linux设备模型之i2c子系统

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      I2c子系统将i2c控制器(i2c寄存器所在的那块电路)抽象出来，用adapter（适配器）这个结构来描述，可以说一个适配器就代表一条i2c总 线，而挂接在i2c总线上的设备是用client这个结构体来表述，另外i2c_bus上的设备链表挂接的不单单是连接的这条i2c上的client，同 样adapter也作为一个设备挂在其所在的i2c_bus，也就是说控制器和设备都作为i2c_bus上的设备连接在设备链表，他们用内嵌的 device的type这个成员来区分，适配器的类型为i2c_adapter_type，client的类型为i2c_client_type。

一、i2c相关的描述结构

     首先看一下i2c子系统给adapter定义的描述结构：

 struct i2c_adapter {
    struct module *owner;
    unsigned int id;
    unsigned int class；                     // 适配器支持的类型，如传感器，eeprom等
    const struct i2c_algorithm *algo;        //该适配器的通信函数
    void *algo_data;
    /* data fields that are valid for all devices   */
    struct rt_mutex bus_lock;
    int timeout;                             //超时时间限定
    int retries;                             //通信重复次数限定
    /*
     * 内嵌的标准device，其中dev->type标识该设备
     * 是个adapter，其值为i2c_adapter_type
     */
    struct device dev;

    int nr;                                  //适配器编号也是bus编号，第几条i2c总线
    char name[48];                           //名字
    struct completion dev_released;
    struct mutex userspace_clients_lock;
    struct list_head userspace_clients;
};

   再来看一下client的描述结构：

 struct i2c_client {
    unsigned short flags;                   //设备的标志，如唤醒标志等等

    /* chip address - NOTE: 7bit    */
    /* addresses are stored in the  */
    /* _LOWER_ 7 bits       */
    unsigned short addr;                    //设备的地址
    char name[I2C_NAME_SIZE];               //设备的名字
    struct i2c_adapter *adapter;            //设备所属的适配器
    struct i2c_driver *driver;              //设备的driver

    /*
     * 内嵌的标准device模型，其中dev->type标识该设备
     * 是个client，其值为i2c_client_type
     */
    struct device dev;      /* the device structure */
    int irq;                               //中断号
    struct list_head detected;             //挂接点，挂接在adapter
};

    下面是driver的表述结构i2c_driver：

 struct i2c_driver {
unsigned int class;                                   //支持的类型，与adapter的class相对
/* 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 *);          //旧式探测函数
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);
 /*
 * 内嵌的标准driver，driver的of_match_table成员也用于标识其支持
 * 的设备，并且优先级高于id_table
 */
struct device_driver driver;

const struct i2c_device_id *id_table;                            //支持的client信息表
/* Device detection callback for automatic device creation */

int (*detect)(struct i2c_client *, struct i2c_board_info *);     //探测函数
const unsigned short *address_list;                              //driver支持的client地址
struct list_head clients;                                        //挂接其探测到的支持的设备
;

      另外client端有一条全局链表，用于串联所有i2c的client设备，为__i2c_board_list，也就是说client可以静态注册亦可动态
被探测，静态注册挂接在该链表上的结构为：

struct i2c_devinfo {
    struct list_head    list;                     //连接指针指向前后设备
    int         busnum;                           //所在bus的编号
    struct i2c_board_info   board_info;           //板级平台信息相关的结构体
};
//其中 i2c_board_info结构的源码为：
struct i2c_board_info {
    char        type[I2C_NAME_SIZE];             //名字
    unsigned short  flags;                       //标志
    unsigned short  addr;                        //地址
    void        *platform_data;                  //私有特殊数据
    struct dev_archdata *archdata;
#ifdef CONFIG_OF
    struct device_node *of_node;                 //节点
#endi
    int     irq;                                 //中断号
};

    i2c_devinfo结构静态注册的信息最后都会被整合集成到client中，形成一个标准的i2c_client设备并注册。

二、i2c核心初始化代码分析

    首先看一下i2c平台无关的核心初始化，代码位于drivers/i2c/i2c-core.c下：

static int __init i2c_init(void)
{
    int retval;
   /*
    * 注册i2c_bus
    */
    retval = bus_register(&i2c_bus_type);
    if (retval)
        return retval;
#ifdef CONFIG_I2C_COMPAT
   /*
    * 在sys/class下创建适配器目录
    */
    i2c_adapter_compat_class = class_compat_register("i2c-adapter");
    if (!i2c_adapter_compat_class) {
        retval = -ENOMEM;
        goto bus_err;
    }

