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;
	else
		up->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 flags
			if (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。