查看“Serial TTY overview”的源代码

本文提供有关 Linux<sup>&reg;</sup> 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。<br />

==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。
<br/>
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。
<br/>
'''TTY子系统不支持STM32 USART外设的同步模式。'''
<br/>
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。
<br/>
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。
<br/>

从内核4.12版本开始，串行设备总线（也称为Serdev）已在TTY框架中引入，以改善为连接到串行端口的设备（例如：蓝牙，NFC，FM无线电和GPS设备）提供的接口 学科“驾驶员”有一些已知的局限性：
* 设备是在用户空间而不是固件中编码的（ACPI的设备树）
* “驱动程序”不是内核驱动程序，而是用户空间守护程序
* 内核空间中没有描述相关的资源（GPIO和中断，调节器，时钟，音频接口），这会影响电源管理
* 打开端口时会注册“驱动程序”

Serdev允许将设备连接到UART，而无需了解线路规则限制：
* 新总线类型：串行
* Serdev控制器
* Serdev设备（客户端或从设备）
* Serdev TTY-端口控制器
** 仅内核内控制器实现
** 定义客户端时，由TTY驱动程序注册
** 客户端由固件（设备树或ACPI）描述
<br/>

STMicroelectronics提供的USART低级驱动程序（“drivers/tty/serial/stm32 USART.c”）支持
[https://en.wikipedia.org/wiki/RS-232 RS-232] standard (for serial communication transmission of data), and [https://en.wikipedia.org/wiki/RS-485 RS-485] standard (for [https://en.wikipedia.org/wiki/Modbus modbus]协议应用程序为例）。<br/>
 Linux<sup>&reg;</sup>低级驱动程序不支持USART同步模式。<br/>
<br/>
在以下用例中，TTY框架用于访问串行设备：
* Linux启动过程中的“'tty虚拟控制台”'
* '''pts pseudo-terminal'''通过终端访问
* '''用户空间应用程序'''

==System overview==
{{
ImageMap|Image:Serial TTY overview.png{{!}} frame {{!}} center{{!}} Serial TTY architecture overview
rect 261 75 412 116 [[How to use TTY from an application | How to use TTY from an application ]]
rect 452 75 603 116 [[How to use TTY with User Terminal | How to use TTY from a user terminal]]
rect 676 75 828 116 [[Bluetooth_overview#System_overview | Example: Bluetooth_overview ]]
rect 268 130 453 220 [http://man7.org/linux/man-pages/man3/termios.3.html termios]
rect 350 314 502 355 {{CodeSource | Linux kernel | drivers/tty/tty_io.c | tty char driver}}
rect 350 381 502 423 [[Serial TTY line discipline| Line discipline]]
rect 677 439 827 479 {{CodeSource | Linux kernel | drivers/tty/serdev | serdev framework}}
rect 350 439 502 480 {{CodeSource | Linux kernel | drivers/tty | tty framework}}
rect 463 514 615 554 {{CodeSource | Linux kernel | drivers/tty/serial/serial_core.c | serial_core framework}}
rect 463 578 615 620 {{CodeSource | Linux kernel | drivers/tty/serial/stm32-usart.c | stm32-usart driver}}
rect 463 653 615 694 [[USART internal peripheral| USART internal peripheral]]
}}
<br/>

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. 
<br/>

===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: [[Bluetooth_overview#System_overview | Bluetooth_overview ]]).

* TTY framework: high-level TTY structures management, including {{CodeSource | Linux kernel | drivers/tty/tty_io.c | '''tty character device driver'''}}, {{CodeSource | Linux kernel | drivers/tty | '''TTY core functions'''}}, [[Serial TTY line discipline| '''line discipline''']] and {{CodeSource | Linux kernel | drivers/tty/serdev | '''Serdev core functions'''}} management.

* Serial framework: low-level serial driver management, including the {{CodeSource | Linux kernel | drivers/tty/serial/serial_core.c | '''serial core''' functions}}.

* USART driver: {{CodeSource | Linux kernel | drivers/tty/serial/stm32-usart.c | stm32-usart}} '''low-level serial driver''' for all STM32 family devices.

* [[USART internal peripheral| 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.<br/>

The '''termios API''' <ref>[http://man7.org/linux/man-pages/man3/termios.3.html termios API], Linux Programmer's Manual termios API Documentation (user land API with serial devices)</ref> is a user land API, and its functions describe a general terminal interface that is provided to control asynchronous communications ports. <br/>

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''' <ref> [https://bootlin.com/doc/legacy/accessing-hardware/accessing-hardware.pdf Character device API overview], ''Accessing hardware from userspace'' training, Bootlin documentation</ref> ioctl operations.<br/>

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 [[Serial TTY line discipline|'''line discipline''']] or a '''Serdev''' client. <br/>
* The '''line discipline''' will be responsible for:
** creating this new kernel API
** routing data flow between the serial core and the new kernel API <br/>
*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 or how to configure kernel | Menuconfig ]] tool.

