查看“I2C overview”的源代码

本文提供有关I2C系统以及如何插入I2C STM32驱动程序的基本信息。

==Framework purpose==
本文旨在解释如何更准确地使用I2C： 
* 如何在 Linux<sup>&reg;</sup> BSP 上激活I2C接口
* 如何从内核空间访问I2C
* 如何从用户空间访问I2C。


本文介绍了在'''master''' 和 '''slave'''模式下的 Linux<sup>&reg;</sup> I<sup>2</sup>C<ref name="More about I2C">http://www.i2c-bus.org/</ref>接口。

通过该外部资源，提出了I<sup>2</sup>C<ref name="I2C introduction">https://bootlin.com/doc/training/linux-kernel/</ref> 的简介。

有关 '''slave''' 接口的说明，请参见'''slave-interface'''<ref>{{CodeSource | Linux kernel | Documentation/i2c/slave-interface}} slave interface description</ref>.<br />

==System overview==
I<sup>2</sup>C 是“ IC间总线”的缩写，“ Inter-IC”总线是一种简单的总线协议，广泛用于低数据速率通信就足够了。<br />
I2C是微处理器 I<sup>2</sup>C 外围设备接口的缩写。

在微处理器设备周围，用户可以添加许多 I<sup>2</sup>C 外部设备来创建定制板。 可以通过I2C从用户空间或内核空间访问每个外部设备。

{{ImageMap|
Image: i2c-overview-new.png {{!}} frame {{!}} center {{!}} Implementation architecture

rect 430 114 548 157 [[I2C overview#i2c-tools | I2C Tools]]
rect 641 113 759 156 [[I2C overview#Application | Application ]]
circle 489 183 10 [[I2C overview#libi2c | I2C Library]]
rect 430 205 548 248 [[I2C overview#i2c-tools | I2C Tools]]
circle 489 309 10 [[I2C overview#User_space_application | User Space Application]]
rect 312 332 430 375 [[I2C overview#Board_peripheral_drivers | Board peripheral drivers]]
rect 548 332 666 375 [[I2C overview#Board_peripheral_drivers | Board peripheral drivers]]
rect 747 363 865 406 [[I2C overview#Board_peripheral_drivers | Board peripheral drivers]]
rect 361 406 615 452 [[I2C overview#i2c-core | I2C core engine]]
circle 489 408 10 [[I2C overview#Kernel_space_peripheral_driver | Kernel space peripheral driver]]
rect 430 469 548 512 [[I2C overview#i2c-stm32 | STM32 I2C driver]]
rect 430 550 548 593 [[I2C_overview#STM32_I2C_internal_peripheral_controller | STM32 I2C peripheral]]
rect 746 550 863 593 [[I2C_overview#STM32_I2C_internal_peripheral_controller | STM32 I2C peripheral]]
rect 312 642 430 687 [[I2C_overview#Board_external_I2C_devices | Board peripheral]]
rect 548 642 666 687 [[I2C_overview#Board_external_I2C_devices | Board peripheral]]
rect 747 642 865 687 [[I2C_overview#Board_external_I2C_devices | Board peripheral]]
}}

===Component description===
====Board external I<sup>2</sup>C devices====
* 从设备X是相对于STM32表现为从设备的物理设备(通过 I<sup>2</sup>C 总线连接到STM32)。<br/>
 STM32仍是 I<sup>2</sup>C 总线上的主机。
* 主设备X是相对于STM32充当主设备的物理设备 (通过 I<sup>2</sup>C 总线连接到STM32) 。<br/>
在这种情况下，STM32充当 I<sup>2</sup>C 总线上的从设备。

====STM32 I2C internal peripheral controller====

它对应于STM32 I2C适配器，该适配器处理与同一总线上连接的任何外部设备的通信。 <br/>它管理从设备（如果有的话），并且如果连接了外部主设备，则可以充当从设备。
STM32微处理器设备通常嵌入 [[I2C internal peripheral]] 的多个实例，以管理多个I2C总线。 提供了一个驱动程序，用于控制硬件。

