Linux UART驱动分析

1. 介绍

8250是IBM PC及兼容机使用的一种串口芯片; 16550是一种带先进先出（FIFO）功能的8250系列串口芯片; 16550A则是16550的升级版本, 修复了FIFO相关BUG, 也是目前比较常见的串口芯片.

本文介绍的是Xilinx UART 驱动分析, 因为没有找到其datasheet, 硬件操作部分分析16550的实现.

Xilinx UART驱动主要由drivers/tty/serial/xilinx_uartps.c来实现
其相关配置和基本信息可参考<Zynq UART>

2. 结构体

uart_driver和console结构变量, 以及实现了uart_ops函数操作集定义如下图所示

static struct uart_driver cdns_uart_uart_driver = {
    .owner          = THIS_MODULE,
    .driver_name    = CDNS_UART_NAME,          /* xuartps */
    .dev_name       = CDNS_UART_TTY_NAME,      /* ttyPS */
    .major          = CDNS_UART_MAJOR,         /* 0, 注册时动态分配 */
    .minor          = CDNS_UART_MINOR,         /* 0, 注册时动态分配*/
    .nr             = CDNS_UART_NR_PORTS,      /* 2 */
#ifdef CONFIG_SERIAL_XILINX_PS_UART_CONSOLE
    .cons           = &cdns_uart_console,      /* ttyPS */
#endif
};

static struct console cdns_uart_console = {
    .name           = CDNS_UART_TTY_NAME,      /* ttyPS */
    .write          = cdns_uart_console_write,
    .device         = uart_console_device,
    .setup          = cdns_uart_console_setup,
    .flags          = CON_PRINTBUFFER,
    .index          = -,                      /* 由cmdline指定(e.g. console=ttyPS ) */
    .data           = &cdns_uart_uart_driver,
};

static const struct uart_ops cdns_uart_ops = {
    .set_mctrl     = cdns_uart_set_mctrl,
    .get_mctrl     = cdns_uart_get_mctrl,
    .start_tx      = cdns_uart_start_tx,
    .stop_tx       = cdns_uart_stop_tx,
    .stop_rx       = cdns_uart_stop_rx,
    .tx_empty      = cdns_uart_tx_empty,
    .break_ctl     = cdns_uart_break_ctl,
    .set_termios   = cdns_uart_set_termios,
    .startup       = cdns_uart_startup,
    .shutdown      = cdns_uart_shutdown,
    .pm            = cdns_uart_pm,
    .type          = cdns_uart_type,
    .verify_port   = cdns_uart_verify_port,
    .request_port  = cdns_uart_request_port,
    .release_port  = cdns_uart_release_port,
    .config_port   = cdns_uart_config_port,
#ifdef CONFIG_CONSOLE_POLL
    .poll_get_char = cdns_uart_poll_get_char,
    .poll_put_char = cdns_uart_poll_put_char,
#endif
};

3. 初始化

模块入口为cdns_uart_init
首先注册UART驱动

uart_register_driver(&cdns_uart_uart_driver);

随后又注册platform驱动

platform_driver_register(&cdns_uart_platform_driver);

其中cdns_uart_platform_driver定义如下

static const struct of_device_id cdns_uart_of_match[] = {
    { .compatible = "xlnx,xuartps", },
    { .compatible = "cdns,uart-r1p8", },
    { .compatible = "cdns,uart-r1p12", .data = &zynqmp_uart_def },
    { .compatible = "xlnx,zynqmp-uart", .data = &zynqmp_uart_def },
    {}
};

static struct platform_driver cdns_uart_platform_driver = {
    .probe   = cdns_uart_probe,
    .remove  = cdns_uart_remove,
    .driver  = {
        .name = CDNS_UART_NAME,
        .of_match_table = cdns_uart_of_match,
        .pm = &cdns_uart_dev_pm_ops,
        },
};

而在arch/arm/boot/dts/zynq-7000.dtsi中, 定义了uart设备树相关信息

uart0: serial@e0000000 {
    compatible = "xlnx,xuartps", "cdns,uart-r1p8";
    status = "disabled";
    clocks = <&clkc >, <&clkc >;
    clock-names = "uart_clk", "pclk";
    reg = <0xE0000000 0x1000>;
    interrupts = <  >;
};

uart1: serial@e0001000 {
    compatible = "xlnx,xuartps", "cdns,uart-r1p8";
    status = "disabled";
    clocks = <&clkc >, <&clkc >;
    clock-names = "uart_clk", "pclk";
    reg = <0xE0001000 0x1000>;
    interrupts = <  >;
};

关于设备树, 可参考<Linux设备树解析>
从文章中我们知道内核会将设备树解析为platform_device, 匹配后则会调用cdns_uart_probe
下面以uart0驱动probe分析一下该函数

int cdns_uart_probe(struct platform_device *pdev)
{
    int id, irq;
    struct uart_port *port;
    struct resource *res;
    struct cdns_uart *cdns_uart_data;

    /* 分配驱动私有数据结构体 */
    cdns_uart_data = devm_kzalloc(&pdev->dev, sizeof(*cdns_uart_data), GFP_KERNEL);

    /* 从dts获取时钟(clocks), pclk=40, uart_clk=23 */
    cdns_uart_data->pclk = devm_clk_get(&pdev->dev, "pclk");
    cdns_uart_data->uartclk = devm_clk_get(&pdev->dev, "uart_clk");
    /* 准备时钟源 */
    clk_prepare(cdns_uart_data->pclk);
    clk_prepare(cdns_uart_data->uartclk);