#endif
   /*
    * 增加一个虚拟的driver
    */
    retval = i2c_add_driver(&dummy_driver);
    if (retval)
        goto class_err;
    return 0;
class_err:
#ifdef CONFIG_I2C_COMPAT
    class_compat_unregister(i2c_adapter_compat_class);
bus_err:
#endif
    bus_unregister(&i2c_bus_type);
    return retval;
}
//其中的i2c_bus_type原型为：
struct bus_type i2c_bus_type = {
    .name       = "i2c",
    .match      = i2c_device_match,
    .probe      = i2c_device_probe,
    .remove     = i2c_device_remove,
    .shutdown   = i2c_device_shutdown,
    .pm     = &i2c_device_pm_ops,
};

三、i2c_add_driver分析

    驱动端的统一接口为i2c_add_driver：

static inline int i2c_add_driver(struct i2c_driver *driver)
{
    /*
     *注册i2c driver，可能是adapter的，也可能是client的
     */
    return i2c_register_driver(THIS_MODULE, driver);
}
int i2c_register_driver(struct module *owner, struct i2c_driver *driver)
{
    int res;
    /* Can't register until after driver model init*/
    if (unlikely(WARN_ON(!i2c_bus_type.p)))
        return -EAGAIN;
    /* add the driver to the list of i2c drivers in the driver core */

    /*
     * i2c_driver内嵌的标准driver赋值，其bus指定为i2c_bus_type
     */
    driver->driver.owner = owner;
    driver->driver.bus = &i2c_bus_type;
    /* When registration returns, the driver core
     * will have called probe() for all matching-but-unbound devices.
     */

    /*注册标准的driver，driver注册后会去i2c_bus_type的设备链表上匹配
     *设备，匹配函数用的是bus端的，也就是i2c_device_match，如果匹配成功
     *将建立标准关联，并且将调用bus端的probe函数初始化这个设备，即
     *函数i2c_device_probe，下面会逐个分析
     */
    res = driver_register(&driver->driver);
    if (res)
        return res;
    pr_debug("i2c-core: driver [%s] registered/n", driver->driver.name);
  /*
   * 把该driver的clients初始化，该成员连接着这个driver可以操作的具
   * 体设备
   */
    INIT_LIST_HEAD(&driver->clients);
    /* Walk the adapters that are already present */
    mutex_lock(&core_lock);

  /*
   * 遍历挂接在该i2c设备链表上的设备，并对其都调用__process_new_driver
   * 函数
   */
    bus_for_each_dev(&i2c_bus_type, NULL, driver, __process_new_driver);
    mutex_unlock(&core_lock);
    return 0;
}
/****************************
* 匹配函数i2c_device_match *
****************************/
static int i2c_device_match(struct device *dev, struct device_driver *drv)
{
    /*
     * i2c_verify_client检查匹配的这个设备是否为i2c_client_type
     * 类型，如果不是则返回NULL，此处的匹配只是针对i2c设备的
     * 不是适配器
     */

    struct i2c_client   *client = i2c_verify_client(dev);
    struct i2c_driver   *driver;
      /*
     * 如果不是i2c设备类型就返回
     */
    if (!client)
        return 0;
    /* Attempt an OF style match */

    /*
     * 如果定义了CONFIG_OF_DEVICE，那么就利用
     * drv.of_match_table成员表进行匹配
     */
    if (of_driver_match_device(dev, drv))
        return 1;
      /*
     * 由内嵌的driver得到外面封装的i2c_driver
     */
    driver = to_i2c_driver(drv);