====i2c-stm32====
内部STM32 I2C控制器驱动程序向基于i2c-core-base的ST I2C内部外围控制器抽象层提供了支持。<br/> 它定义了I2C核心基础要使用的所有I2C传输方法算法，其中包括I2C和SMBus<ref name="More about SMBus">https://www.i2c-bus.org/smbus/</ref> 传输API ，注册/注销从属API和功能检查。<br/> 即使I2C Core可以在整个标准I2C消息中模拟SMBus协议，所有SMBus功能都在驱动程序中实现。

====i2c-core====
这是通信的大脑：它实例化和管理所有总线和外围设备。
* 如其名称所述，为'''{{Highlight|i2c-core}}''' ，它是I2C核心引擎，但它也负责解析适配器和/或设备的设备树条目
* '''{{Highlight|i2c-core-smbus}}'''处理所有与SMBus相关的API。
* '''{{Highlight|i2c-core-slave}}''' 管理充当STM32中的从设备的I2C设备。
* '''{{Highlight|i2c-smbus}}'''处理特定的协议SMBus警报。 (由I2C核心库处理的SMBus主机通知)

====Board peripheral drivers====
该层表示与物理外围设备关联的所有驱动程序。<br/>
外围设备驱动程序可以编译为内核模块，也可以直接编译为内核(也称为内置).

====i2c-dev====
i2c-dev是用户与外围设备之间的接口。
它是一个内核驱动程序，它使用此dev-interface API提供对用户空间应用程序的I2C总线访问。
请参见示例[[I2C overview#API_description|API Description]].

====i2c-tools====
[[I2C_i2c-tools | I2C Tools]]软件包提供了：
* shell命令通过i2c-dev通过SMBus协议访问I2C
* library 将SMBus函数用于用户空间应用程序，所有这些函数都在这个SMBus协议API中进行了描述。<br/>
'''Note''' : 某些外围设备无需SMBus协议即可工作。

====Application====
应用程序可以使用 [[I2C_i2c-tools | I2C Tools]], libI2C ([[I2C_i2c-tools|I2C Tools]]), i2c-dev.

===API description===

====libi2c====
I2C工具<ref name="I2C tools">https://i2c.wiki.kernel.org/index.php/I2C_Tools</ref> 软件包提供了一组Shell命令，这些命令主要使用SMBus协议访问I2C和API， 开发一个应用程序（libi2c）。<br/>
所有工具和libi2c均依赖SMBus API，但[[I2C_i2c-tools#I2C_Transfer| i2ctransfer]]不是，因为它依赖于标准I2C协议。<br/>
工具和libi2c通过'''devfs'''读/写/ ioctl调用访问SMBus和I2C API。<br/><br/>

SMBus协议构成 I<sup>2</sup>C 规范中定义的数据传输格式的子集。 <br/>
SMBus规范中定义的标准方法无法访问不符合这些协议的I2C外设。 <br/>
有关 '''I<sup>2</sup>C'''<ref name="I2C summary">{{CodeSource | Linux kernel | Documentation/i2c/summary}} I2C and SMBus summary</ref> 和'''SMBus协议'''<ref name="SMBus protocol">{{CodeSource | Linux kernel | Documentation/i2c/smbus-protocol}} SMBus protocol summary</ref>的更多详细信息，请参见外部参考。

libi2c API模拟“SMBus协议”<ref name="SMBus protocol">{{CodeSource | Linux kernel | Documentation/i2c/smbus-protocol}} SMBus protocol summary</ref> ，但在用户空间级别。<br/>
此库中的API与 ''SMBus protocol''<ref name="SMBus protocol">{{CodeSource | Linux kernel | Documentation/i2c/smbus-protocol}} SMBus protocol summary</ref>中的API相同。除了特定的SMBus协议API（例如SMBus Host Notify和SMBus Alert）外，所有SMBus API均在此处重复。

====User space application====
[[#user_space_application| User space application]] 正在使用内核驱动程序（i2c-dev），该驱动程序通过devfs提供I2C访问。<br />
'''支持的系统调用: '''open(), close(), read(), write(), ioctl(), llseek(), release().<br />
{|
|+ Supported ioctls commands
|-
!Constant
!Description
|-
|I2C_SLAVE/I2C_SLAVE_FORCE
|Sets slave address for read/write operations
|-
|I2C_FUNCS
|Gets bus functionalities
|-
|I2C_TENBIT
|10bits address support
|-
|I2C_RDWR
|Combined R/W transfer (one STOP only)
|-
|I2C_SMBUS
|Perform an SMBus transfer instead of standard I<sup>2</sup>C
|}
以上命令是主要命令(框架中定义了更多命令)：
请参见'''dev-interface API'''<ref name="dev-interface API">{{CodeSource | Linux kernel | Documentation/i2c/dev-interface}} dev-interface API</ref> 获取完整列表。
====Kernel space peripheral driver====
[[#Kernel_space_driver| Kernel space peripheral driver]] 同时访问 I<sup>2</sup>C 和SMBus设备，并使用以下 '''I2C核心API'''<ref name="I2C core API">https://www.kernel.org/doc/html/latest/driver-api/i2c.html</ref>

