I2C（Inter-Integrated Circuit，内置集成电路）最初为飞利浦所开发，为2线接口。这2根线为时钟线和双向数据线，分别被称为SCL（Serial CLock，串行时钟）和SDA（Serial DAta，串行数据）。由于I2C总线仅需要一对总线，因此在电路板上占用的空间更少，带来的问题是带宽较窄。即使I2C支持双向数据交换，由于仅有一根数据线，故通信是半双工的。
由于I2C被大量的微控制器所支持，在当前的市场上可找到大量便宜的I2C设备。
I2C主机：master，一般来说是主控制器，也可以理解为CPU
I2C从机：slave，一般来说是外设
I2C 总线上有3 种类型的信号，分别是：开始信号，结束信号和应答信号。这些信号都是由SDA和SCL上的电平变化来表示的。
开始信号（S）：当SCL为高电平时，SDA由高电平向低电平跳变，表示开始传输数据。
结束信号（P）：当SCL为高电平时，SDA由低电平向高电平跳变，表示结束传输数据。
相应信号（ACK）：从机接收到8位数据后，在第9个时钟周期，拉低SDA电平，表示已经接收到数据。
当总线空闲时，SDA 和SCL 都处于高电平，主机检测到总线空闲就可以向从机发送数据。主机首先发送开始信号S，接着发出8位数据（包括前7位的从机地址和1 为的方向位），然后等待从机发回确认信号ACK.
当第8位为0时，表示向从机传输数据，主机收到确认信号后就可以连续的向从机写入8 位数据；当第8 位为1时，表示向从读取数据，这时主机就可以接收来自从机的一系列数据。最后当总个数据传输过程完成后，由主机发送结束信号P，表示本次的数据传输完成。
说到I2C总线，就不得不提一下它的一个好基友——SMBus。
SMBus (System Management Bus,系统管理总线) 是1995年由Intel提出的。它大部分基于I2C总线规范，和I2C一样，SMBus不需要增加额外的引脚。
当然I2C和SMBus还是有很多差异的，比如以运作频率，I2C此方面相当宽裕，最低频可至0Hz（直流状态，等于时间暂停），高可至100kHz（Standard Mode）、400kHz（Fast Mode）、乃至3.4MHz（High Speed Mode），相对的SMBus就很局限，最慢不慢于10kHz，最快不快于100kHz。很明显的，I2C与SMBus的交集运作频率即是10kHz〜100kHz间。其它还有不少差异，这里不做细论。
I2C设备挂载到I2C总线下，是需要一个确定的地址的。一般来说I2C都使用7位地址，协议也支持10位地址，但大多数的设备仅响应7位地址，因此在总线上最多有127个设备，0号"设备"作为群呼地址。
I2C设备，第一个字节(为slave address)由7位地址和一位R/W读写位组成的，这字节是个器件地址。
首先，你要知道：常用IIC接口通用器件的器件地址是由种类型号，及寻址码组成的，共7位。
如格式如下：
D7 D6 D5 D4 D3 D2 D1 D0
器件类型由：D7-D4 共4位决定的。这是由半导公司生产时就已固定此类型的了，也就是说这4位已是固定的。
用户自定义地址码：D3-D1共3位。这是由用户自己设置的，通常的作法如EEPROM这些器件是由外部IC的3个引脚所组合电平决定的（用常用的名字如A0,A1,A2）。这也就是寻址码。
所以为什么同一IIC总线上同一型号的IC只能最多共挂8片同种类芯片的原因了。
最低一位：就是R/W位。
就I2C的整体结构而言，I2C可以分为核心层、总线层、设备层。
简单来说：
核心层提供统一的接口。
总线层是用来驱动I2C总线的。
设备层这是用来驱动I2C设备的。
一般来说的I2C设备只要关注设备层的内容即可。
在kernel-4.9/include/linux/i2c.h中定义了几个关键的结构。
表示设备的i2c_clinet：
/**
* struct i2c_client - represent an I2C slave device
* @flags: I2C_CLIENT_TEN indicates the device uses a ten bit chip address;
* I2C_CLIENT_PEC indicates it uses SMBus Packet Error Checking
* @addr: Address used on the I2C bus connected to the parent adapter.
