circ = state->info->xmit;

if (!circ->buf)

return 0;

spin_lock_irqsave(port->lock, flags);

while (1) {

c = CIRC_SPACE_TO_END(circ->head, circ->tail, UART_XMIT_SIZE);

if (count c)

c = count;

if (c = 0)

break;

memcpy(circ->buf + circ->head, buf, c);

circ->head = (circ->head + c) (UART_XMIT_SIZE - 1);

buf += c;

count -= c;

ret += c;

}

spin_unlock_irqrestore(port->lock, flags);

uart_start(tty);

return ret;

}

Uart_start()代碼如下：

static void uart_start(struct tty_struct *tty)

{

struct uart_state *state = tty->driver_data;

struct uart_port *port = state->port;

unsigned long flags;

spin_lock_irqsave(port->lock, flags);

__uart_start(tty);

spin_unlock_irqrestore(port->lock, flags);

}

static void __uart_start(struct tty_struct *tty)

{

struct uart_state *state = tty->driver_data;

struct uart_port *port = state->port;

if (!uart_circ_empty(state->info->xmit) state->info->xmit.buf

!tty->stopped !tty->hw_stopped)

port->ops->start_tx(port);

}

顯然，對于write操作而言，它就是將數據copy到環形緩存區。然后調用port->ops->start_tx()將數據寫到硬件寄存器。

八：Read操作

Uart的read操作同Tty的read操作相同，即都是調用ldsic->read()讀取read_buf中的內容。有對這部份內容不太清楚的，參閱《 linux設備模型之tty驅動架構》.

九：小結

本小節是分析serial驅動的基礎。在理解了tty驅動架構之后，再來理解uart驅動架構應該不是很難。隨著我們在linux設備驅動分析的深入，越來越深刻的體會到，linux的設備驅動架構很多都是相通的。只要深刻理解了一種驅動架構。舉一反三。也就很容易分析出其它架構的驅動了。