Linux I2C子系统架构全解析

I2C子系统架构概述

I2C子系统是Linux内核中用于管理I2C总线设备的框架，采用分层设计模式，分为核心层、适配器层和设备驱动层。核心层提供注册和管理功能，适配器层处理硬件通信，设备驱动层实现具体设备功能。

核心层功能

核心层位于drivers/i2c/i2c-core.c，提供总线类型注册、设备匹配和通信接口。关键结构体i2c_adapter表示物理适配器，i2c_client表示连接的设备。核心层通过i2c_add_adapter()和i2c_register_driver()实现组件注册。

适配器层实现

适配器层由SoC厂商实现，需填充i2c_algorithm结构体定义传输方法：

struct i2c_algorithm {
    int (*master_xfer)(struct i2c_adapter *adap, 
                      struct i2c_msg *msgs, int num);
};

典型实现包括寄存器配置、时序控制和中断处理。例如STM32的硬件抽象层需配置CR1/CR2寄存器控制时钟频率。

设备驱动开发

设备驱动通过i2c_driver结构体注册，实现probe()/remove()方法：

static struct i2c_driver foo_driver = {
    .driver = { .name = "foo" },
    .probe = foo_probe,
    .remove = foo_remove,
    .id_table = foo_ids,
};

数据传输使用i2c_transfer()或封装函数如i2c_smbus_read_byte()。时钟频率需在设备树中通过clock-frequency属性指定。

设备树配置

典型I2C节点定义包含总线参数和子设备：

i2c1: i2c@40005400 {
    compatible = "st,stm32-i2c";
    reg = <0x40005400 0x400>;
    clocks = <&rcc 0 21>;
    #address-cells = <1>;
    #size-cells = <0>;
    eeprom@50 {
        compatible = "atmel,24c02";
        reg = <0x50>;
    };
};

调试技巧

使用i2c-tools工具包进行总线检测：

i2cdetect -y 1  # 扫描总线1上的设备
i2cget -y 1 0x50  # 读取0x50地址寄存器

内核调试可启用CONFIG_I2C_DEBUG_CORE选项，通过dmesg查看传输日志。示波器测量SCL/SDA信号可验证时序是否符合标准模式（100kHz）或快速模式（400kHz）。

性能优化策略

采用DMA传输减少CPU占用，使用i2c_put_dma_safe_msg_buf()处理缓存对齐。高频通信时需优化i2c_msg缓冲区分配，避免频繁内存申请。对于多设备系统，可通过i2c_new_dummy_device()创建虚拟客户端实现多路复用。

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