* @name: Indicates the type of the device, usually a chip name that's
* generic enough to hide second-sourcing and compatible revisions.
* @adapter: manages the bus segment hosting this I2C device
* @dev: Driver model device node for the slave.
* @irq: indicates the IRQ generated by this device (if any)
* @detected: member of an i2c_driver.clients list or i2c-core's
* userspace_devices list
* @slave_cb: Callback when I2C slave mode of an adapter is used. The adapter
* calls it to pass on slave events to the slave driver.
*
* An i2c_client identifies a single device (i.e. chip) connected to an
* i2c bus. The behaviour exposed to Linux is defined by the driver
* managing the device.
*/
struct i2c_client {
unsigned short flags; /* div., see below */
unsigned short addr; /* chip address - NOTE: 7bit */
/* addresses are stored in the */
/* _LOWER_ 7 bits */
char name[I2C_NAME_SIZE];
struct i2c_adapter *adapter; /* the adapter we sit on */
struct device dev; /* the device structure */
int irq; /* irq issued by device */
struct list_head detected;
#if IS_ENABLED(CONFIG_I2C_SLAVE)
i2c_slave_cb_t slave_cb; /* callback for slave mode */
#endif
};代表驱动的i2c_driver：
/**
* struct i2c_driver - represent an I2C device driver
* @class: What kind of i2c device we instantiate (for detect)
* @attach_adapter: Callback for bus addition (deprecated)
* @probe: Callback for device binding
* @remove: Callback for device unbinding
* @shutdown: Callback for device shutdown
* @alert: Alert callback, for example for the SMBus alert protocol
* @command: Callback for bus-wide signaling (optional)
* @driver: Device driver model driver
* @id_table: List of I2C devices supported by this driver
* @detect: Callback for device detection
* @address_list: The I2C addresses to probe (for detect)
* @clients: List of detected clients we created (for i2c-core use only)
*
* The driver.owner field should be set to the module owner of this driver.
* The driver.name field should be set to the name of this driver.
*
* For automatic device detection, both @detect and @address_list must
* be defined. @class should also be set, otherwise only devices forced
* with module parameters will be created. The detect function must
* fill at least the name field of the i2c_board_info structure it is
* handed upon successful detection, and possibly also the flags field.
*
* If @detect is missing, the driver will still work fine for enumerated
* devices. Detected devices simply won't be supported. This is expected
* for the many I2C/SMBus devices which can't be detected reliably, and
* the ones which can always be enumerated in practice.
*
* The i2c_client structure which is handed to the @detect callback is
* not a real i2c_client. It is initialized just enough so that you can
* call i2c_smbus_read_byte_data and friends on it. Don't do anything
* else with it. In particular, calling dev_dbg and friends on it is
* not allowed.
*/
struct i2c_driver {
unsigned int class;
/* Notifies the driver that a new bus has appeared. You should avoid
* using this, it will be removed in a near future.
*/
int (*attach_adapter)(struct i2c_adapter *) __deprecated;
/* Standard driver model interfaces */
int (*probe)(struct i2c_client *, const struct i2c_device_id *);
int (*remove)(struct i2c_client *);
/* driver model interfaces that don't relate to enumeration */
void (*shutdown)(struct i2c_client *);
/* Alert callback, for example for the SMBus alert protocol.
* The format and meaning of the data value depends on the protocol.
* For the SMBus alert protocol, there is a single bit of data passed
* as the alert response's low bit ("event flag").
* For the SMBus Host Notify protocol, the data corresponds to the
* 16-bit payload data reported by the slave device acting as master.
*/
void (*alert)(struct i2c_client *, enum i2c_alert_protocol protocol,
unsigned int data);
/* a ioctl like command that can be used to perform specific functions
* with the device.