    /* 从dts获取编址(reg), start=0xE0000000,end=0xE0001000 */
    platform_get_resource(pdev, IORESOURCE_MEM, );

    /* 从dts获取中断(interrupts), 中断号为27 !!! */
    platform_get_irq(pdev, );

    /* 获取设备编号, 此处为0 */
    id = of_alias_get_id(pdev->dev.of_node, "serial");

    /* 初始化uart端口 */
    port = cdns_uart_get_port(id);

    /* 设置uart端口硬件相关参数 */
    port->mapbase = res->start;
    port->irq = irq;
    port->dev = &pdev->dev;
    port->uartclk = clk_get_rate(cdns_uart_data->uartclk);
    port->private_data = cdns_uart_data;
    cdns_uart_data->port = port;
    platform_set_drvdata(pdev, port);

    /* 添加uart端口 */
    uart_add_one_port(&cdns_uart_uart_driver, port);
}

static struct uart_port cdns_uart_port[CDNS_UART_NR_PORTS]; /* 2 */
struct uart_port *cdns_uart_get_port(int id)
{
    struct uart_port *port;

    /* 获取本地定义的uart_port结构体变量 */
    port = &cdns_uart_port[id];

    spin_lock_init(&port->lock);
    port->membase  = NULL;
    port->irq      = ;
    port->type     = PORT_UNKNOWN;        /* 会在config_port中设置为PORT_XUARTPS */
    port->iotype   = UPIO_MEM32;          /* 串口接口寄存器的地址类型 */
    port->flags    = UPF_BOOT_AUTOCONF;   /* 该标志会使uart_add_one_port调用config_port */
    port->ops      = &cdns_uart_ops;      /* 即前面定义的uart_ops函数操作集 */
    port->fifosize = CDNS_UART_FIFO_SIZE; /* 64 */
    port->line     = id;                  /* 0 */
    port->dev      = NULL;
    return port;
}

4. 16550介绍

16550寄存器信息如下

RBF定义如下

THR定义如下

IER定义如下

IIR定义如下

FCR定义如下

LCR定义如下

MCR定义如下

LSR定义如下

MSR定义如下

SCR定义如下

5. 硬件操作实现

这里分析8250/16550对uart_ops的实现serial8250_pops
主要代码位于drivers/tty/serial/8250/8250_port.c

tx_empty: serial8250_tx_empty
读取并判断LSR的第THRE、TEMT位是否为1

set_mctrl: serial8250_set_mctrl
将位设置(RTS、DTR、OUT1、OUT2、LOOP)写入MCR

get_mctrl: serial8250_get_mctrl
读取MSR, 即Modem Interface的当前状态

stop_tx: serial8250_stop_tx
禁用IER的THRI/ETBEI位

start_tx: serial8250_start_tx
启用IER的THRI/ETBEI位; 当LSR的THRE位为1, 通过操作THR将circ_buf的数据搬运至UART

stop_rx: serial8250_stop_rx
禁用IER的RLSI/ELSI和RDI/ERBFI位

enable_ms: serial8250_enable_ms
启用IER的MSI/EDSSI

break_ctl: serial8250_break_ctl
启动或者禁用LCR的SBC/SetBreak位

startup: serial8250_startup
1. 设置FCR清空FIFO缓冲区, 清空中断寄存器(LSR、RX、IIR、MSR), 初始化相关寄存器
2. 调用uart_8250_ops::setup_irq(univ8250_setup_irq)
3. 设置MCR寄存器
4. 为TX/RX请求DMA通道

univ8250_setup_irq
  serial_link_irq_chain
    request_irq
      serial8250_interrupt
        dw8250_handle_irq
        /* 即uart_port::handle_irq */
          serial8250_handle_irq
            handle_rx_dma(Running here???)
              serial8250_rx_dma
              /* uart_8250_port::uart_8250_dma::rx_dma */
                __dma_rx_complete
                  tty_insert_flip_string
                  /* 将数据插入接收数据缓冲区 */
                  tty_flip_buffer_push
                  /* 将数据搬至线路规程层 */
                    tty_schedule_flip
                      flush_to_ldisc
            serial8250_rx_chars
              serial8250_read_char
                uart_insert_char
                  tty_insert_flip_char
                  /* 将数据插入接收数据缓冲区 */
              tty_flip_buffer_push
              /* 将数据搬至线路规程层 */
                tty_schedule_flip
                  flush_to_ldisc

shutdown: serial8250_shutdown
初始化寄存器(...), 注销中断处理程序(???)

set_termios: serial8250_set_termios
设置相关寄存器(...)

set_ldisc: serial8250_set_ldisc
如果没有设置了Modem状态, 则禁用IER的MSI位

pm: serial8250_pm
休眠(???)

type: serial8250_type
获取硬件名称

release_port: serial8250_release_port
释放端口占用物理资源, 如Memory, I/O

request_port: serial8250_request_port
请求物理资源

config_port: serial8250_config_port
按照传入参数配置端口

verify_port: serial8250_verify_port
校验端口配置是否有效

参考:
<Xinu uart-ns16550>
<AXI UART 16550 v2.0>
<XPS 16550 UART v3.00>
<dw_apb_uart Databook>
<Serial UART information>