Serial TTY overview

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本文提供有关 Linux® TTY框架的信息。 它解释了如何激活“”UART“”接口，以及如何从用户和内核空间访问它。

Framework purpose

TTY子系统控制UART设备与使用这些设备的程序之间的通信。
TTY子系统负责：

  • 控制异步线路上的物理数据流（包括传输速度，字符大小和线路可用性）。
  • 通过识别特殊字符并适应本国语言来解释数据。
  • 通过控制终端的概念来控制作业和终端访问。


TTY子系统不支持STM32 USART外设的同步模式。
控制终端管理一组进程的输入和输出操作。 TTY特殊文件（ttyX文件系统条目）支持控制终端接口。
为了执行其任务，TTY子系统由模块（也称为学科）组成。 模块是一组处理规则，它们控制计算机与异步设备之间的通信接口。 可以为每个TTY动态添加和删除模块。

TTY子系统支持三种主要类型的模块：

  • TTY驱动程序：TTY驱动程序或硬件规则直接控制硬件（TTY设备）或伪硬件（PTY设备）。 它们通过为适配器上面的模块提供服务来执行对适配器的实际输入和输出。 打开端口时，这些服务是流控制和特殊语义。
  • 线学科：线学科提供编辑，作业控制和特殊字符解释。 它们执行入站和出站数据流上发生的所有转换。 生产线规程还为TTY驱动程序执行大多数错误处理和状态监视。
  • 转换器模块：转换器模块或映射学科，翻译或映射输入和输出字符。


Since kernel 4.12 version, the serial device bus (also called Serdev) has been introduced in the TTY framework to improve the interface offered to devices attached to a serial port (ex: Bluetooth, NFC, FM Radio and GPS devices), as the line disciplines "drivers" have some known limitations:

  • the devices are encoded in the user space rather than in the firmware (Device Tree of ACPI)
  • "drivers" are not kernel drivers but user space daemons
  • the associated ressources (GPIOs and interrupts, regulators, clocks, audio interface) are not described in the kernel space, which impacts power management
  • "drivers" are registered when a port is opened

The Serdev allows a device to be attached on UART without known the line disciplines limitations:

  • New bus type: serial
  • Serdev controllers
  • Serdev devices (clients or slaves)
  • Serdev TTY-port controller
    • Only in-kernel controller implementation
    • Registered by TTY driver when client is defined
    • clients are described by firmware (Device Tree or ACPI)


The USART low-level driver provided by STMicroelectronics, (drivers/tty/serial/stm32-usart.c) supports RS-232 standard (for serial communication transmission of data), and RS-485 standard (for modbus protocol applications as example).
The Synchronous mode of USART is not supported by Linux® low-level driver .

The TTY framework is used to access the serial device in the following use cases:

  • tty virtual console during Linux boot sequence
  • pts pseudo-terminal to access over a terminal
  • user space application

System overview

How to use TTY from an application How to use TTY from a user terminal Example: Bluetooth_overview termios drivers/tty/tty_io.c | |}} tty char driver Line discipline drivers/tty/serdev | |}} serdev framework drivers/tty | |}} tty framework drivers/tty/serial/serial_core.c | |}} serial_core framework drivers/tty/serial/stm32-usart.c | |}} stm32-usart driver USART internal peripheral
Serial TTY architecture overview


Note: during boot, while a serial device is probed, the serial framework instantiates an associated tty terminal as a virtual device. Then the system sees this tty virtual device as a child of the associated serial device.

Components description

From client application to hardware

  • Application: customer application to read/write data from the peripheral connected on the serial port.
  • TTY tools: tools provided by Linux community, such as stty, ldattach, inputattach, tty, ttys, agetty, mingetty, kermit and minicom.
  • Termios: API which offers an interface to develop an application using serial drivers.
  • Client subsystem: kernel subsystem client of serdev core (Example: 蓝牙概述).
  • STM32 USART: STM32 frontend IP connected to the external devices through a serial port.

APIs description

The TTY provides only character device interface (named /dev/ttyX) to the user space. The main API for user space TTY client applications is provided by the portable POSIX terminal interface termios, which relies on /dev/ttyX interface for TTY link configuration.

The termios API [1] is a user land API, and its functions describe a general terminal interface that is provided to control asynchronous communications ports.

The POSIX termios API abstracts the low-level details of the hardware, and provides a simple, yet complete, programming interface that can be used for advanced projects. It is a wrapper on character device API [2] ioctl operations.

Note: If a serial interface is needed at kernel level (to control an external device through U(S)ART by a kernel driver for example), the customer can use a line discipline or a Serdev client.

  • The line discipline will be responsible for:
    • creating this new kernel API
    • routing data flow between the serial core and the new kernel API
  • The Serdev provides an interface to kernel drivers.
    • This interface resembles line-discipline operations: open and close, terminal settings, write, modem control, read (callback), and write wakeup (callback)

Configuration

This section describes how to configure a device on a serial port.

