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2 I2C子系统

2.1 LinuxI2C子系统架构

在内核中已经提供I2C子系统，所以在做I2C驱动之前，就必须要熟悉该子系统。

2.2 三大组成部分

1、I2C核心(i2c-core)

I2C核心提供了I2C总线驱动和设备驱动的注册、注销方法，I2C通信方法(algorithm)上层的、与具体适配器无关的代码以及探测设备、检测设备地址的上层代码等。

2、I2C总线驱动(I2Cadapter/Algo driver)

I2C总线驱动是I2C适配器的软件实现，提供I2C适配器与从设备间完成数据通信的能力。

I2C总线驱动由i2c_adapter和i2c_algorithm来描述

3、I2C客户驱动程序(I2Cclient driver)

I2C客户驱动是对I2C从设备的软件实现，一个具体的I2C客户驱动包括两个部分：一部分是i2c_driver，用于将设备挂接于i2c总线；另一部分是设备本身的驱动。

I2C客户驱动程序由i2c_driver和i2c_client来描述

2.3 所有的I2C驱动代码位于drivers/i2c目录下

I2c-core.c 实现I2C核心的功能

I2c-dev.c 通用的从设备驱动

Chips 特定的I2C设备驱动

Busses I2C适配器的驱动

Algos 实现了一些I2C总线适配器的algorithm

2.4 I2C驱动编写的两种方法

从上面的图我们可以看到两种编写驱动方法，一种是利用系统提供的i2c-dev.c来实现一个i2c适配器的设备文件，然后通过在应用层操作I2C适配器来控制I2C设备；另一种是为I2C从设备独立编写一个设备驱动，不需要i2c-dev.c文件。

2.5 重要的数据结构

每次分析子系统免不了分析它的数据结构，OK我们先来分析一下。

I2c_adapter结构体代表I2C总线控制器

[cpp]

struct i2c_adapter {
struct module *owner;
unsigned int class; /*classes to allow probing for */
const struct i2c_algorithm*algo; /* 总线上数据传输的算法*/
void *algo_data; /* algorithm 数据 */
int timeout; /* injiffies */
int retries; /* 重试次数 */
struct device dev; /* the adapter device */
int nr;
char name[48]; /* 适配器名字 */
struct completion dev_released; /* 用于同步 */
};

struct i2c_adapter {
   struct module *owner;
   unsigned int class;       /*classes to allow probing for */
    const struct i2c_algorithm*algo; /* 总线上数据传输的算法*/
   void *algo_data;              /* algorithm 数据 */
 
   int timeout;            /* injiffies */
   int retries;             /* 重试次数 */
    struct device dev;      /* the adapter device */
 
   int nr;
   char name[48];                 /* 适配器名字 */
   struct completion dev_released;   /* 用于同步 */
};

I2c_algorithm对应一套通信方法

[cpp]

struct i2c_algorithm {
int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs, intnum);
int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,
unsigned short flags, charread_write,
u8 command, int size, unioni2c_smbus_data *data);
u32 (*functionality) (structi2c_adapter *);
};

struct i2c_algorithm {
    int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs, intnum);
   int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,
               unsigned short flags, charread_write,
               u8 command, int size, unioni2c_smbus_data *data);
    u32 (*functionality) (structi2c_adapter *);
};

Functionality 函数用于返回algorithm所支持的通信协议，比如I2C_FUNC_I2C,I2C_FUNC_10BIT_ADDR等。

Master_xfer 函数实现总线上数据传输，与具体的适配器有关

Master_xfer函数实现模板

[cpp]

