/** 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;         /*该结构体所属模块*/unsigned int id __deprecated;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;   /*控制并发访问的自旋锁*/int timeout;              /* in jiffies */int retries;            /*重试次数*/struct device dev;          /* the adapter device */int nr;char name[48];           /*适配器的名称*/struct completion dev_released;   /*完成量，用于同步*/struct mutex userspace_clients_lock;struct list_head userspace_clients;  /*client链表头*/
};

/** 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.*/
struct i2c_algorithm {/* If an adapter algorithm can't do I2C-level access, set master_xferto NULL. If an adapter algorithm can do SMBus access, setsmbus_xfer. If set to NULL, the SMBus protocol is simulatedusing common I2C messages *//* master_xfer should return the number of messages successfullyprocessed, or a negative value on error */int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,int num);                       /*i2c传输函数指针*/int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,unsigned short flags, char read_write,u8 command, int size, union i2c_smbus_data *data); /*smbus传输函数指针*//* To determine what the adapter supports */u32 (*functionality) (struct i2c_adapter *);             /*适配器支持的功能*/
};

/*** struct i2c_driver - represent an I2C device driver* @class: What kind of i2c device we instantiate (for detect)* @attach_adapter: Callback for bus addition (for legacy drivers)* @detach_adapter: Callback for bus removal (for legacy drivers)* @probe: Callback for device binding* @remove: Callback for device unbinding* @shutdown: Callback for device shutdown* @suspend: Callback for device suspend* @resume: Callback for device resume* @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_data 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 or is about to be* removed. You should avoid using this if you can, it will probably* be removed in a near future.*/int (*attach_adapter)(struct i2c_adapter *);int (*detach_adapter)(struct i2c_adapter *);/* 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 *);int (*suspend)(struct i2c_client *, pm_message_t mesg);int (*resume)(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").*/void (*alert)(struct i2c_client *, 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;
};

/*** 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* @driver: device's driver, hence pointer to access routines* @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** 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 i2c_driver *driver;    /* and our access routines  */struct device dev;        /* the device structure     */int irq;          /* irq issued by device     */struct list_head detected;
};

2 结构体之间的关系

struct i2c_msg {__u16 addr;    /* slave address            */__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;      /* msg length               */__u8 *buf;        /* pointer to msg data          */
};

i2c_client对应一个物理的i2c设备，i2c_driver则是i2c设备对应的驱动。在i2c_driver注册时，attach_adapter()函数运行并探测设备，若找到client，会将i2c_client结构体中的adapter指针指向对应的i2c_adapter，driver指针指向该i2c_driver，并调用i2c_adapter中的client_register()函数。

2.3 struct i2c_adapter与struct i2c_client