/*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;//tempif (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);
}