*/
int (*command)(struct i2c_client *client, unsigned int cmd, void *arg);
struct device_driver driver;
const struct i2c_device_id *id_table;
/* Device detection callback for automatic device creation */
int (*detect)(struct i2c_client *, struct i2c_board_info *);
const unsigned short *address_list;
struct list_head clients;
};总线的基本工作方式是将设备和驱动都注册到总线上，然后总线负责它们的匹配工作。
这里已经有了设备和驱动，下面就看一下总线了，它的结构体是i2c_adapter，其中成员algo是传输数据的算法：
/*
* i2c_adapter is the structure used to identify a physical i2c bus along
* with the access algorithms necessary to access it.
*/
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 */
const struct i2c_lock_operations *lock_ops;
struct rt_mutex bus_lock;
struct rt_mutex mux_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;
struct i2c_bus_recovery_info *bus_recovery_info;
const struct i2c_adapter_quirks *quirks;
};
/**
* struct i2c_algorithm - represent I2C transfer method
* @master_xfer: Issue a set of i2c transactions to the given I2C adapter
* defined by the msgs array, with num messages available to transfer via
* the adapter specified by adap.
* @smbus_xfer: Issue smbus transactions to the given I2C adapter. If this
* is not present, then the bus layer will try and convert the SMBus calls
* into I2C transfers instead.
* @functionality: Return the flags that this algorithm/adapter pair supports
* from the I2C_FUNC_* flags.
* @reg_slave: Register given client to I2C slave mode of this adapter
* @unreg_slave: Unregister given client from I2C slave mode of this adapter
*
* The following structs are for those who like to implement new bus drivers:
* i2c_algorithm is the interface to a class of hardware solutions which can
* be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584
* to name two of the most common.
*
* The return codes from the @master_xfer field should indicate the type of
* error code that occurred during the transfer, as documented in the kernel
* Documentation file Documentation/i2c/fault-codes.
*/
struct i2c_algorithm {
/* If an adapter algorithm can't do I2C-level access, set master_xfer
to NULL. If an adapter algorithm can do SMBus access, set
smbus_xfer. If set to NULL, the SMBus protocol is simulated
using common I2C messages */
/* master_xfer should return the number of messages successfully
processed, or a negative value on error */
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 *);
#if IS_ENABLED(CONFIG_I2C_SLAVE)
int (*reg_slave)(struct i2c_client *client);
int (*unreg_slave)(struct i2c_client *client);
#endif
};添加驱动到总线i2c_add_driver：
/* use a define to avoid include chaining to get THIS_MODULE */
#define i2c_add_driver(driver) \
i2c_register_driver(THIS_MODULE, driver)添加设备到总线i2c_register_board_info，这里有个宏I2C_BOARD_INFO可以帮忙快速赋值：
/**
* i2c_register_board_info - statically declare I2C devices
* @busnum: identifies the bus to which these devices belong
* @info: vector of i2c device descriptors
* @len: how many descriptors in the vector; may be zero to reserve
* the specified bus number.
*
* Systems using the Linux I2C driver stack can declare tables of board info
* while they initialize. This should be done in board-specific init code
* near arch_initcall() time, or equivalent, before any I2C adapter driver is
* registered. For example, mainboard init code could define several devices,
* as could the init code for each daughtercard in a board stack.
*
* The I2C devices will be created later, after the adapter for the relevant
* bus has been registered. After that moment, standard driver model tools
* are used to bind "new style" I2C drivers to the devices. The bus number
* for any device declared using this routine is not available for dynamic
* allocation.
*
* The board info passed can safely be __initdata, but be careful of embedded
* pointers (for platform_data, functions, etc) since that won't be copied.
*/
int i2c_register_board_info(int busnum, struct i2c_board_info const *info, unsigned len)
/**
* I2C_BOARD_INFO - macro used to list an i2c device and its address
* @dev_type: identifies the device type
* @dev_addr: the device's address on the bus.
*
* This macro initializes essential fields of a struct i2c_board_info,
* declaring what has been provided on a particular board. Optional
* fields (such as associated irq, or device-specific platform_data)
* are provided using conventional syntax.
*/
#define I2C_BOARD_INFO(dev_type, dev_addr) \
.type = dev_type, .addr = (dev_addr)