Kernel Configuration

The serial driver, serial framework, and TTY framework are activated by default in ST deliveries. Nevertheless, if a specific configuration is needed, this section indicates how IIO can be activated/deactivated in the kernel.

Activate the device TTY in kernel configuration with Linux Menuconfig tool.

For TTY, select: 

Device Drivers  --->
   Character devices  --->
     [*]   Enable TTY                           

Allows to remove the TTY support which can save space, and blocks features that require TTY from inclusion in the kernel.
The TTY is required for any text terminals or serial port communication. Most users should leave this enabled.

For the STM32 serial driver, select: 

Device Drivers  --->
   Character devices  --->
      Serial drivers  --->
     <*> STMicroelectronics STM32 serial port support
     [*]   Support for console on STM32

This driver is for the on-chip serial controller on STMicroelectronics STM32 MCUs. 
The USART supports Rx and Tx functionality. It supports all industry standard baud rates.

Device tree configuration

The UART configuration thanks to the device tree is described in the dedicated article Serial TTY device tree configuration.

How to use TTY

This section describes how to use TTY from the user land (from a terminal or an application) and from the kernel space, based on the two following use cases:

  • How to configure the serial port by using the termios structure
  • How to send/receive data

The termios structure allows to configure communication ports with many settings, such as :

  • Baud rate
  • Character size mask
  • Parity bit enabling
  • Parity and framing errors detection settings
  • Start/stop input and output control
  • RTS/CTS (hardware) flow control
  • ...

As the USART internal peripheral supports 7, 8 and 9 word length data, the following termios character size and parity bit configurations are supported:

  • CS6 with parity bit
  • CS7 with or without parity bit
  • CS8 with or without parity bit

Tips to use TTY:

How to trace and debug the framework

How to monitor

As Debugfs does not propose any information about serial or TTY frameworks, the way to monitor Serial and TTY frameworks is to use the linux kernel log method (based on printk) described in Dmesg和Linux内核日志 article.

How to trace

Kernel boot log

The following extract of kernel boot log shows a serial driver properly probed: 

[    0.793340] STM32 USART driver initialized
[    0.798779] 4000f000.serial: ttySTM1 at MMIO 0x4000f000 (irq = 25, base_baud = 4000000) is a stm32-usart
[    0.808875] stm32-usart 4000f000.serial: interrupt mode used for rx (no dma)
[    0.816106] stm32-usart 4000f000.serial: interrupt mode used for tx (no dma)
[    0.824253] 40010000.serial: ttySTM0 at MMIO 0x40010000 (irq = 27, base_baud = 4000000) is a stm32-usart
[    0.833796] console [ttySTM0] enabled
[    0.833796] console [ttySTM0] enabled
[    0.840862] bootconsole [earlycon0] disabled
[    0.840862] bootconsole [earlycon0] disabled
[    0.850132] stm32-usart 40010000.serial: interrupt mode used for rx (no dma)
[    0.855755] stm32-usart 40010000.serial: interrupt mode used for tx (no dma)

dmesg output information

The system log shows the UART devices and associated TTY terminals registered during the probe. 

Board $> dmesg | grep ttySTM*
[    0.000000] Kernel command line: root=/dev/mmcblk0p5 rootwait rw earlyprintk console=ttySTM1,115200
# ttySTM1 terminal is associated with usart3 (4000f000.serial) # 
[    0.798779] 4000f000.serial: ttySTM1 at MMIO 0x4000f000 (irq = 25, base_baud = 4000000) is a stm32-usart
# ttySTM0 terminal is associated with uart4 (40010000.serial) for console#
[    0.824253] 40010000.serial: ttySTM0 at MMIO 0x40010000 (irq = 27, base_baud = 4000000) is a stm32-usart      
# ttySTM0 terminal is activated by default for console #
[    0.833796] console [ttySTM0] enabled

Dynamic trace

A detailed dynamic trace is available in How to use the kernel dynamic debug

 Board $> echo  "file drivers/tty/* +p" > /sys/kernel/debug/dynamic_debug/control

This command enables all the traces related to the TTY core and drivers at runtime.
A finer selection can be made by choosing only the files to trace.

Info.png Reminder: loglevel needs to be increased to 8 by using either boot arguments or the dmesg -n 8 command through the console

How to debug

While a TTY serial port is instantiated, the TTY core exports different files through devfs, sysfs and procfs.

