I2C体系结构
linux的I2C体系结构分为3个组成部分：
(1) I2C核心。提供I2C总线驱动和设备驱动的注册和注销方法。
(2) I2C总线驱动。对适配器端的实现。
(3) I2C设备驱动。设备端的实现。
整个体系架构如图：
I2C设备在sysfs文件系统中显示在sys/bus/i2c目录，例如：
理解i2c体系结构，首先要理解i2c_driver、i2c_client、i2c_adapter和i2c_algorithm这四个数据结构。这四个数据结构均在i2c.h在定义：
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 *);
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 {
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;
};
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;
};
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 *);
};
看代码，稍作分析：
1. i2c_adapter对应物理上一个适配器，而i2c_algorithm对应一套通信方法。一个i2c适配器需要i2c_algorithm中提供的通信函数来控制适配器上产生特定的访问周期。i2c_algorithm中关键函数master_xfer用于产生i2c访问周期需要的信号，以i2c_msg为单位。
2. i2c_driver对应一套驱动方法，辅助作用的数据结构，不对应任何物理实体。i2c_client对应于真实的物理设备，每个i2c设备都需要一个i2c_client来描述。i2c_driver的attach_adapter函数运行时将i2c_driver和i2c_client关联起来。
3. i2c_adapter和i2c_client的关系与i2c硬件体系中适配器和设备的关系一致，即i2c_client依附于i2c_adapter。
I2C核心
i2c核心中提供了一组不依赖于硬件平台的接口函数，具体可以查看dirvers/i2c/i2c-core.c文件。主要是增加删除i2c_driver和增加删除i2c_adapter，i2c传输发送接收等。主要功能函数必须看代码理解，不太需要我们自己去改。
I2C总线驱动
适配器的加载主要是初始化i2c适配器所使用的硬件资源，如申请I/O地址、中断后等，通过i2c_add_adapter添加i2c_adapter数据结构。卸载工作则与之相反。
i2c适配器通信方法，我们需要根据特定的适配器实现i2c_algorithm中的master_xfer方法，master_xfer方法用于传输i2c消息，内核源码中有很多可以参考的实例，这里摘取一个：
static int i2c_xfer(struct i2c_adapter *i2c_adap, struct i2c_msg *msgs, int num)
{
struct saa7164_i2c *bus = i2c_adap->algo_data;
struct saa7164_dev *dev = bus->dev;
int i, retval = 0;
dprintk(DBGLVL_I2C, "%s(num = %d)\n", __func__, num);
for (i = 0 ; i < num; i++) {
dprintk(DBGLVL_I2C, "%s(num = %d) addr = 0x%02x len = 0x%x\n",
__func__, num, msgs[i].addr, msgs[i].len);
if (msgs[i].flags & I2C_M_RD) {
/* Unsupported - Yet*/
printk(KERN_ERR "%s() Unsupported - Yet\n", __func__);
continue;
} else if (i + 1 < num && (msgs[i + 1].flags & I2C_M_RD) &&
msgs[i].addr == msgs[i + 1].addr) {
/* write then read from same address */
retval = saa7164_api_i2c_read(bus, msgs[i].addr,
msgs[i].len, msgs[i].buf,
msgs[i+1].len, msgs[i+1].buf
);
i++;
if (retval < 0)
goto err;
} else {
/* write */
retval = saa7164_api_i2c_write(bus, msgs[i].addr,
msgs[i].len, msgs[i].buf);
}
if (retval < 0)
goto err;
}
return num;
err:
return retval;
}
I2C设备驱动
i2c设备驱动需要使用i2c_driver和i2c_client数据结构并填充其中的成员函数。
i2c驱动的注册加载，仍然从源码中摘取一段，看看：
static struct i2c_driver adnp_i2c_driver = {
.driver = {
.name = "gpio-adnp",
.owner = THIS_MODULE,
.of_match_table = adnp_of_match,
},
.probe = adnp_i2c_probe,
.remove = adnp_i2c_remove,
.id_table = adnp_i2c_id,
};
i2c设备的注册，直接看代码：
int saa7164_i2c_register(struct saa7164_i2c *bus)
{
struct saa7164_dev *dev = bus->dev;
dprintk(DBGLVL_I2C, "%s(bus = %d)\n", __func__, bus->nr);
bus->i2c_adap = saa7164_i2c_adap_template;
bus->i2c_client = saa7164_i2c_client_template;
bus->i2c_adap.dev.parent = &dev->pci->dev;
strlcpy(bus->i2c_adap.name, bus->dev->name,
sizeof(bus->i2c_adap.name));
bus->i2c_adap.algo_data = bus;
i2c_set_adapdata(&bus->i2c_adap, bus);
i2c_add_adapter(&bus->i2c_adap);
bus->i2c_client.adapter = &bus->i2c_adap;
if (0 != bus->i2c_rc)
printk(KERN_ERR "%s: i2c bus %d register FAILED\n",
dev->name, bus->nr);
return bus->i2c_rc;
}
剩下的就是提供文件接口了。
总得说来，我们需要完成i2c适配器的硬件驱动、探测等，根据特定的传输规则实现algorithm中的master_xfer方法，实现设备与驱动的接口，attach_adapter。最后实现i2c设备驱动的文件操作接口。
这些理论还是通过实例会理解的更深刻一些。i2c的实例很普遍，还是通过实例学习好一些。改日通过一个显示屏的驱动实例再来过一遍吧。。