static int i2c_adapter_xxx_xfer(structi2c_adapter *adap, struct i2c_msg *msgs, int num)
{
......
for (i = 0; i < num; i++) {
i2c_adapter_xxx_start(); /*产生起始位*/
if (msgs[i]->flags & I2C_M_RD) { /*读取*/
i2c_adapter_xxx_setaddr((msg->addr << 1) | 1); /*发送从设备地址*/
i2c_adapter_xxx_wait_ack(); /*获得从设备的ACK*/
i2c_adapter_xxx_readbytes(msgs[i]->buf,msgs[i]->len); /*读取len长度的数据到buf中*/
} else {
i2c_adapter_xxx_setaddr(msg->addr << 1);
i2c_adapter_xxx_wait_ack();
i2c_adapter_xxx_writebytes(msgs[i]->buf, msgs[i]->len);
}
}
i2c_adapter_xxx_stop(); /*产生停止位*/
}

static int i2c_adapter_xxx_xfer(structi2c_adapter *adap, struct i2c_msg *msgs, int num)
{
   ......
   for (i = 0; i < num; i++) {
       i2c_adapter_xxx_start();         /*产生起始位*/
       if (msgs[i]->flags & I2C_M_RD) {    /*读取*/
           i2c_adapter_xxx_setaddr((msg->addr << 1) | 1);  /*发送从设备地址*/
           i2c_adapter_xxx_wait_ack();   /*获得从设备的ACK*/
i2c_adapter_xxx_readbytes(msgs[i]->buf,msgs[i]->len);  /*读取len长度的数据到buf中*/
       } else {
           i2c_adapter_xxx_setaddr(msg->addr << 1);
           i2c_adapter_xxx_wait_ack();
           i2c_adapter_xxx_writebytes(msgs[i]->buf, msgs[i]->len);
       }
    }
   i2c_adapter_xxx_stop(); /*产生停止位*/
}

上面调用的函数用于完成适配器的底层硬件操作，与I2C适配器和CPU的具体硬件直接相关，需要由工程师根据芯片的数据手册来实现。在内核源码中，针对不同的I2C适配器都有master_xfer的实现，风格与模板不尽相同，但是可以用该模板作为参考来看源代码，受益匪浅。

I2c_driver代表I2C从设备驱动

[cpp]

struct i2c_driver {
unsignedint class;
int(*attach_adapter)(struct i2c_adapter *) __deprecated; /*依附i2c适配器函数指针*/
int(*detach_adapter)(struct i2c_adapter *) __deprecated;/*脱离i2c适配器函数指针*/
int (*probe)(struct i2c_client*, const struct i2c_device_id *);
int (*remove)(struct i2c_client*);
int(*suspend)(struct i2c_client *, pm_message_t mesg);
int(*resume)(struct i2c_client *);
void(*alert)(struct i2c_client *, 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; /* 该驱动所支持的设备ID表 */
/*Device detection callback for automatic device creation */
int(*detect)(struct i2c_client *, struct i2c_board_info *);
constunsigned short *address_list;
structlist_head clients;
};

struct i2c_driver {
         unsignedint class;
 
         int(*attach_adapter)(struct i2c_adapter *) __deprecated; /*依附i2c适配器函数指针*/
         int(*detach_adapter)(struct i2c_adapter *) __deprecated;/*脱离i2c适配器函数指针*/
 
         int (*probe)(struct i2c_client*, const struct i2c_device_id *);
         int (*remove)(struct i2c_client*);
 
         int(*suspend)(struct i2c_client *, pm_message_t mesg);
         int(*resume)(struct i2c_client *);
         void(*alert)(struct i2c_client *, 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;  /* 该驱动所支持的设备ID表 */
 
         /*Device detection callback for automatic device creation */
         int(*detect)(struct i2c_client *, struct i2c_board_info *);
         constunsigned short *address_list;
         structlist_head clients;
};