devfs

  • The repository /dev contains all the probed TTY serial devices.
Board $> ls /dev/ttySTM*
# ttySTM1 and ttySTM0 terminals are probed #
/dev/ttySTM1  /dev/ttySTM0

sysfs

  • /sys/class/tty/ lists all TTY devices which ttySx correspond to serial port devices.
Board $> ls /sys/class/tty/*/device/driver
/sys/class/tty/ttySTM1/device/driver -> ../../../../bus/platform/drivers/stm32-usart
/sys/class/tty/ttySTM0/device/driver -> ../../../../bus/platform/drivers/stm32-usart
  • /sys/devices/platform/soc/ lists all the usart devices probed
Board $> ls -d /sys/devices/platform/soc/*.serial
# Serial devices 4000f000.serial (usart3) and 40010000.serial (uart4) are probed #
/sys/devices/platform/soc/4000f000.serial  /sys/devices/platform/soc/40010000.serial
  • /sys/devices/platform/soc/device.serial/tty lists the TTY terminal associated to a serial device
Board $> ls /sys/devices/platform/soc/4000f000.serial/tty/      
# ttySTM1 is associated to serial device 4000f000.serial (usart3)  #
ttySTM1

procfs

  • The repository /proc/device-tree lists all the usart devices declared in the device-tree, including the disabled ones.
Board $> ls -d /proc/device-tree/soc/serial@*
/proc/device-tree/soc/serial@4000e000  /proc/device-tree/soc/serial@40010000  /proc/device-
tree/soc/serial@40018000  /proc/device-tree/soc/serial@44003000
/proc/device-tree/soc/serial@4000f000  /proc/device-tree/soc/serial@40011000  /proc/device-
tree/soc/serial@40019000  /proc/device-tree/soc/serial@5c000000

Then for each device listed, device-tree properties are available. 

Board $> ls /proc/device-tree/soc/serial@40010000/
clock-names  compatible       interrupts-extended  name     pinctrl-0  pinctrl-names  reg     wakeup-source
clocks       interrupt-names  linux,phandle        phandle  pinctrl-1  power-domains  status
As an example, the status entry provides the status of the device in the device tree node.
Board $> cat /proc/device-tree/soc/serial@40010000/status
# status of device serial@40010000 (uart4) is set to "okay" in the device tree #
okay


  • The file /proc/interrupts lists the interrupts for active serial ports.
Board $> cat /proc/interrupts | grep serial
26:          0          0     GIC-0  71 Level     4000f000.serial
27:          0          0  stm32-exti-h  28 Edge      4000f000.serial
28:      13509          0     GIC-0  84 Level     40010000.serial
29:          0          0  stm32-exti-h  30 Edge      40010000.serial


  • The file /proc/tty/driver/stm32-usart lists serial core counters and modem information for each serial instance.
Driver information:
  • serial driver name
  • serial device start address
  • irq number
Counters:
  • tx: Number of bytes sent
  • rx: Number of bytes received
  • fe: Number of framing errors received
  • pe: Number of parity errors received
  • brk: Number of break signals received
  • oe: Number of overrun errors received
  • bo: Number of framework buffer overrun errors received
Modem information:
  • RTS: Request To Send
  • CTS: Clear To Send
  • DTR: Data Terminal Ready
  • DSR: Data Set Ready
  • CD: Carrier Detect
  • RI: Ring Indicator
Board $> cat /proc/tty/driver/stm32-usart 
serinfo:1.0 driver revision:
0: uart:stm32-usart mmio:0x40010000 irq:29 tx:22722 rx:2276 RTS|CTS|DTR|DSR|CD
1: uart:stm32-usart mmio:0x4000F000 irq:27 tx:0 rx:1149 fe:121 oe:2 pe:296 brk:3 RTS|CTS|DTR|DSR|CD
3: uart:stm32-usart mmio:0x4000E000 irq:25 tx:0 rx:0 CTS|DSR|CD

How to go further

The Linux community provides many detailed documentation about Linux serial/TTY. Please find below a selection of the most relevant ones:

  • Linux Serial-HOWTO [3] describes how to set up serial ports from both hardware and software perspectives.
  • Serial Programming Guide for POSIX Compliant Operating Systems [4] , by Michael Sweet.


More information can be found in the following web articles in order to get a good understanding of the Linux TTY framework:

  • TTY Subsystem [5], by IBM
  • The TTY demystified [6], by Linus Akesson
  • Serial drivers training [7], by Bootlin
  • Linux Serial drivers [8], by Alessandro Rubini
  • Serial Device Bus [9], by Johan Hovold

References

  1. ↑ termios API, Linux Programmer's Manual termios API Documentation (user land API with serial devices)
  2. ↑ Character device API overview, Accessing hardware from userspace training, Bootlin documentation
  3. ↑ Linux Serial-HOWTO, tdlp.org training document, describes how to set up serial ports from both hardware and software perspectives
  4. ↑ Serial Programming Guide for POSIX Compliant Operating Systems, by Michael Sweet, training document
  5. ↑ TTY Subsystem, by IBM
  6. ↑ The TTY demystified TTY subsystem presentation article, by Linus Akesson
  7. ↑ Linux serial drivers training Linux Serial Drivers training, by Bootlin
  8. ↑ Linux Serial Drivers Serial drivers article describing data flows, by Alessandro Rubini
  9. ↑ The Serial Device Bus Serdev framework presentation, by Johan Hovold

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