TTY设备

TTY 驱动类型如下，串口也属于TTY

/* tty driver types */
#define TTY_DRIVER_TYPE_SYSTEM        0x0001
#define TTY_DRIVER_TYPE_CONSOLE        0x0002
#define TTY_DRIVER_TYPE_SERIAL        0x0003
#define TTY_DRIVER_TYPE_PTY        0x0004
#define TTY_DRIVER_TYPE_SCC        0x0005    /* scc driver */
#define TTY_DRIVER_TYPE_SYSCONS        0x0006

串口驱动程序分析

串口驱动程序层次结构如下图所示。简单来说，串口驱动程序层次结构可以分为两层，下层为串口驱动层，它直接与硬件相接触，需要填充一个 struct uart_ops 的结构体。上层为tty层，包括tty核心层及线路规程，它们各自都有一个 ops 结构体，用户空间可以通过tty注册的字符设备节点来访问串口设备。

芯片厂商定义一个 struct uart_driver 类型全局变量，只是填充了一些名字、设备号等信息，这些都是不涉及底层硬件访问的。

static struct uart_driver imx_uart_uart_driver = {
    .owner          = THIS_MODULE,
    .driver_name    = DRIVER_NAME,
    .dev_name       = DEV_NAME,
    .major          = SERIAL_IMX_MAJOR,
    .minor          = MINOR_START,
    .nr             = ARRAY_SIZE(imx_uart_ports),
    .cons           = IMX_CONSOLE,
};

在struct uart_driver imx_uart_uart_driver 结构体中，有两个成员未被赋值，分别是tty_driver和uart_state。对于tty_driver，代表的是上层，它会在 uart_ register_driver 的过程中赋值。而uart_state ，则代表下层，uart_state也会在uart_ register_driver 的过程中分配空间，但是它里面真正设置硬件相关的东西是 uart_state->uart_port ，这个uart_port 是需要从其它地方调用 uart_add_one_port 来添加的。

调用 uart_register_driver 把 struct uart_driver 类型变量注册到系统中，由下面代码可知，无论芯片有几个串口，uart_register_driver 函数只会执行一遍，共用一套驱动代码，在系统启动时执行

static int __init imx_uart_init(void)
{
    int ret = uart_register_driver(&imx_uart_uart_driver);

    if (ret)
        return ret;

    ret = platform_driver_register(&imx_uart_platform_driver);
    if (ret != 0)
        uart_unregister_driver(&imx_uart_uart_driver);

    return ret;
}

　module_init(imx_uart_init);

uart_register_driver 函数

　　1、根据 uart_driver->nr 来申请 nr 个 uart_state 空间，用来存放驱动所支持的串口（端口）物理信息

　　2、创建一个 struct tty_driver 类型变量 normal，并且调用 tty_register_driver 把 normal 注册到系统中。因为串口属于TTY，注册串口驱动就是注册TTY驱动。芯片上所有的串口设备共用 normal

int uart_register_driver(struct uart_driver *drv)
{
    struct tty_driver *normal;
...

    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 = B9600 | CS8 | CREAD | HUPCL | CLOCAL;
    normal->init_termios.c_ispeed = normal->init_termios.c_ospeed = 9600;
...

    retval = tty_register_driver(normal);
...
}

在 tty_register_driver 内，由于已经指定了 driver->major，driver->flags == TTY_DRIVER_REAL_RAW | TTY_DRIVER_DYNAMIC_DEV，所以功能是

　　1、按芯片串口总数申请设备号

　　2、把 driver 添加到链表 tty_drivers

int tty_register_driver(struct tty_driver *driver)
{
    int error;
    int i;
    dev_t dev;
    struct device *d;

    if (!driver->major) {
...
    } else {
        dev = MKDEV(driver->major, driver->minor_start);
        error = register_chrdev_region(dev, driver->num, driver->name);
    }
...

    mutex_lock(&tty_mutex);
    list_add(&driver->tty_drivers, &tty_drivers);
    mutex_unlock(&tty_mutex);

...
    driver->flags |= TTY_DRIVER_INSTALLED;
    return 0;
...
}

串口设备和驱动通过platform总线进行匹配，当设备树的串口设备和驱动匹配，执行 imx_uart_probe

如下设备树有两个串口设备

                uart7: serial@2018000 {
                    compatible = "fsl,imx6ul-uart",
                             "fsl,imx6q-uart";
                    reg = <0x02018000 0x4000>;
                    interrupts = <GIC_SPI 39 IRQ_TYPE_LEVEL_HIGH>;
                    clocks = <&clks IMX6UL_CLK_UART7_IPG>,
                         <&clks IMX6UL_CLK_UART7_SERIAL>;
                    clock-names = "ipg", "per";
                    dmas = <&sdma 43 4 0>, <&sdma 44 4 0>;
                    dma-names = "rx", "tx";
                    status = "disabled";
                };

                uart1: serial@2020000 {
                    compatible = "fsl,imx6ul-uart",
                             "fsl,imx6q-uart";
                    reg = <0x02020000 0x4000>;
                    interrupts = <GIC_SPI 26 IRQ_TYPE_LEVEL_HIGH>;
                    clocks = <&clks IMX6UL_CLK_UART1_IPG>,
                         <&clks IMX6UL_CLK_UART1_SERIAL>;
                    clock-names = "ipg", "per";
                    status = "disabled";
                };

static const struct of_device_id imx_uart_dt_ids[] = {
    { .compatible = "fsl,imx6q-uart", .data = &imx_uart_devdata[IMX6Q_UART], },
    { .compatible = "fsl,imx53-uart", .data = &imx_uart_devdata[IMX53_UART], },
    { .compatible = "fsl,imx1-uart", .data = &imx_uart_devdata[IMX1_UART], },
    { .compatible = "fsl,imx21-uart", .data = &imx_uart_devdata[IMX21_UART], },
    { /* sentinel */ }
};

imx_uart_probe() 功能：

　　1、创建一个 struct imx_port 类型变量 sport，并添加到数组 imx_uart_ports[] 内，数组容量大小等于芯片串口总数。

　　2、调用 uart_add_one_port() ，令imx_uart_uart_driver->state[串口号]->uart_port 指向 sport->port，一个 sport->port 对应一个串口设备

uart_add_one_port ---> tty_port_register_device_attr_serdev

分析可知，serdev_tty_port_register 返回值为 -ENODEV，所以会执行 tty_register_device_attr()

struct device *tty_port_register_device_attr_serdev(struct tty_port *port,
        struct tty_driver *driver, unsigned index,
        struct device *device, void *drvdata,
        const struct attribute_group **attr_grp)
{
    struct device *dev;

    tty_port_link_device(port, driver, index);

    dev = serdev_tty_port_register(port, device, driver, index);
    if (PTR_ERR(dev) != -ENODEV) {
        /* Skip creating cdev if we registered a serdev device */
        return dev;
    }

    return tty_register_device_attr(driver, index, device, drvdata,
            attr_grp);
}