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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); int c; 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, int count) { struct uart_state *state = tty->driver_data; struct uart_port *port; struct circ_buf *circ; unsigned long flags; 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。
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原文地址：http://www.cnblogs.com/51qianrushi/p/4324845.html
