Characteres to ignore*/up->port.ignore_status_mask = 0;if (termios->c_iflag & IGNPAR)up->port.ignore_status_mask |= FEF | PEF;if (termios->c_iflag & IGNBRK) {up->port.ignore_status_mask |= BIF;/** If we're ignoring parity and break indicators,* ignore overruns too (for real raw support).*/if (termios->c_iflag & IGNPAR)up->port.ignore_status_mask |= RX_OVER_IF;}if (termios->c_cflag & CRTSCTS)up->mcr |= UART_MCR_AFE;elseup->mcr &= ~UART_MCR_AFE;nuc970serial_set_mctrl(&up->port, up->port.mctrl);serial_out(up, UART_REG_BAUD, quot | 0x30000000);serial_out(up, UART_REG_LCR, lcr);     spin_unlock_irqrestore(&up->port.lock, flags);}

可以看到在上面有对termios->c_cflag字段进行判断，最终转换为up->port.ignore_status_mask屏蔽字，关于该屏蔽字的功能，主要解决串口驱动在接收到数据时，出现奇偶校验错误的判断，可以看下如下的串口驱动的接收函数

static void
receive_chars(struct uart_nuc970_port *up)
{unsigned char ch;unsigned int fsr;int max_count = 256;char flag;do {ch = (unsigned char)serial_in(up, UART_REG_RBR);fsr = serial_in(up, UART_REG_FSR);flag = TTY_NORMAL;up->port.icount.rx++;if (unlikely(fsr & (BIF | FEF | PEF | RX_OVER_IF))) {if (fsr & BIF) {serial_out(up, UART_REG_FSR, BIF);up->port.icount.brk++;if (uart_handle_break(&up->port))continue;}if (fsr & FEF) {serial_out(up, UART_REG_FSR, FEF);up->port.icount.parity++;}if (fsr & PEF) {serial_out(up, UART_REG_FSR, PEF);up->port.icount.frame++;}if (fsr & RX_OVER_IF) {serial_out(up, UART_REG_FSR, RX_OVER_IF);up->port.icount.overrun++;}// FIXME: check port->read_status_mask to determin report flagsif (fsr & BIF)flag = TTY_BREAK;if (fsr & PEF)flag = TTY_PARITY;if (fsr & FEF)flag = TTY_FRAME;}if (uart_handle_sysrq_char(&up->port, ch))continue;uart_insert_char(&up->port, fsr, RX_OVER_IF, ch, flag);} while (!(fsr & RX_EMPTY) && (max_count-- > 0));spin_lock(&up->port.lock);tty_flip_buffer_push(&up->port.state->port); spin_unlock(&up->port.lock);
}

通过如下函数进行奇偶校验

uart_insert_char(&up->port, fsr, RX_OVER_IF, ch, flag);

void uart_insert_char(struct uart_port *port, unsigned int status,unsigned int overrun, unsigned int ch, unsigned int flag)
{struct tty_port *tport = &port->state->port;if ((status & port->ignore_status_mask & ~overrun) == 0)if (tty_insert_flip_char(tport, ch, flag) == 0)++port->icount.buf_overrun;/** Overrun is special.  Since it's reported immediately,* it doesn't affect the current character.*/if (status & ~port->ignore_status_mask & overrun)if (tty_insert_flip_char(tport, 0, TTY_OVERRUN) == 0)++port->icount.buf_overrun;
}

在该函数内部，可以看到前面初始化的成员ignore_status_mask。

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