    /* match on an id table if there is one */

    /*
     * 如果i2c_driver->id_table存在，也就是支持的设备信息表
     * 存在，那么利用这个表进行匹配
     */
    if (driver->id_table)
        return i2c_match_id(driver->id_table, client) != NULL;
    return 0;
}
/**********************************
* 初始化设备函数i2c_device_probe *
**********************************/
static int i2c_device_probe(struct device *dev)
{
    /*
     * 检查如果设备类型不是client则返回
     */
    struct i2c_client   *client = i2c_verify_client(dev);
    struct i2c_driver   *driver;
    int status;
    if (!client)
        return 0;

      /*
     * dev->driver指向匹配完成的driver，根据该标准
     * driver得到其外围封装的i2c_driver
     */
    driver = to_i2c_driver(dev->driver);

    /*
     * 如果该i2c_driver的probe成员或者id_table成员为
     * NULL则退出
     */
    if (!driver->probe || !driver->id_table)
        return -ENODEV;

    /*
     * client的driver成员赋值为该i2c_driver
     */
    client->driver = driver;

    /*
     * 唤醒该设备
     */
    if (!device_can_wakeup(&client->dev))
        device_init_wakeup(&client->dev,
                    client->flags & I2C_CLIENT_WAKE);
    dev_dbg(dev, "probe/n");

     /*
     * 利用i2c_driver的probe成员初始化该设备，此部分为实际平台相关
     */
    status = driver->probe(client, i2c_match_id(driver->id_table, client));

    /*
     * 失败则清除client指定的driver
     */
    if (status) {
        client->driver = NULL;
        i2c_set_clientdata(client, NULL);
    }
    return status;
}
/*************************************************************
* 下面看一下当找到一个dev后调用的__process_new_driver函数 *
*************************************************************/
static int __process_new_driver(struct device *dev, void *data)
{
    /*
     * 设备的类型如果不是i2c_adapter类型就推出
     * 下面的代码是针对i2c适配器的代码
     */
    if (dev->type != &i2c_adapter_type)
        return 0;

    /*
     * 如果这个设备代表i2c适配器，则调用i2c_do_add_adapter
     * 此时的data类型为i2c_driver
     */
    return i2c_do_add_adapter(data, to_i2c_adapter(dev));//根据设备得到他的适配器
                             //i2c_driver。第一个是i2c_driver
}
/*************************
* i2c_do_add_adapter函数 *
*************************/
static int i2c_do_add_adapter(struct i2c_driver *driver,
                  struct i2c_adapter *adap)
{
    /* Detect supported devices on that bus, and instantiate them */

    /*
     * 利用该适配器和该i2c_driver探测该适配器所在的这条i2c总线
     * 找到该driver支持的设备并实例化它
     */
    i2c_detect(adap, driver);

    /* Let legacy drivers scan this bus for matching devices */

    /*
     * 老版本的探测利用i2c_driver的attach_adapter函数
     */
    if (driver->attach_adapter) {
        /* We ignore the return code; if it fails, too bad */
        driver->attach_adapter(adap);
    }
    return 0;
}
/****************************
* 重点看一下i2c_detect函数 *
****************************/
static int i2c_detect(struct i2c_adapter *adapter, struct i2c_driver *driver)
{
    const unsigned short *address_list;
    struct i2c_client *temp_client;
    int i, err = 0;
    int adap_id = i2c_adapter_id(adapter);
  /*
   * 得到该i2c_driver指定的client地址范围
   */
    address_list = driver->address_list;

  /*
   * driver平台相关的detect函数和client地址范围不能为NULL
   */
    if (!driver->detect || !address_list)
        return 0;
    /* Set up a temporary client to help detect callback */

  /*
   * 申请一块client内存
   */
    temp_client = kzalloc(sizeof(struct i2c_client), GFP_KERNEL);
    if (!temp_client)
        return -ENOMEM;

  /*
   * 申请的client结构的adapter成员设置为当前的adapter
   */
    temp_client->adapter = adapter;
    /* Stop here if the classes do not match */

