I2C体系架构
主要由三部分组成：
(1) I2C核心
提供I2C控制器和设备驱动的注册和注销方法，I2C通信方法，与适配器无关的代码以及探测设备等。
(2) I2C控制器驱动(适配器)
(3) I2C设备驱动
重要结构体
i2c_adapter
//i2c控制器(适配器)struct i2c_adapter {struct module *owner;unsigned int class; /* classes to allow probing for */const struct i2c_algorithm *algo; /* 总线通信结构体指针 */void *algo_data; //algorithm数据/* data fields that are valid for all devices *///并发同步，互斥锁const struct i2c_lock_operations *lock_ops;struct rt_mutex bus_lock;struct rt_mutex mux_lock;int timeout; /* in jiffies */int retries; //重试次数struct device dev; /* the adapter device */int nr;char name[48]; //适配器名称struct completion dev_released;struct mutex userspace_clients_lock;struct list_head userspace_clients; //client链表struct i2c_bus_recovery_info *bus_recovery_info;const struct i2c_adapter_quirks *quirks;struct irq_domain *host_notify_domain;};
i2c_algorithm
//I2C传输方法struct i2c_algorithm {//i2c传输函数指针int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,int num);//smbus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);//返回适配器支持的功能u32 (*functionality) (struct i2c_adapter *);//作为从机时使用#if IS_ENABLED(CONFIG_I2C_SLAVE)int (*reg_slave)(struct i2c_client *client);int (*unreg_slave)(struct i2c_client *client);#endif};
SMBus是基于I2C总线规范的，所以上面的传输函数要根据自己的总线来选择，选择其一就可以。
i2c_driver
//I2C驱动，和platform_driver,spi_driver类似struct i2c_driver {unsigned int class;/* Standard driver model interfaces */int (*probe)(struct i2c_client *, const struct i2c_device_id *);int (*remove)(struct i2c_client *);int (*probe_new)(struct i2c_client *);void (*shutdown)(struct i2c_client *);void (*alert)(struct i2c_client *, enum i2c_alert_protocol protocol,unsigned int data);int (*command)(struct i2c_client *client, unsigned int cmd, void *arg);struct device_driver driver;const struct i2c_device_id *id_table; //该设备所支持的设备ID表/* 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; //client链表bool disable_i2c_core_irq_mapping;};
i2c_client
//I2C设备struct i2c_client {unsigned short flags; //标志unsigned short addr; //芯片地址，保存在addr低7位char name[I2C_NAME_SIZE]; //设备名称struct i2c_adapter *adapter; //依附的i2c_adapterstruct device dev; //设备结构体int irq; //设备使用的中断号struct list_head detected; //client链表，和i2c_driver中clients的一样#if IS_ENABLED(CONFIG_I2C_SLAVE)i2c_slave_cb_t slave_cb; //从机模式回调#endif};
i2c_msg
//I2C传输数据结构体，代表一个消息数据struct i2c_msg {__u16 addr; //设备地址__u16 flags; //标志#define I2C_M_RD 0x0001 /* read data, from slave to master *//* I2C_M_RD is guaranteed to be 0x0001! */#define I2C_M_TEN 0x0010 /* this is a ten bit chip address */#define I2C_M_DMA_SAFE 0x0200 /* the buffer of this message is DMA safe *//* makes only sense in kernelspace *//* userspace buffers are copied anyway */#define I2C_M_RECV_LEN 0x0400 /* length will be first received byte */#define I2C_M_NO_RD_ACK 0x0800 /* if I2C_FUNC_PROTOCOL_MANGLING */#define I2C_M_IGNORE_NAK 0x1000 /* if I2C_FUNC_PROTOCOL_MANGLING */#define I2C_M_REV_DIR_ADDR 0x2000 /* if I2C_FUNC_PROTOCOL_MANGLING */#define I2C_M_NOSTART 0x4000 /* if I2C_FUNC_NOSTART */#define I2C_M_STOP 0x8000 /* if I2C_FUNC_PROTOCOL_MANGLING */__u16 len; //消息长度__u8 *buf; //消息数据};
总结上面结构体关系：
1. i2c_adapter和i2c_algorithm
i2c_adapter对应物理上的一个适配器，而i2c_algorithm对应一套通信方法，适配器需要通过i2c_algorithm提供的通信函数来产生对应的访问时序。所以i2c_adapter中包含i2c_algorithm的指针。
i2c_algorithm使用master_xfer()来产生I2C时序，以i2c_msg为单位，i2c_msg代表一次传输的数据。
2. i2c_driver和i2c_client
i2c_driver对应一套驱动方法，包含probe,remove等方法。i2c_clent对应真实的物理设备，每个i2c设备都需要一个i2c_client来描述。i2c_driver与i2c_client是一对多的关系，一个i2c_driver上可以支持多个同类型的i2c_client。
3. i2c_adapter和i2c_client
i2c_adapter与i2c_client的关系和硬件上适配器与设备的关系一致，即i2c_client依附于i2c_adapter。一个适配器可以连接多个设备，所以i2c_adapter中包含i2c_client的链表。
API函数