==Configuration==
===Kernel configuration===
使用Linux Menuconfig工具在内核配置中激活I2C： [[Menuconfig or how to configure kernel]].

 [x] Device Drivers
     [x] I2C support
         [x] I2C device interface
         [ ] I2C Hardware Bus support
             [x] STMicroelectronics STM32F7 I2C support

这可以在您的内核中手动完成：
 CONFIG_I2C=y
 CONFIG_I2C_CHARDEV=y
 CONFIG_I2C_STM32F7=y

如果软件需要SMBus特定协议，例如SMBus Alert协议和SMBus Host Notify协议，则添加：
 [x] Device Drivers
     [x] I2C support
         [x] I2C device interface
         [ ] Autoselect pertinent helper modules
         [x] SMBus-specific protocols
         [ ] I2C Hardware Bus support
             [x] STMicroelectronics STM32F7 I2C support

这可以在您的内核中手动完成：
 CONFIG_I2C_SMBUS=y

===Device tree configuration===
请参考[[I2C device tree configuration]].

==How to use the framework==
本节介绍如何使用框架访问I2C外设。
===i2c-tools package===
在用户空间中将[[I2C_i2c-tools | I2C Tools]] 与基于'''SMBus API协议'''<ref name="SMBus protocol">{{CodeSource | Linux kernel | Documentation/i2c/smbus-protocol}} SMBus protocol summary</ref> 的shell命令配合使用，可以轻松快速地访问I2C，而无需编写任何代码。<br />
'''用例''' 许多外壳命令允许检测I2C总线并通过SMBus协议访问I2C外设。 该软件包包括一个库，以便在C程序中使用SMBus协议。<br />
有关详细说明，请访问[[I2C_i2c-tools| link]]。
[[File:i2c-tools-overview.png|800px|center|link=|Using i2c-tools overview]]

=== User space application ===
允许在用户空间中使用带有此 '''device interface'''的i2c-dev内核驱动程序来开发应用程序。<ref name="dev-interface API">{{CodeSource | Linux kernel | Documentation/i2c/dev-interface}} dev-interface API</ref>.<br />
'''用例：'''通过加载i2c-dev模块，用户可以通过'''/dev'''接口访问I2C。 通过函数open()、ioctl()、read()、write()和lose()可以非常轻松地访问I2C。 如果外围设备兼容，则也可以使用 [[I2C_i2c-tools | I2C Tools]] 库。<br />
[[File:i2c-dev-overview.png|800px|center|link=|Using i2c-dev overview]]

=== Kernel space driver ===
允许使用'''I2C core API'''开发已编译到内核或作为模块插入的驱动程序<ref name="I2C core API"/><br />
Linux内核提供了有关如何编写I2C客户端驱动程序的示例
<ref>https://www.kernel.org/doc/html/latest/i2c/writing-clients.html</ref><br />
'''用例''' : 使用内核空间内的特定驱动程序控制I2C外设。 例如，驱动程序在系统引导时初始化所有参数，并通过sysfs创建对外围数据的访问。<br />
[[File:i2c-driver-overview.png|800 px|center|link=|Using driver i2c overview]]

===Board description===
To instantiate a peripheral, several methods exist: see '''instantiating devices'''<ref ="instantiating-devices">{{CodeSource | Linux kernel | Documentation/i2c/instantiating-devices}} How to instantiate I2C devices</ref> for more details. <br/>

The below information focuses on '''device tree''', '''sysfs''' and '''Application Code'''.
==== Device tree ====
The device tree is a description of the hardware that is used by the kernel to know which devices are connected. In order to add a slave device on an I2C bus, complete the device tree with the information related to the new device.<br />