    /*
     * 当前adapter的类型如果和driver的类型不一样，则退出
     * 例如：适配器的类型可以为传感器，eeprom，driver类型必须
     * 与其匹配
     */
    if (!(adapter->class & driver->class))
        goto exit_free;
  /*
   * 根据指定的支持的地址范围开始逐一探测
   */
    for (i = 0; address_list[i] != I2C_CLIENT_END; i += 1) {
        dev_dbg(&adapter->dev, "found normal entry for adapter %d, "
            "addr 0x%02x/n", adap_id, address_list[i]);
  /*
   * 临时申请的client的地址设置为这次要探测的地址
   */
        temp_client->addr = address_list[i];
        err = i2c_detect_address(temp_client, driver);
        if (err)
            goto exit_free;
    }
 exit_free:
    kfree(temp_client);
    return err;
}
/**********************************
* 继续跟进i2c_detect_address函数 *
**********************************/
static int i2c_detect_address(struct i2c_client *temp_client,
                  struct i2c_driver *driver)
{
    struct i2c_board_info info;
    struct i2c_adapter *adapter = temp_client->adapter;
    int addr = temp_client->addr;
    int err;
    /* Make sure the address is valid */

  /*
   * 检查该地址是否有效，小于0x08或者大于0x77都是无效
   * 地址，该函数在后面介绍
   */
    err = i2c_check_addr_validity(addr);
    if (err) {
        dev_warn(&adapter->dev, "Invalid probe address 0x%02x/n",
             addr);
        return err;
    }
    /* Skip if already in use */

  /*
   * 如果地址在使用中则跳过
   */
    if (i2c_check_addr_busy(adapter, addr))
        return 0;
    /* Make sure there is something at this address */

  /*
   * 默认初始化探测，确定该地址上有设备存在
   */
    if (!i2c_default_probe(adapter, addr))
        return 0;
    /* Finally call the custom detection function */

  /*
   * 走到这里将调用平台相关的自定义探测函数去探测该地址
   * 上是否设备，并填充i2c_board_info结构体
   */
    memset(&info, 0, sizeof(struct i2c_board_info));
    info.addr = addr;
    err = driver->detect(temp_client, &info);
    if (err) {
        /* -ENODEV is returned if the detection fails. We catch it
           here as this isn't an error. */
        return err == -ENODEV ? 0 : err;
    }
    /* Consistency check */

    /*
     * 填充的info名字为空，则结束否则实例化这个设备
     */
    if (info.type[0] == '/0') {
        dev_err(&adapter->dev, "%s detection function provided "
            "no name for 0x%x/n", driver->driver.name,
            addr);
    } else {
        struct i2c_client *client;
        /* Detection succeeded, instantiate the device */
        dev_dbg(&adapter->dev, "Creating %s at 0x%02x/n",
            info.type, info.addr);

    /*
     * 根据当前适配器和填充的info实例化该地址上探测到的设备
     */
        client = i2c_new_device(adapter, &info);

