一、前言
在linux中，serial也对应着终端你，通常被称为串口终端。在shell上，我们看到得/dev/ttyS*就是串口终端所对应的设备节点。
uart(Universal Asynchronous Receicer and transmitter)即为“通用异步收发器”。它是串口设备驱动的封装层。
二、Uart驱动架构概貌
从上图可以看到，uart设备是继tty_drivers的又一层封装。实际上uart_driver就是对应tty_driver.在它的操作函数中，将操作转入uart_port.
- 在写操作的时候，先将数据放入一个叫做circ_buf的环形缓存区。然后uart_port从缓存区中取数据，将其写入到串口设备中。
- 当uart_port从serial设备接收到数据时，会将设备放入对应line discipline的缓存区中。这样，用户在编写串口驱动的时候，只先要注册一个uart_driver.它的主要作用是定义设备节点号。然后将对设备的各项操作封装在uart_port.驱动工程师没必要关心上层的流程，只需按硬件规范将uart_port中的接口函数完成就可以了。
三、uart驱动代码分析
1、uart驱动注册函数
drivers/tty/serial/serial_core.c
/**
* uart_register_driver - register a driver with the uart core layer
* @drv: low level driver structure
*
* Register a uart driver with the core driver. We in turn register
* with the tty layer, and initialise the core driver per-port state.
*
* We have a proc file in /proc/tty/driver which is named after the
* normal driver.
*
* drv->port should be NULL, and the per-port structures should be
* registered using uart_add_one_port after this call has succeeded.
*/
int uart_register_driver(struct uart_driver *drv)
{
struct tty_driver *normal;
int i, retval = -ENOMEM;
BUG_ON(drv->state);
/*
* Maybe we should be using a slab cache for this, especially if
* we have a large number of ports to handle.
*/
drv->state = kcalloc(drv->nr, sizeof(struct uart_state), GFP_KERNEL);
if (!drv->state)
goto out;
normal = alloc_tty_driver(drv->nr);
if (!normal)
goto out_kfree;
drv->tty_driver = normal;
normal->driver_name = drv->driver_name;
normal->name = drv->dev_name;
normal->major = drv->major;
normal->minor_start = drv->minor;
normal->type = TTY_DRIVER_TYPE_SERIAL;
normal->subtype = SERIAL_TYPE_NORMAL;
normal->init_termios = tty_std_termios;
normal->init_termios.c_cflag = B4000000 | CS8 | CREAD | HUPCL | CLOCAL;
normal->init_termios.c_ispeed = normal->init_termios.c_ospeed = 4000000; //默认的波特率
normal->flags = TTY_DRIVER_REAL_RAW | TTY_DRIVER_DYNAMIC_DEV;
normal->driver_state = drv;
tty_set_operations(normal, &uart_ops);
/*
* Initialise the UART state(s).
*/
for (i = 0; i < drv->nr; i++) {
struct uart_state *state = drv->state + i;
struct tty_port *port = &state->port;
tty_port_init(port); //端口初始化
port->ops = &uart_port_ops;
}
//驱动注册
retval = tty_register_driver(normal);
if (retval >= 0)
return retval;
for (i = 0; i < drv->nr; i++)
tty_port_destroy(&drv->state[i].port);
put_tty_driver(normal);
out_kfree:
kfree(drv->state);
out:
return retval;
}
2、添加端口函数uart_add_one_port（）
在uart_driver增加一个port
/**
* uart_add_one_port - attach a driver-defined port structure
* @drv: pointer to the uart low level driver structure for this port
* @uport: uart port structure to use for this port.
*
* This allows the driver to register its own uart_port structure
* with the core driver. The main purpose is to allow the low
* level uart drivers to expand uart_port, rather than having yet
* more levels of structures.