'''Example :''' with an EEPROM

<syntaxhighlight lang="c" line highlight="10-14">
&i2c4 {
    status = "okay";
    i2c-scl-rising-time-ns = <185>;
    i2c-scl-falling-time-ns = <20>;

    dmas = <&mdma1 36 0x0 0x40008 0x0 0x0 0>,
           <&mdma1 37 0x0 0x40002 0x0 0x0 0>;
    dma-names = "rx", "tx";
    
    eeprom@50 {
    compatible = "at,24c256";
        pagesize = <64>;
        reg = <0x50>;
    };
};
</syntaxhighlight>

The EEPROM is now instantiated on the bus i2c-X (X depends on how many adapters are probed at runtime) at address 0x50 and it is compatible with the driver registered with the same property.<br/>
Please note the driver specifies a SCL rising/falling time as input.<br/>

Please refer to [[I2C device tree configuration]] for proper configuration and explanation.

Be aware the I2C specification reserves a range of addresses for special purposes, see '''slave addressing'''<ref name="slave addressing">http://www.totalphase.com/support/articles/200349176-7-bit-8-bit-and-10-bit-I2C-Slave-Addressing Slave addressing</ref>.<br />
The below figure shows the relation between the device tree and how it is used :<br />

[[File:i2c-overview-dt.png|840 px|center|link=|relation between device tree and kernel]]

====sysfs====
Through sysfs, i2c-core offers the possibility to instantiate and remove a peripheral: <br /><br />

Add a peripheral "myPeripheralName" attached to the bus x at the address 0xAA<br />
'''Note''' that the field "myPeripheralName" should have the same name as the compatible driver string so that they match one another.
<pre>
echo myPeripheralName 0xAA > i2c-x/new_device
</pre>

Remove a peripheral attached to the bus x at the address 0xAA
<pre>
echo 0xAA > i2c-x/delete_device
</pre>

Into each driver directory ('''/sys/bus/i2c/drivers/at24/''' for the EEPROM peripheral example), it is possible to:<br />
bind a peripheral with a driver
<pre>
echo 3-0050 > bind
</pre>

unbind a peripheral with a driver
<pre>
echo 3-0050 > unbind
</pre>

==== Application code ====
Here is a minimalist code to register a new slave device onto I2C adapter without Device Tree usage.

<syntaxhighlight lang="c" line highlight="4-6,18,20">
#include <linux/i2c.h>

/* Create a device with slave address <0x42> */
static struct i2c_board_info stm32_i2c_test_board_info = {
    I2C_BOARD_INFO("i2c_test07", 0x42);
};

/* 
   Module define creation skipped
*/

static int __init i2c_test_probe(void)
{
    struct i2c_adapter *adap;
    struct i2c_client *client;

    /* Get I2C controller */
    adap = i2c_get_adapter(i);
    /* Build new devices */
    client = i2c_new_device(adap,&stm32_i2c_test_board_info);
}
</syntaxhighlight>

==How to trace and debug the framework ==
In Linux<sup>&reg;</sup> kernel, there are standard ways to debug and monitor I2C. The debug can take place at different levels: hardware and software.

===How to trace===
====Dynamic trace====
Detailed dynamic trace is available here [[How to use the kernel dynamic debug]]<br/>
  {{Board$}} echo  "file i2c-* +p" > /sys/kernel/debug/dynamic_debug/control
This command enables all traces related to I2C core and drivers at runtime.<br/><br/>
Nonetheless at [[Menuconfig or how to configure kernel | Linux® Kernel menu configuration]] level, it provides the granularity for debugging: Core and/or Bus.<br/>
 Device Drivers -> 
     [*] I2C support ->
         [*] I2C Core debugging messages
         [*] I2C Bus debugging messages

* I2C Core debugging messages (CONFIG_I2C_DEBUG_CORE)<br> Compile I2C engine with DEBUG flag.
* I2C Bus debugging messages (CONFIG_I2C_DEBUG_BUS)<br> Compile I2C drivers with DEBUG flag.

Having both '''I2C Core''' and '''I2C Bus''' debugging messages is equivalent to using the above dynamic debug command: the dmesg output will be the same.

====Bus snooping====
Bus snooping is really convenient for viewing I2C protocol and see what has been exchanged between the STM32 and the devices.<br/>
As this debug feature uses [[Ftrace]], please refer to the [[Ftrace]] article for enabling it.