    /*
     * 实例化成功将该client挂到该driver的clients链表上
     */
        if (client)
            list_add_tail(&client->detected, &driver->clients); //驱动挂到driver下
        else
            dev_err(&adapter->dev, "Failed creating %s at 0x%02x/n",
                info.type, info.addr);
    }
    return 0;
}
/******************************
* i2c_check_addr_validity函数 *
******************************/
static int i2c_check_addr_validity(unsigned short addr)
{
    /*
     * Reserved addresses per I2C specification:
     *  0x00       General call address / START byte
     *  0x01       CBUS address
     *  0x02       Reserved for different bus format
     *  0x03       Reserved for future purposes
     *  0x04-0x07  Hs-mode master code
     *  0x78-0x7b  10-bit slave addressing
     *  0x7c-0x7f  Reserved for future purposes
     */
    if (addr < 0x08 || addr > 0x77)
        return -EINVAL;
    return 0;
}
/*********************************
* 再看一下i2c_default_probe函数 *
*********************************/
static int i2c_default_probe(struct i2c_adapter *adap, unsigned short addr)
{
    int err;
    union i2c_smbus_data dummy;
#ifdef CONFIG_X86
        /*
     * 这里是对intel特殊设备的检查，就不深入看下去了
     */
    if (addr == 0x73 && (adap->class & I2C_CLASS_HWMON)
     && i2c_check_functionality(adap, I2C_FUNC_SMBUS_READ_BYTE_DATA))
        err = i2c_smbus_xfer(adap, addr, 0, I2C_SMBUS_READ, 0,
                     I2C_SMBUS_BYTE_DATA, &dummy);
    else
#endif
        /*
     * 对特殊设备的检查
     */
    if (!((addr & ~0x07) == 0x30 || (addr & ~0x0f) == 0x50)
     && i2c_check_functionality(adap, I2C_FUNC_SMBUS_QUICK))
        err = i2c_smbus_xfer(adap, addr, 0, I2C_SMBUS_WRITE, 0,
                     I2C_SMBUS_QUICK, NULL);
  /*
   * i2c_check_functionality函数确定该i2c适配器所支持的通信方式
   * 如果支持该方式则调用i2c_smbus_xfer函数
   */
    else if (i2c_check_functionality(adap, I2C_FUNC_SMBUS_READ_BYTE))
        err = i2c_smbus_xfer(adap, addr, 0, I2C_SMBUS_READ, 0,
                     I2C_SMBUS_BYTE, &dummy);
    else {
        dev_warn(&adap->dev, "No suitable probing method supported/n");
        err = -EOPNOTSUPP;
    }
    return err >= 0;
}
/******************************
* i2c_check_functionality函数 *
******************************/
static inline int i2c_check_functionality(struct i2c_adapter *adap, u32 func)
{
    return (func & i2c_get_functionality(adap)) == func;
}
static inline u32 i2c_get_functionality(struct i2c_adapter *adap)
{
   /*
    * 最终会调用adapter通信函数里面的functionality函数确定支持的
    * 通信方式
    */
    return adap->algo->functionality(adap);
}
/*********************
* i2c_smbus_xfer函数 *
*********************/
s32 i2c_smbus_xfer(struct i2c_adapter *adapter, u16 addr, unsigned short flags,
           char read_write, u8 command, int protocol,
           union i2c_smbus_data *data)
{
    unsigned long orig_jiffies;
    int try;
    s32 res;
    flags &= I2C_M_TEN | I2C_CLIENT_PEC;

   /*
    * 如果适配器通信函数中的smbus_xfer函数存在，则直接利用它进行发送
    */
    if (adapter->algo->smbus_xfer) {
        i2c_lock_adapter(adapter);
        /* Retry automatically on arbitration loss */
        orig_jiffies = jiffies;
        for (res = 0, try = 0; try <= adapter->retries; try++) {
            res = adapter->algo->smbus_xfer(adapter, addr, flags,
                            read_write, command,
                            protocol, data);
            if (res != -EAGAIN)
                break;
            if (time_after(jiffies,
                       orig_jiffies + adapter->timeout))
                break;
        }
        i2c_unlock_adapter(adapter);
    } else

   /*
    * 否则利用i2c_smbus_xfer_emulated处理,此处也就是不支持smbus，
    * 则得利用i2c模拟smbus命令
    */
        res = i2c_smbus_xfer_emulated(adapter, addr, flags, read_write,
                          command, protocol, data);
    return res;
}
/******************************
* i2c_smbus_xfer_emulated函数 *
******************************/
static s32 i2c_smbus_xfer_emulated(struct i2c_adapter *adapter, u16 addr,
                   unsigned short flags,
                   char read_write, u8 command, int size,
                   union i2c_smbus_data *data)
{
    /* So we need to generate a series of msgs. In the case of writing, we
      need to use only one message; when reading, we need two. We initialize
      most things with sane defaults, to keep the code below somewhat
      simpler. */

  /*
   * 为了进行通信我们必须创建msgs结构，当写时，我们需要一个这样的结构就
   * 够了，当读的时候，我们需要两个
   */
    unsigned char msgbuf0[I2C_SMBUS_BLOCK_MAX+3];
    unsigned char msgbuf1[I2C_SMBUS_BLOCK_MAX+2];

  /*
   * 读的时候需要两次
   */
    int num = read_write == I2C_SMBUS_READ ? 2 : 1;

  /*
   * 填充需要的两个msg结构
   */
    struct i2c_msg msg[2] = { { addr, flags, 1, msgbuf0 },
                              { addr, flags | I2C_M_RD, 0, msgbuf1 }
                            };
    int i;
    u8 partial_pec = 0;
    int status;
  /*
   * 将要发送的命令填充到msg0
   */
    msgbuf0[0] = command;
    switch (size) {

