学习笔记 --- LINUX TTY子系统框架分析

入口：tty_init

1 创建tty_cdev字符设备
cdev_init     /*设置字符设备操作函数为tty_fops*/
cdev_add        /* 注册字符设备*/

2 创建console_cdev字符设备（用于终端打印）
cdev_init     /*设置字符设备操作函数为console_fops*/
cdev_add      /* 注册字符设备*/

操作tty就是操作字符设备那样，这里以open为例说明。

打开一个tty设备时，调用tty_fops里的：

open
ret = __tty_open(inode, filp);
dev_t device = inode->i_rdev; //通过设备节点找到设备号
driver = get_tty_driver(device, &index)  //通过设备号，找到驱动与index
list_for_each_entry(p, &tty_drivers, tty_drivers) {  //遍历链表
dev_t base = MKDEV(p->major, p->minor_start);
if (device < base || device >= base + p->num)
continue;
*index = device - base;
return tty_driver_kref_get(p);
}
tty = tty_init_dev(driver, index, 0)  //用driver数据初始化一个新的tty_struct
tty = alloc_tty_struct();
initialize_tty_struct(tty, driver, idx);
tty->driver = driver;
tty->ops = driver->ops;
tty->index = idx;
retval = tty_driver_install_tty(driver, tty); //安装tty_struct
retval = tty->ops->open(tty, filp);   //driver->ops里面的open

上面可以知道：操作tty时，通过设备节点找到设备号，再找到驱动，再调用驱动的抽象操作driver->ops，那么这个驱动怎样注册的？

以uart为例，注册驱动时：

ret = uart_register_driver(&s3c24xx_uart_drv)  //uart_driver转换成tty_driver再注册
normal  = alloc_tty_driver(drv->nr);
drv->tty_driver = normal;
normal->driver_state    = drv; //s3c24xx_uart_drv设置为tty_driver的driver_state
tty_set_operations(normal, &uart_ops)
driver->ops = op;  //设置好驱动的操作抽象：driver->ops
retval = tty_register_driver(normal)
list_add(&driver->tty_drivers, &tty_drivers);  //添加到链表

上面的操作抽象driver->ops里面的接口最终调用normal->driver_state的操作函数，而normal->driver_state= drv，所以这里调用的s3c24xx_uart_drv；

比如之前说过再open一个tty时，调用的是driver->ops里面的open，这里就是uart_ops里面的uart_open，这个uart_open是怎么找到他对应的硬件操作的？

uart_open
struct uart_driver *drv = (struct uart_driver *)tty->driver->driver_state
/*driver_state就是s3c24xx_uart_drv，在uart_register_driver(&s3c24xx_uart_drv)的时候设置的:normal->driver_state=drv;*/
line = tty->index
state = uart_get(drv, line);  //state = drv->state + line;
uart_startup
struct uart_port *uport = state->uart_port;
retval = uport->ops->startup(uport); //uart_open的实际硬件操作