In order to access to events for I2C bus snooping, the following kernel configuration is necessary:

 Kernel hacking -> 
     [*] Tracers ->
         [*] Trace process context switches and events

Depending on the protocol being used, it is necessary to enable i2c and/or smbus tracers as follow:
 echo 1 > /sys/kernel/debug/tracing/events/i2c/enable
 echo 1 > /sys/kernel/debug/tracing/events/smbus/enable

Then tracing is enabled using the following command:
 echo 1 > /sys/kernel/debug/tracing/tracing_on

After a transaction, trace can be read by looking at the trace file:
 cat /sys/kernel/debug/tracing/trace

Here is part of the output, and how it looks like when using ''i2cdetect'' command on the i2c-0 bus:
 ... smbus_write: i2c-0 a=003 f=0000 c=0 QUICK l=0 []
 ... smbus_result: i2c-0 a=003 f=0000 c=0 QUICK wr res=-6
 ... smbus_write: i2c-0 a=004 f=0000 c=0 QUICK l=0 []
 ... smbus_result: i2c-0 a=004 f=0000 c=0 QUICK wr res=-6

{{Info|Notice that i2cdetect, i2cget/i2cput, i2cdump are doing smbus protocol based transactions.}}

On the contrary, below output shows the result of a transaction done in I2C protocol mode:
 ... i2c_write: i2c-1 #0 a=042 f=0000 l=1 [45]
 ... i2c_result: i2c-1 n=1 ret=1
 ... i2c_write: i2c-2 #0 a=020 f=0000 l=1 [45]
 ... i2c_result: i2c-2 n=1 ret=1

The utilization of traces of I2C bus is well described here '''I2C bus snooping'''<ref name="I2C bus snooping">https://linuxtv.org/wiki/index.php/Bus_snooping/sniffing#i2c I2C Bus Snooping</ref>.

===How to debug===
====Detect I2C configuration====
=====sysfs=====
When a peripheral is instantiated, i2c-core and the kernel export different files through sysfs :<br />
'''/sys/class/i2c-adapter/i2c-x''' shows all instantiated I2C buses with 'x' being the I2C bus number.<br />
'''/sys/bus/i2c/devices''' lists all instantiated peripherals. For example, there is a directory named '''3-0050''' that corresponds to the EEPROM peripheral at address 0x50 on bus number 3.<br />
'''/sys/bus/i2c/drivers''' lists all instantiated drivers. Directory named '''at24/''' is the driver of EEPROM.<br />

<pre>
/sys/bus/i2c/devices/3-0050/
            /       /
            /       /i2c-3/3-0050/
            /
            /drivers/at24/3-0050/
</pre>

<pre>
/sys/class/i2c-adapter/i2c-0/
                      /i2c-1/
                      /i2c-2/
                      /i2c-3/3-0050/
                      /i2c-4/
                      /i2c-5/
</pre>

====devfs====
If i2c-dev driver is compiled into the kernel, the directory '''dev''' contains all I2C bus names numbered i2c-0 to i2c-n.
<pre>
/dev/i2c-0
    /i2c-1
    /i2c-2
    /i2c-3
    /i2c-4
    /i2c-n
</pre>

====i2c-tools====
Check all I2C instantiated adapters:
 {{Board$}}i2cdetect -l

See [[I2C_i2c-tools| i2c-tools]] for full description.

==Source code location==
* I2C Framework driver is in {{CodeSource | Linux kernel | drivers/i2c drivers/i2c}}
* I2C STM32 Driver is in {{CodeSource | Linux kernel | drivers/i2c/busses/i2c-stm32f7.c}}
* User API for I2C bus is in {{CodeSource | Linux kernel | include/uapi/linux/i2c.h}} and I2C dev is {{CodeSource | Linux kernel | include/uapi/linux/i2c-dev.h}}.

==To go further==
Bootlin has written a nice walkthrough article: 
''Building a Linux system for the STM32MP1: connecting an I2C sensor''<ref name="Building a Linux system for the STM32MP1: connecting an I2C sensor">https://bootlin.com/blog/building-a-linux-system-for-the-stm32mp1-connecting-an-i2c-sensor/</ref>

==References==
<references />