Serial TTY line discipline

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

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 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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• TTY framework: high-level TTY structures management, including drivers/tty/tty_io.c | |}} tty character device driver , drivers/tty | |}} TTY core functions , line discipline.

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• Serial framework: low-level serial driver management, including the drivers/tty/serial/serial_core.c | |}} serial core functions .

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• USART driver: drivers/tty/serial/stm32-usart.c | |}} stm32-usart low-level serial driver for all stm32 family devices.

• 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 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 ioctl operations.

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