Serial TTY line discipline

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本文提供有关 Linux® TTY 框架的信息。 它说明了如何使用线路规则激活“ UART”接口,以及如何从用户和内核空间访问它。

Framework purpose

UART是几种设备类型(例如蓝牙,NFC,FM收音机和GPS设备)的通用接口。
从内核版本4.12开始,TTY框架中引入了串行设备总线(也称为Serdev),以改善提供给连接到串行端口的设备的接口。 请参考 Serial TTY overview文章,以详细了解TTY框架的用途。尽管如此,仍然有可能(即使不推荐)使用行规“驱动程序”。 本文重点介绍提供给连接到串行端口的设备的线路规范接口。

System overview

Serial TTY Line Discipline overview.png


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.

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  • Termios: API[1] which offers an interface to develop an application using serial drivers.
  • Client subsystem: kernel subsystem client of TTY core (Example: a Bluetooth device)

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  • STM32 USART: STM32 frontend IP connected to the external devices through a serial port

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APIs description

The TTY provides only character device interface (named /dev/ttyX) to 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.

Info.png A line discipline driver can be used when a serial interface is needed at kernel level.

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  • The line discipline will be responsible for:
    • creating this new kernel API
    • routing data flow between serial core and new kernel API

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References

  1. 1.01.1 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