I2C总线驱动

Linux内核将SOC的I2C适配器抽象为i2c_adapter，include/linux/i2c.h。

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;

	struct i2c_bus_recovery_info *bus_recovery_info;
	const struct i2c_adapter_quirks *quirks;
};

i2c_algorithm类型的指针变量algo，是I2C适配器与I2C设备通信的方法，include/linux/i2c.h。

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
};

master_xfer：I2C适配器传输函数，与I2C设备进行通信。
smbus_xfer：SMBUS总线的传输函数。

I2C总线驱动首先初始化i2c_adapter结构体变量，然后设置i2c_algorithm中的master_xfer函数，最后通过i2c_add_adapter或i2c_add_numbered_adapter函数向Linux系统注册。

int i2c_add_adapter(struct i2c_adapter *adapter);
int i2c_add_numbered_adapter(struct i2c_adapter *adap);

adapter/adap：要注册的I2C适配器。
返回值：0，成功；负值，失败。

使用i2c_del_adapter删除I2C适配器。

void i2c_del_adapter(struct i2c_adapter *adap);

adap：要删除的I2C适配器。
返回值：无。

I2C设备驱动

Linux定义i2c_client结构体描述设备信息，include/linux/i2c.h。

/**
 * 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.
 */
struct i2c_client {
	unsigned short flags;		/* 标志 */
	unsigned short addr;		/* 芯片地址，低7位 */
	char name[I2C_NAME_SIZE];   /* 名字*/
	struct i2c_adapter *adapter;/* I2C适配器 */
	struct device dev;		    /* 设备结构体 */
	int irq;			        /* 设备中断 */
	struct list_head detected;  /* 成员列表 */
#if IS_ENABLED(CONFIG_I2C_SLAVE)
	i2c_slave_cb_t slave_cb;	/* 回调	*/
#endif
};

一个设备对应一个i2c_client，每检测到一个I2C设备就给这个I2C设备分配一个i2c_client。

Linux定义i2c_driver结构体描述驱动信息，include/linux/i2c.h。

/**
 * 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").
	 */
	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;
};

probe和remove函数在I2C设备匹配成功和关闭驱动后执行。
device_driver：驱动结构体，使用设备树时需要设置of_match_table成员变量。
id_table：未使用设备树的匹配列表。

i2c_driver初始化完成后，使用i2c_register_driver向Linux内核注册。

int i2c_register_driver(struct module     *owner,
                        struct i2c_driver *driver);

owner：THIS_MODULE。
driver：要注册的i2c_driver。
返回值：0，成功；负值，失败。

也可使用宏i2c_add_driver注册，对i2c_register_driver进行封装，只有一个driver参数。

#define i2c_add_driver(driver) i2c_register_driver(THIS_MODULE, driver)

使用i2c_del_driver注销I2C设备驱动。

void i2c_del_driver(struct i2c_driver *driver);

driver：要注销的i2c_driver。
返回值：无。

i2c_driver注册流程

static int xxx_probe(struct i2c_client *client, 
                     const struct i2c_device_id *id)
{
    /* ... */
    return 0;
}

static int xxx_remove(struct i2c_client *client)
{
    /* ... */
    return 0;
}

/* 无设备树匹配ID列表 */
static const struct i2c_device_id xxx_id[] = {
    {"xxx", 0},
    {}
};

/* 设备树匹配列表 */
static const struct of_device_id xxx_of_match[] = {
    {.compatible = "xxx"},
    {}
};

/* I2C驱动结构体 */
static struct i2c_driver xxx_driver = {
    .probe = xxx_probe,
    .remove = xxx_remove,
    .driver = {
        .owner = THIS_MODULE,
        .name = "xxx",
        .of_match_table = xxx_of_match,
    },
    .id_table = xxx_id,
};

/* 驱动入口函数 */
static int __init xxx_init(void)
{
    int ret = 0;
    ret = i2c_add_driver(&xxx_driver);
    return ret;    
}

/* 驱动出口函数 */
static void __exit xxx_exit(void)
{
    i2c_del_driver(&xxx_driver);
}

module_init(xxx_init);
module_exit(xxx_exit);