/*和read的分析过程一样， 我们首先分析tty_write*/

/*最重要的就是do_tty_write函数。 前面都是一些合法性判断*/
static ssize_t tty_write(struct file *file, const char __user *buf,size_t count, loff_t *ppos)
{
	struct inode *inode = file->f_path.dentry->d_inode;
	struct tty_struct *tty = file_tty(file);
 	struct tty_ldisc *ld;
	ssize_t ret;

	if (tty_paranoia_check(tty, inode, "tty_write"))
		return -EIO;
	if (!tty || !tty->ops->write ||
		(test_bit(TTY_IO_ERROR, &tty->flags)))
			return -EIO;
	/* Short term debug to catch buggy drivers */
	if (tty->ops->write_room == NULL)
		printk(KERN_ERR "tty driver %s lacks a write_room method.\n",
			tty->driver->name);
	ld = tty_ldisc_ref_wait(tty);
	if (!ld->ops->write)
		ret = -EIO;
	else
		/*调用tty_ldisc_N_TTY中的write函数*/
		ret = do_tty_write(ld->ops->write, tty, file, buf, count);
	tty_ldisc_deref(ld);
	return ret;
}

/*调用uart_ops中的write函数*/
static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,const unsigned char *buf, size_t nr)
{
	c = tty->ops->write(tty, b, nr);
}

static int uart_write(struct tty_struct *tty,const unsigned char *buf, int count)
{
	struct uart_state *state = tty->driver_data;
	struct uart_port *port;
	struct circ_buf *circ;
	unsigned long flags;
	int c, ret = 0;

	/*
	 * This means you called this function _after_ the port was
	 * closed.  No cookie for you.
	 */
	if (!state) {
		WARN_ON(1);
		return -EL3HLT;
	}

  /*取出所对应的port和循环缓冲buf*/
	port = state->uart_port;
	circ = &state->xmit;

	if (!circ->buf)
		return 0;

	spin_lock_irqsave(&port->lock, flags);
	while (1) {
		/*计算循环缓冲的剩余空间 */
		c = CIRC_SPACE_TO_END(circ->head, circ->tail, UART_XMIT_SIZE);
		if (count < c)
			c = count;
		if (c <= 0)
			break;
		/*拷贝数据到循环缓冲区*/
		memcpy(circ->buf + circ->head, buf, c);
		/*调正循环缓冲head的位置*/
		circ->head = (circ->head + c) & (UART_XMIT_SIZE - 1);
		/*挪动buf当前的指针位置*/
		buf += c;
		count -= c;
		ret += c;
	}
	spin_unlock_irqrestore(&port->lock, flags);

	uart_start(tty);
	return ret;
}

/*判断循环缓冲是否为空等。 然后调用注册驱动时的ops。 也就是s3c24xx_serial_ops*/
static void __uart_start(struct tty_struct *tty)
{
	struct uart_state *state = tty->driver_data;
	struct uart_port *port = state->uart_port;

	if (!uart_circ_empty(&state->xmit) && state->xmit.buf &&
	    !tty->stopped && !tty->hw_stopped)
		port->ops->start_tx(port);
}

/*判断端口是否使能，如果没使能则使能端口。 然后使能tx中断
* 使能tx中断，则当有数据来时，则会触发tx中断，调用中断函数
*/
static void s3c24xx_serial_start_tx(struct uart_port *port)
{
	struct s3c24xx_uart_port *ourport = to_ourport(port);
	static int a =1;//temp
	if (port->line == 3) {
//		printk("485_start_tx\n");

		if(a){
			s3c_gpio_cfgpin(S3C64XX_GPK(5), S3C_GPIO_SFN(1));
			a=0;
		}
		gpio_set_value(S3C64XX_GPK(5), 1);
	}
	if (!tx_enabled(port)) {
		if (port->flags & UPF_CONS_FLOW)
			s3c24xx_serial_rx_disable(port);

		enable_irq(ourport->tx_irq);
		tx_enabled(port) = 1;
	}
}

/*tx中断触发函数*/
static irqreturn_t s3c24xx_serial_tx_chars(int irq, void *id)
{
	struct s3c24xx_uart_port *ourport = id;
	struct uart_port *port = &ourport->port;
	struct circ_buf *xmit = &port->state->xmit;
	int count = 256;

	/*判断x_char是否存在，如果存在则写进UTXH寄存器。清空x_char*/
	if (port->x_char) {
		wr_regb(port, S3C2410_UTXH, port->x_char);
		port->icount.tx++;
		port->x_char = 0;
		goto out;
	}

	/* if there isn't anything more to transmit, or the uart is now
	 * stopped, disable the uart and exit
	*/

	/*判断循环缓冲是否为空，或者tx是否为停止状态。 如果是则停止发送*/
	if (uart_circ_empty(xmit) || uart_tx_stopped(port)) {
		s3c24xx_serial_stop_tx(port);
		goto out;
	}

	/* try and drain the buffer... */

	/*当循环缓冲buff不为空，而且count是大于0的。则进入while循环*/
	while (!uart_circ_empty(xmit) && count-- > 0) {
		
		/*首先读取UFSTAT寄存器，然后判断tx_fifo是否为0.是则退出*/
		if (rd_regl(port, S3C2410_UFSTAT) & ourport->info->tx_fifofull)
			break;

		/*然后将循环buff中的数据读出到UTXH寄存器。然后设置tail的指针位置*/
		wr_regb(port, S3C2410_UTXH, xmit->buf[xmit->tail]);
		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
		port->icount.tx++;
	}

	/*判断循环缓冲中的数据是否小于WAKEUP_CHARS, 小于则启动接受*/
	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
		uart_write_wakeup(port);
		
	/*如果循环buff为空，则停止发送，退出*/
	if (uart_circ_empty(xmit))
		s3c24xx_serial_stop_tx(port);
}