一、应用程序中write函数到底层驱动历程

和前文提到的一样，首先先注册串口，使用uart_register_driver函数，依次分别为tty_register_driver，cdev_init函数，找到使用的file_operations，即应用程序与tty架构的统一接口。步骤不再赘述。

static const struct file_operations tty_fops ={

.llseek=no_llseek,

.read=tty_read,

.write=tty_write,

.poll=tty_poll,

.unlocked_ioctl=tty_ioctl,

.compat_ioctl=tty_compat_ioctl,

.open=tty_open,

.release=tty_release,

.fasync=tty_fasync,

};

tty_write函数

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;

...

ret= do_tty_write(ld->ops->write, tty, file, buf, count);

...

}

这里通过do_tty_write函数调用到了线路规程(ldisc)中的函数，结构名为tty_ldisc_N_TTY。

struct tty_ldisc_ops tty_ldisc_N_TTY ={

.magic=TTY_LDISC_MAGIC,

.name= "n_tty",

.open=n_tty_open,

.close=n_tty_close,

.flush_buffer=n_tty_flush_buffer,

.chars_in_buffer=n_tty_chars_in_buffer,

.read=n_tty_read,

.write=n_tty_write,

.ioctl=n_tty_ioctl,

.set_termios=n_tty_set_termios,

.poll=n_tty_poll,

.receive_buf=n_tty_receive_buf,

.write_wakeup=n_tty_write_wakeup

};

n_tty_write函数

static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,const unsigned char *buf, size_t nr)

{const unsigned char *b =buf;

DECLARE_WAITQUEUE(wait, current);intc;

ssize_t retval= 0;

...

c = tty->ops->write(tty, b, nr);

...

}

ops为struct tty_operations类型，由上文可知该结构名为

static const struct tty_operations uart_ops ={

....write=uart_write,

...};

uart_write函数

static int uart_write(struct tty_struct *tty,const unsigned char *buf, intcount)

{struct uart_state *state = tty->driver_data;struct uart_port *port;struct circ_buf *circ;

unsignedlongflags;int c, ret = 0;

...

uart_start(tty);

...

}

uart_start函数中又调用了__uart_start函数

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);

}

这里的port就是uart_port类型的了，终于到达底层驱动了，好累。。又是这个数组，同样的函数操作集

static struct s3c24xx_uart_port s3c24xx_serial_ports[CONFIG_SERIAL_SAMSUNG_UARTS] ={

[0] ={

.port={

.lock = __SPIN_LOCK_UNLOCKED(s3c24xx_serial_ports[0].port.lock),

.iotype=UPIO_MEM,

.irq=IRQ_S3CUART_RX0,

.uartclk= 0,

.fifosize= 16,

.ops= &s3c24xx_serial_ops,

.flags=UPF_BOOT_AUTOCONF,

.line= 0,

}

},

...

}

所以在底层驱动与之对应的就是s3c24xx_serial_start_tx这个函数。层层追溯下来，最终与应用程序中的write函数千里相会。。。

static void s3c24xx_serial_start_tx(struct uart_port *port)

{struct s3c24xx_uart_port *ourport =to_ourport(port);if (!tx_enabled(port)) {if (port->flags &UPF_CONS_FLOW)

s3c24xx_serial_rx_disable(port);

enable_irq(ourport->tx_irq);

tx_enabled(port)= 1;

}

}

但是要让各位看官失望了。这个函数很简单，功能上来说就是如果没有打开发送使能就去打开。那么串口驱动又是在什么地方去完成发送数据相关的操作呢？

预知后事如何，且听下回分析。

如果有疑问或建议，欢迎指出。

原文：http://www.cnblogs.com/51qianrushi/p/4324845.html