  /*
   * 快速传输，多用于确定该地址有应答
   */
    case I2C_SMBUS_QUICK:
        msg[0].len = 0;
        /* Special case: The read/write field is used as data */
        msg[0].flags = flags | (read_write == I2C_SMBUS_READ ?
                    I2C_M_RD : 0);
        num = 1;
        break;

  /*
   * 字节传输，一次读写一个字节
   */
    case I2C_SMBUS_BYTE:
        if (read_write == I2C_SMBUS_READ) {
            /* Special case: only a read! */
            msg[0].flags = I2C_M_RD | flags;
            num = 1;
        }
        break;

  /*
   * 命令+单字节形式传输
   */
    case I2C_SMBUS_BYTE_DATA:
        if (read_write == I2C_SMBUS_READ)
            msg[1].len = 1;
        else {
            msg[0].len = 2;
            msgbuf0[1] = data->byte;
        }
        break;

  /*
   * 命令+字形式传输
   */
    case I2C_SMBUS_WORD_DATA:
        if (read_write == I2C_SMBUS_READ)
            msg[1].len = 2;
        else {
            msg[0].len = 3;
            msgbuf0[1] = data->word & 0xff;
            msgbuf0[2] = data->word >> 8;
        }
        break;

  /*
   * 命令+字形式，需要应答
   */
    case I2C_SMBUS_PROC_CALL:
        num = 2; /* Special case */
        read_write = I2C_SMBUS_READ;
        msg[0].len = 3;
        msg[1].len = 2;
        msgbuf0[1] = data->word & 0xff;
        msgbuf0[2] = data->word >> 8;
        break;

  /*
   * 多字节数据模式，字节数传输中不确定
   */
    case I2C_SMBUS_BLOCK_DATA:
        if (read_write == I2C_SMBUS_READ) {
            msg[1].flags |= I2C_M_RECV_LEN;
            msg[1].len = 1; /* block length will be added by
                       the underlying bus driver */
        } else {
            msg[0].len = data->block[0] + 2;
            if (msg[0].len > I2C_SMBUS_BLOCK_MAX + 2) {
                dev_err(&adapter->dev,
                    "Invalid block write size %d/n",
                    data->block[0]);
                return -EINVAL;
            }
            for (i = 1; i < msg[0].len; i++)
                msgbuf0[i] = data->block[i-1];
        }
        break;

  /*
   * 多字节数据传输，需要应答
   */
    case I2C_SMBUS_BLOCK_PROC_CALL:
        num = 2; /* Another special case */
        read_write = I2C_SMBUS_READ;
        if (data->block[0] > I2C_SMBUS_BLOCK_MAX) {
            dev_err(&adapter->dev,
                "Invalid block write size %d/n",
                data->block[0]);
            return -EINVAL;
        }
        msg[0].len = data->block[0] + 2;
        for (i = 1; i < msg[0].len; i++)
            msgbuf0[i] = data->block[i-1];
        msg[1].flags |= I2C_M_RECV_LEN;
        msg[1].len = 1; /* block length will be added by
                   the underlying bus driver */
        break;

  /*
   * 多字节数据传输，传输字节数确定
   */
    case I2C_SMBUS_I2C_BLOCK_DATA:
        if (read_write == I2C_SMBUS_READ) {
            msg[1].len = data->block[0];
        } else {
            msg[0].len = data->block[0] + 1;
            if (msg[0].len > I2C_SMBUS_BLOCK_MAX + 1) {
                dev_err(&adapter->dev,
                    "Invalid block write size %d/n",
                    data->block[0]);
                return -EINVAL;
            }
            for (i = 1; i <= data->block[0]; i++)
                msgbuf0[i] = data->block[i];
        }
        break;
    default:
        dev_err(&adapter->dev, "Unsupported transaction %d/n", size);
        return -EOPNOTSUPP;
    }
    i = ((flags & I2C_CLIENT_PEC) && size != I2C_SMBUS_QUICK
                      && size != I2C_SMBUS_I2C_BLOCK_DATA);
    if (i) {
        /* Compute PEC if first message is a write */
        if (!(msg[0].flags & I2C_M_RD)) {
            if (num == 1) /* Write only */
                i2c_smbus_add_pec(&msg[0]);
            else /* Write followed by read */
                partial_pec = i2c_smbus_msg_pec(0, &msg[0]);
        }
        /* Ask for PEC if last message is a read */
        if (msg[num-1].flags & I2C_M_RD)
            msg[num-1].len++;
    }
  /*
   * 调用i2c_transfer传输
   */
    status = i2c_transfer(adapter, msg, num);
    if (status < 0)
        return status;
    /* Check PEC if last message is a read */
    if (i && (msg[num-1].flags & I2C_M_RD)) {
        status = i2c_smbus_check_pec(partial_pec, &msg[num-1]);
        if (status < 0)
            return status;
    }