For TTY, select:
<pre>
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.
</pre>

For the STM32 serial driver, select:
<pre>
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.</pre>

===Device tree configuration ===
The UART configuration thanks to the device tree is described in the dedicated article [[Serial TTY device tree configuration|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:
<span id{{=}}"CSIZE_configurations"></span>
* CS6 with parity bit
* CS7 with or without parity bit
* CS8 with or without parity bit

Tips to use TTY:
* '''How to use TTY from user terminal''': TTY usage from a user terminal is described in a dedicated article, [[How to use TTY with User Terminal|How to use TTY from a user terminal]]
* '''How to use TTY from an application''': TTY usage from an application is described in a dedicated article, [[How_to_use_TTY_from_an_application|How to use TTY from an application]]
* '''TTY line discipline''': TTY line discipline is described in a dedicated article,  [[Serial TTY line discipline|Serial TTY line discipline]]

==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_and_Linux_kernel_log]] article.

=== How to trace ===
==== Kernel boot log ====
The following extract of '''kernel boot log''' shows a serial driver properly probed:

<pre>
[    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)
</pre>

==== 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
 {{highlight|# ttySTM1 terminal is associated with usart3 (4000f000.serial) #}} 
 [    0.798779] 4000f000.serial: {{Green|'''ttySTM1'''}} at MMIO 0x4000f000 (irq = 25, base_baud = 4000000) is a stm32-usart
 {{highlight|# ttySTM0 terminal is associated with uart4 (40010000.serial) for console#}}
 [    0.824253] 40010000.serial: {{Green|'''ttySTM0'''}} at MMIO 0x40010000 (irq = 27, base_baud = 4000000) is a stm32-usart      
 {{highlight|# ttySTM0 terminal is activated by default for console #}}
 [    0.833796] console [{{Green|'''ttySTM0'''}}] enabled

==== Dynamic trace ====
A detailed dynamic trace is available in [[How to use the kernel dynamic debug]]<br/>
  {{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.<br/>
A finer selection can be made by choosing only the files to trace.<br/>

{{Info|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*
 {{highlight|# ttySTM1 and ttySTM0 terminals are probed #}}
 /dev/{{Green|'''ttySTM1'''}}  /dev/{{Green|'''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/{{Green|'''ttySTM1'''}}/device/driver -> ../../../../bus/platform/drivers/{{Green|'''stm32-usart'''}}
 /sys/class/tty/{{Green|'''ttySTM0'''}}/device/driver -> ../../../../bus/platform/drivers/{{Green|'''stm32-usart'''}}

* '''/sys/devices/platform/soc/''' lists all the usart devices probed
 
 {{Board$}} ls -d /sys/devices/platform/soc/*.serial
 {{highlight|# Serial devices 4000f000.serial (usart3) and 40010000.serial (uart4) are probed #}}
 /sys/devices/platform/soc/{{Green|'''4000f000.serial'''}}  /sys/devices/platform/soc/{{Green|'''40010000.serial'''}}

* '''/sys/devices/platform/soc/''device''.serial/tty''' lists the TTY terminal associated to a serial device

 {{Board$}} ls /sys/devices/platform/soc/{{Green|'''4000f000.serial'''}}/tty/      
 {{highlight|# ttySTM1 is associated to serial device 4000f000.serial (usart3)  #}}
 {{Green|'''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/{{Green|'''serial@40010000'''}}/status
 {{highlight|# status of device serial@40010000 (uart4) is set to "okay" in the device tree #}}
 {{Green|'''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


<span id{{=}}"procfs_serialcounters"></span>
* 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 <ref>[http://tldp.org/HOWTO/Serial-HOWTO.html Linux Serial-HOWTO], tdlp.org training document, describes how to set up serial ports from both hardware and software perspectives</ref> describes how to set up serial ports from both hardware and software perspectives.
* Serial Programming Guide for POSIX Compliant Operating Systems <ref>[https://www.cmrr.umn.edu/~strupp/serial.html Serial Programming Guide for POSIX Compliant Operating Systems], by Michael Sweet, training document</ref> , 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 <ref> [https://www.ibm.com/support/knowledgecenter/en/ssw_aix_71/com.ibm.aix.genprogc/ttysys.htm TTY Subsystem], by IBM </ref>, by IBM
* The TTY demystified <ref>[http://www.linusakesson.net/programming/tty/ The TTY demystified] TTY subsystem presentation article, by Linus Akesson </ref>, by Linus Akesson
* Serial drivers training <ref>[http://bootlin.com/doc/legacy/serial-drivers/linux-serial-drivers.pdf Linux serial drivers training] Linux Serial Drivers training, by Bootlin </ref>, by Bootlin
* Linux Serial drivers <ref>[http://www.linux.it/~rubini/docs/serial/serial.html Linux Serial Drivers] Serial drivers article describing data flows, by Alessandro Rubini</ref>, by Alessandro Rubini
* Serial Device Bus <ref>[https://elinux.org/images/0/0e/Serdev-elce-2017-2.pdf The Serial Device Bus] Serdev framework presentation, by Johan Hovold</ref>, by Johan Hovold

==References==
<references />

<noinclude>
[[Category:Serial TTY|1]]
{{PublicationRequestId | 15053 | 2020-02-21 | 8368 (BrunoB - 2018-08-31)}}
{{ArticleBasedOnModel | Framework overview article model}}
</noinclude>