1、I2C核心 I2C核心提供了I2C总线驱动和设备驱动的注册、注销方法，I2C通信方法。 2、I2C总线驱动 I2C总线驱动是对I2C硬件体系结构中适配器的实现。 I2C总线驱动主要包含I2C适配器数据结构i2c_adapter、I2C适配器的algorithm数据结构i2c_algorithm和控制I2C适配器产生通信信号的函数。 经由I2C总线驱动的代码，我们可以控制I2C适配器以主控方式产生开始位、停止位、读写周期，以及以从设备方式被读写、产生ACK等。 3、I2C设备驱动 I2C设备驱动主要包含数据结构i2c_driver和i2c_client。
代码路径：drivers/I2C/busses/i2c_tegra.c 每个代码架构都有他对应的总线代码，下面介绍i2c_adapter、i2c_algorithm、i2c_msg三个结构体 定义总线适配器
struct i2c_adapter { struct module *owner; unsigned int class; /* classes to allow probing for */ const struct i2c_algorithm *algo; /* the algorithm to access the bus */ void *algo_data; /* data fields that are valid for all devices */ struct rt_mutex bus_lock; int timeout; /* in jiffies */ int retries; struct device dev; /* the adapter device */ int nr; char name[48]; struct completion dev_released; struct mutex userspace_clients_lock; struct list_head userspace_clients; };数据传输结构体，决定I2C的通信方式
struct i2c_algorithm { int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs, int num); int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr, unsigned short flags, char read_write, u8 command, int size, union i2c_smbus_data *data); /* To determine what the adapter supports */ u32 (*functionality) (struct i2c_adapter *); };数据格式
struct i2c_msg { __u16 addr; /* 从地址 */ __u16 flags; #define I2C_M_TEN 0x0010 /* this is a ten bit chip address */ #define I2C_M_RD 0x0001 /* read data, from slave to master */ #define I2C_M_NOSTART 0x4000 /* if I2C_FUNC_PROTOCOL_MANGLING */ #define I2C_M_REV_DIR_ADDR 0x2000 /* if I2C_FUNC_PROTOCOL_MANGLING */ #define I2C_M_IGNORE_NAK 0x1000 /* if I2C_FUNC_PROTOCOL_MANGLING */ #define I2C_M_NO_RD_ACK 0x0800 /* if I2C_FUNC_PROTOCOL_MANGLING */ #define I2C_M_RECV_LEN 0x0400 /* length will be first received byte */ __u16 len; /* 数据长度 */ __u8 *buf; /* 数据指针 */ };I2C总线的初始化
static int __init tegra_i2c_init_driver(void) { return platform_driver_register(&tegra_i2c_driver); } subsys_initcall(tegra_i2c_init_driver); int platform_driver_register(struct platform_driver *drv) { drv->driver.bus = &platform_bus_type; if (drv->probe) drv->driver.probe = platform_drv_probe; if (drv->remove) drv->driver.remove = platform_drv_remove; if (drv->shutdown) drv->driver.shutdown = platform_drv_shutdown; return driver_register(&drv->driver); } EXPORT_SYMBOL_GPL(platform_driver_register);总线会像普通设备那个去注册驱动，并调用probe函数。 tegra_i2c_probe函数用于初始化i2c_adapter和i2c_algorithm结构体
static int tegra_i2c_probe(struct platform_device *pdev) { struct tegra_i2c_dev *i2c_dev; struct tegra_i2c_platform_data *pdata = pdev->dev.platform_data; struct resource *res; struct resource *iomem; struct clk *clk; struct clk *i2c_clk; void *base; int irq; int ret = 0; res = platform_get_resource(pdev, IORESOURCE_MEM, 0); …… res = platform_get_resource(pdev, IORESOURCE_IRQ, 0); …… i2c_dev = kzalloc(sizeof(struct tegra_i2c_dev), GFP_KERNEL); if (!i2c_dev) { ret = -ENOMEM; goto err_i2c_clk_put; } i2c_dev->base = base; i2c_dev->clk = clk; i2c_dev->i2c_clk = i2c_clk; i2c_dev->iomem = iomem; i2c_dev->adapter.algo = &tegra_i2c_algo; i2c_dev->irq = irq; i2c_dev->cont_id = pdev->id; i2c_dev->dev = &pdev->dev; i2c_dev->bus_clk_rate = pdata ? pdata->bus_clk_rate : 100000; if (pdev->id == 3) i2c_dev->is_dvc = 1; init_completion(&i2c_dev->msg_complete); platform_set_drvdata(pdev, i2c_dev); ret = tegra_i2c_init(i2c_dev); …… i2c_set_adapdata(&i2c_dev->adapter, i2c_dev); i2c_dev->adapter.owner = THIS_MODULE; i2c_dev->adapter.class = I2C_CLASS_HWMON; strlcpy(i2c_dev->adapter.name, "Tegra I2C adapter", sizeof(i2c_dev->adapter.name)); i2c_dev->adapter.algo = &tegra_i2c_algo; i2c_dev->adapter.dev.parent = &pdev->dev; i2c_dev->adapter.nr = pdev->id; ret = i2c_add_numbered_adapter(&i2c_dev->adapter); if (ret) { dev_err(&pdev->dev, "Failed to add I2C adapter\n"); goto err_free_irq; } return 0; …… }通过i2c_set_adapdata函数设置i2c_adapter； 通过i2c_dev->adapter.algo = &tegra_i2c_algo去指定i2c_algorithm结构体
/*drivers/I2C/busses/i2c-tegra.c*/ static const struct i2c_algorithm tegra_i2c_algo = { .master_xfer = tegra_i2c_xfer, .functionality = tegra_i2c_func, };master_xfer定义了数据传输函数。
在at24cxx_read函数中会调用i2c_transfer函数，进行数据的读取。
/*drivers/I2C/i2c-core.c*/ int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num) { unsigned long orig_jiffies; int ret, try; if (adap->algo->master_xfer) { if (in_atomic() || irqs_disabled()) { ret = i2c_trylock_adapter(adap); if (!ret) /* I2C activity is ongoing. */ return -EAGAIN; } else { i2c_lock_adapter(adap); } /* Retry automatically on arbitration loss */ orig_jiffies = jiffies; for (ret = 0, try = 0; try <= adap->retries; try++) { ret = adap->algo->master_xfer(adap, msgs, num); if (ret != -EAGAIN) break; if (time_after(jiffies, orig_jiffies + adap->timeout)) break; } i2c_unlock_adapter(adap); return ret; } else { dev_dbg(&adap->dev, "I2C level transfers not supported\n"); return -EOPNOTSUPP; } } EXPORT_SYMBOL(i2c_transfer);在i2c_transfer函数中调用adap->algo->master_xfer所指定的函数进行数据的传输。即调用总线去传输数据。
I2C设备的整体运行过程如下： 上层通过read、write函数去调用驱动中对应的接口，然后由接口函数再去调用总线的相关函数，然后实现与设备的通信。