在新内核中，attach_adapter和detach_adapter已经被probe和remove取代

Id_table用于i2c_driver和i2c_client的匹配

I2c_client代表I2C从设备

[cpp]

struct i2c_client {
unsigned short flags; /*I2C_CLIENT_TEN：使用10位从地址，I2C_CLIENT_PEC：使用SMBus包错误检测*/
unsignedshort addr; /* chipaddress - NOTE: 7bit */
charname[I2C_NAME_SIZE];
struct i2c_adapter *adapter; /* 依附的i2c_adapter */
struct i2c_driver *driver; /* 依附的i2c_driver*/
structdevice dev; /* the devicestructure */
intirq; /* irq issuedby device */
structlist_head detected;
};

struct i2c_client {
unsigned short flags;                 /*I2C_CLIENT_TEN：使用10位从地址，I2C_CLIENT_PEC：使用SMBus包错误检测*/
         unsignedshort addr;                 /* chipaddress - NOTE: 7bit    */
         charname[I2C_NAME_SIZE];
         struct i2c_adapter *adapter; /* 依附的i2c_adapter   */
         struct i2c_driver *driver;         /* 依附的i2c_driver*/
         structdevice dev;             /* the devicestructure             */
         intirq;                         /* irq issuedby device               */
         structlist_head detected;
};

2.6 核心层提供的接口函数

1、增加/删除I2C适配器

int i2c_add_adapter(struct i2c_adapter *adapter)

int i2c_del_adapter(struct i2c_adapter *adap)

[cpp]

static int i2c_register_adapter(struct i2c_adapter *adap)
{
res = device_register(&adap->dev);
if (adap->nr <__i2c_first_dynamic_bus_num)
i2c_scan_static_board_info(adap);
bus_for_each_drv(&i2c_bus_type, NULL, adap, __process_new_adapter);
}

 
static int i2c_register_adapter(struct i2c_adapter *adap)
{
res = device_register(&adap->dev);
 
if (adap->nr <__i2c_first_dynamic_bus_num)
           i2c_scan_static_board_info(adap);
 
bus_for_each_drv(&i2c_bus_type, NULL, adap, __process_new_adapter);
}

Device_register(&adap->dev) 向I2C总线注册一个adapter设备

i2c_scan_static_board_info(adap) 注册所有已知的i2c_client

2、增加/删除I2C从设备驱动

[cpp]

int i2c_add_driver(struct i2c_driver *driver)
void i2c_del_driver(struct i2c_driver *driver)
inti2c_register_driver(struct module *owner, struct i2c_driver *driver)
{
/* add the driver to the list of i2c drivers inthe driver core */
driver->driver.owner = owner;
driver->driver.bus = &i2c_bus_type;
res = driver_register(&driver->driver);
/* Walk the adapters that are already present*/
i2c_for_each_dev(driver, __process_new_driver);
}

int i2c_add_driver(struct i2c_driver *driver)
void i2c_del_driver(struct i2c_driver *driver)
 
inti2c_register_driver(struct module *owner, struct i2c_driver *driver)
{
 
/* add the driver to the list of i2c drivers inthe driver core */
driver->driver.owner = owner;
driver->driver.bus = &i2c_bus_type;
 
res = driver_register(&driver->driver);
 
/* Walk the adapters that are already present*/
i2c_for_each_dev(driver, __process_new_driver);
}

driver_register(&driver->driver) 向I2C总线注册一个i2c_driver

3、 i2c传输，发送和接收

[cpp]

int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg*msgs, int num)
int i2c_master_send(const struct i2c_client *client, constchar *buf, int count)
int i2c_master_recv(const struct i2c_client *client, char*buf, int count)
int i2c_transfer(structi2c_adapter *adap, struct i2c_msg *msgs, int num)
{
if (adap->algo->master_xfer) {
for (ret = 0, try = 0; try <=adap->retries; try++) {
ret = adap->algo->master_xfer(adap, msgs,num);
}
}
}

int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg*msgs, int num)
int i2c_master_send(const struct i2c_client *client, constchar *buf, int count)
int i2c_master_recv(const struct i2c_client *client, char*buf, int count)
 
int i2c_transfer(structi2c_adapter *adap, struct i2c_msg *msgs, int num)
{
if (adap->algo->master_xfer) {
           for (ret = 0, try = 0; try <=adap->retries; try++) {
                    ret = adap->algo->master_xfer(adap, msgs,num);
           }
}
}

最终会调用到适配器实现的master_xfer函数来完成数据传输工作

参考资料：《essential Linux device drivers》

《linux设备驱动开发详解》

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