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(duì)于write操作而言，它就是將數(shù)據(jù)copy到環(huán)形緩存區(qū)。然后調(diào)用port->ops->start_tx()將數(shù)據(jù)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(diào)用ldsic->read()讀取read_buf中的內(nèi)容。有對(duì)這部份內(nèi)容不太清楚的，參閱《 linux設(shè)備模型之tty驅(qū)動(dòng)架構(gòu)》.

九：小結(jié)

本小節(jié)是分析serial驅(qū)動(dòng)的基礎(chǔ)。在理解了tty驅(qū)動(dòng)架構(gòu)之后，再來(lái)理解uart驅(qū)動(dòng)架構(gòu)應(yīng)該不是很難。隨著我們?cè)趌inux設(shè)備驅(qū)動(dòng)分析的深入，越來(lái)越深刻的體會(huì)到，linux的設(shè)備驅(qū)動(dòng)架構(gòu)很多都是相通的。只要深刻理解了一種驅(qū)動(dòng)架構(gòu)。舉一反三。也就很容易分析出其它架構(gòu)的驅(qū)動(dòng)了。