//增加/删除i2c_adapterint i2c_add_adapter(struct i2c_adapter *adapter)void i2c_del_adapter(struct i2c_adapter *adap)//增加/删除i2c_driverint i2c_register_driver(struct module *owner, struct i2c_driver *driver)void i2c_del_driver(struct i2c_driver *driver)#define i2c_add_driver(driver) \i2c_register_driver(THIS_MODULE, driver)//i2c传输、发送和接收//完成I2C总线和I2C设备之间的一定数目的I2C message交互int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)//通过封装i2c_transfer()完成一次I2c发送操作int i2c_master_send(const struct i2c_client *client,const char *buf, int count)//通过封装i2c_transfer()完成一次I2c接收操作int i2c_master_recv(const struct i2c_client *client,char *buf, int count)
i2c_transfer()函数本身不具备驱动适配器物理硬件以完成消息交互的能力，它只是寻找到与i2c_adapter对应的i2c_algorithm, 并使用i2c_algorithm的master_xfer()函数真正驱动硬件流程。
追踪i2c_transfer()的源码会发现下面的代码
for (ret = 0, try = 0; try <= adap->retries; try++) {ret = adap->algo->master_xfer(adap, msgs, num); //真正发送的函数if (ret != -EAGAIN)break;if (time_after(jiffies, orig_jiffies + adap->timeout))break;}
适配器驱动
由于I2C控制器通常是在内存上的，所以它本身也连接在platform总线上的，通过platform_driver和platform_device的匹配还执行。
(1) probe()完成如下工作：
初始化I2C控制器所使用的硬件资源，如申请IO地址，中断号，时钟等。
为特定I2C控制器实现通信方法，主要是实现i2c_algorithm的master_xfer()和functionality()函数。
通过i2c_add_adapter()添加i2c_adapter的数据结构(i2c_adapter成员已被初始化)。
模板代码：
static const struct i2c_algorithm xxx_i2c_algo = {.master_xfer = xxx_i2c_master_xfer,.functionality = xxx_i2c_func,};static u32 xxx_i2c_func(struct i2c_adapter *adap){return I2C_FUNC_I2C | I2C_FUNC_10BIT_ADDR |(I2C_FUNC_SMBUS_EMUL & ~I2C_FUNC_SMBUS_QUICK) |I2C_FUNC_SMBUS_BLOCK_DATA;}static int xxx_i2c_master_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs,int num){int ret, ;u32 reg;struct xxx_i2c *id = adap->algo_data;/* Process the msg one by one */for (i = 0; i < num; i++, msgs++) {i2c_adapter_xxx_start(); /*产生开始位*//*是读消息*/if (msgs[i]->flags &I2C_M_RD){i2c_adapter_xxx_setaddr((msg->addr << 1) | 1); /*发送从设备读地址*/i2c_adapter_xxx_wait_ack(); /*获得从设备的ack*/i2c_adapter_xxx_readbytes(msgs[i]->buf, msgs[i]->len); /*读取msgs[i]->len长的数据到msgs[i]->buf*/}else/*是写消息*/{i2c_adapter_xxx_setaddr(msg->addr << 1); /*发送从设备写地址*/i2c_adapter_xxx_wait_ack(); /*获得从设备的ack*/i2c_adapter_xxx_writebytes(msgs[i]->buf, msgs[i]->len); /*读取msgs[i]->len长的数据到msgs[i]->buf*/}}i2c_adapter_xxx_stop(); /*产生停止位*/return num;}static int xxx_i2c_probe(struct platform_device *pdev) // dts里的设备信息传递进来了{struct resource *r_mem;struct xxx_i2c *id;int ret;id = devm_kzalloc(&pdev->dev, sizeof(*id), GFP_KERNEL);if (!id)return -ENOMEM;platform_set_drvdata(pdev, id);xxx_adapter_hw_init(); //通常初始化iic适配器使用的硬件资源，如申请IO地址、中断号、时钟等id->adap.dev.of_node = pdev->dev.of_node;id->adap.algo = &xxx_i2c_algo; // 把altorithm连进来id->adap.timeout = XXX_I2C_TIMEOUT;id->adap.retries = 3; /* Default retry value. */id->adap.algo_data = id;id->adap.dev.parent = &pdev->dev;ret = i2c_add_adapter(&id->adap);...}static int xxx_i2c_remove(struct platform_device *pdev){struct xxx_i2c *id = platform_get_drvdata(pdev);i2c_del_adapter(&id->adap);xxx_adapter_hw_free(); // 硬件相关资源的freereturn 0;}static const struct of_device_id xxx_i2c_of_match[] = {{ .compatible = "cdns,i2c-r1p10", },{ /* end of table */ }};MODULE_DEVICE_TABLE(of, xxx_i2c_of_match);static struct platform_driver xxx_i2c_drv = {.driver = {.name = DRIVER_NAME,.owner = THIS_MODULE,.of_match_table = xxx_i2c_of_match, // dts匹配的依据.pm = &xxx_i2c_dev_pm_ops,},.probe = xxx_i2c_probe,.remove = xxx_i2c_remove,};module_platform_driver(xxx_i2c_drv);
xxx_adapter_hw_init实现和具体的CPU和I2C控制器硬件相关的初始化。
functionality() 函数比较简单，返回支持的通信协议。
master_xfer() 函数在适配器上完成i2c_msg的数据传输。
设备驱动
i2c_dirver就是i2c标准总线设备驱动模型中的驱动部分，i2c_client可理解为i2c总线上挂的外设。
模板代码：
static struct i2c_driver xxx_driver = {.driver = {.name = "xxx",.of_match_table = xxx_of_match,.acpi_match_table = ACPI_PTR(xxx_acpi_ids),},.probe_new = xxx_probe,.remove = xxx_remove,.id_table = xxx_ids,};static int __init xxx_init(void){.......return i2c_add_driver(&xxx_driver); // 匹配后，driver中的probe就能执行}static void __exit xxx_exit(void){i2c_del_driver(&xxx_driver);}module_exit(xxx_exit);module_init(xxx_init);
总结
上面是对Linux中I2C相关架构的分析，后面会拿出一些相对应的驱动来进行具体分析。希望能做到理论和实践相结合！
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