*/
int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport)
{
struct uart_state *state;
struct tty_port *port;
int ret = 0;
struct device *tty_dev;
int num_groups;
BUG_ON(in_interrupt());
if (uport->line >= drv->nr)
return -EINVAL;
state = drv->state + uport->line;
port = &state->port;
mutex_lock(&port_mutex);
mutex_lock(&port->mutex);
if (state->uart_port) {
ret = -EINVAL;
goto out;
}
/* Link the port to the driver state table and vice versa */
atomic_set(&state->refcount, 1);
init_waitqueue_head(&state->remove_wait);
state->uart_port = uport;
uport->state = state;
state->pm_state = UART_PM_STATE_UNDEFINED;
uport->cons = drv->cons;
uport->minor = drv->tty_driver->minor_start + uport->line;
uport->name = kasprintf(GFP_KERNEL, "%s%d", drv->dev_name,
drv->tty_driver->name_base + uport->line);
if (!uport->name) {
ret = -ENOMEM;
goto out;
}
/*
* If this port is a console, then the spinlock is already
* initialised.
*/
if (!(uart_console(uport) && (uport->cons->flags & CON_ENABLED))) {
spin_lock_init(&uport->lock);
lockdep_set_class(&uport->lock, &port_lock_key);
}
if (uport->cons && uport->dev)
of_console_check(uport->dev->of_node, uport->cons->name, uport->line);
tty_port_link_device(port, drv->tty_driver, uport->line);
uart_configure_port(drv, state, uport);
port->console = uart_console(uport);
num_groups = 2;
if (uport->attr_group)
num_groups++;
uport->tty_groups = kcalloc(num_groups, sizeof(*uport->tty_groups),
GFP_KERNEL);
if (!uport->tty_groups) {
ret = -ENOMEM;
goto out;
}
uport->tty_groups[0] = &tty_dev_attr_group;
if (uport->attr_group)
uport->tty_groups[1] = uport->attr_group;
/*
* Register the port whether it's detected or not. This allows
* setserial to be used to alter this port's parameters.
*/
tty_dev = tty_port_register_device_attr_serdev(port, drv->tty_driver,
uport->line, uport->dev, port, uport->tty_groups);
if (!IS_ERR(tty_dev)) {
device_set_wakeup_capable(tty_dev, 1);
} else {
dev_err(uport->dev, "Cannot register tty device on line %d\n",
uport->line);
}
/*
* Ensure UPF_DEAD is not set.
*/
uport->flags &= ~UPF_DEAD;
out:
mutex_unlock(&port->mutex);
mutex_unlock(&port_mutex);
return ret;
}
首先这个函数不能在中断环境中使用.Uart_port-> line就是对uart设备文件序号.它对应的也就是uart_driver-> state数组中的uart_port->line项.
它主要初始化对应uart_driver-> state项.接着调用uart_configure_port()进行port的自动配置.然后注册 tty_device.如果用户空间运行了udev或者已经配置好了hotplug.就会在/ dev下自动生成设备文件了.
3、串口的文件操作集合
static const struct tty_operations uart_ops = {
.install = uart_install,
.open = uart_open,
.close = uart_close,
.write = uart_write,
.put_char = uart_put_char, //单字符写入
.flush_chars = uart_flush_chars,
.write_room = uart_write_room, //检测缓冲区的剩余空间
.chars_in_buffer= uart_chars_in_buffer, //检测包含数据缓冲区的数量
.flush_buffer = uart_flush_buffer, //刷新缓冲区并丢弃其中的数据
.ioctl = uart_ioctl,
.throttle = uart_throttle,
.unthrottle = uart_unthrottle,
.send_xchar = uart_send_xchar,
.set_termios = uart_set_termios,
.set_ldisc = uart_set_ldisc, //设置线路规程的函数
.stop = uart_stop,
.start = uart_start,
.hangup = uart_hangup,
.break_ctl = uart_break_ctl,
.wait_until_sent= uart_wait_until_sent, //用来向硬件发送数据
#ifdef CONFIG_PROC_FS
.proc_show = uart_proc_show,
#endif
.tiocmget = uart_tiocmget, //获取特定tty设备当前的线路设置
.tiocmset = uart_tiocmset, //设置特定tty设备当前的线路
.set_serial = uart_set_info_user,
.get_serial = uart_get_info_user,
.get_icount = uart_get_icount,
#ifdef CONFIG_CONSOLE_POLL
.poll_init = uart_poll_init,
.poll_get_char = uart_poll_get_char,
.poll_put_char = uart_poll_put_char,
#endif
};