  /*
   * 将得到的数据回传给data
   */
    if (read_write == I2C_SMBUS_READ)
        switch (size) {
        case I2C_SMBUS_BYTE:
            data->byte = msgbuf0[0];
            break;
        case I2C_SMBUS_BYTE_DATA:
            data->byte = msgbuf1[0];
            break;
        case I2C_SMBUS_WORD_DATA:
        case I2C_SMBUS_PROC_CALL:
            data->word = msgbuf1[0] | (msgbuf1[1] << 8);
            break;
        case I2C_SMBUS_I2C_BLOCK_DATA:
            for (i = 0; i < data->block[0]; i++)
                data->block[i+1] = msgbuf1[i];
            break;
        case I2C_SMBUS_BLOCK_DATA:
        case I2C_SMBUS_BLOCK_PROC_CALL:
            for (i = 0; i < msgbuf1[0] + 1; i++)
                data->block[i] = msgbuf1[i];
            break;
        }
    return 0;
}
/*******************
* i2c_transfer函数 *
*******************/
int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
{
    unsigned long orig_jiffies;
    int ret, try;
    /* REVISIT the fault reporting model here is weak:
     *
     *  - When we get an error after receiving N bytes from a slave,
     *    there is no way to report "N".
     *
     *  - When we get a NAK after transmitting N bytes to a slave,
     *    there is no way to report "N" ... or to let the master
     *    continue executing the rest of this combined message, if
     *    that's the appropriate response.
     *
     *  - When for example "num" is two and we successfully complete
     *    the first message but get an error part way through the
     *    second, it's unclear whether that should be reported as
     *    one (discarding status on the second message) or errno
     *    (discarding status on the first one).
     */
  /*
   * 如果适配器的adap->algo->master_xfer函数存在，则调用它把
   * 该信息发送出去
   */
    if (adap->algo->master_xfer) {
#ifdef DEBUG
        for (ret = 0; ret < num; ret++) {
            dev_dbg(&adap->dev, "master_xfer[%d] %c, addr=0x%02x, "
                "len=%d%s/n", ret, (msgs[ret].flags & I2C_M_RD)
                ? 'R' : 'W', msgs[ret].addr, msgs[ret].len,
                (msgs[ret].flags & I2C_M_RECV_LEN) ? "+" : "");
        }
#endif
        if (in_atomic() || irqs_disabled()) {
            ret = i2c_trylock_adapter(adap);
            if (!ret)
                /* I2C activity is ongoing. */
                return -EAGAIN;
        } else {
            i2c_lock_adapter(adap);
        }
        /* Retry automatically on arbitration loss */
        orig_jiffies = jiffies;
        for (ret = 0, try = 0; try <= adap->retries; try++) {
            ret = adap->algo->master_xfer(adap, msgs, num);
            if (ret != -EAGAIN)
                break;
            if (time_after(jiffies, orig_jiffies + adap->timeout))
                break;
        }
        i2c_unlock_adapter(adap);
        return ret;
    } else {
        dev_dbg(&adap->dev, "I2C level transfers not supported/n");
        return -EOPNOTSUPP;
    }
}
/**********************************************
* 回过头来看一下i2c_new_device这个实例化函数*
**********************************************/
struct i2c_client *
i2c_new_device(struct i2c_adapter *adap, struct i2c_board_info const *info)
{
    struct i2c_client   *client;
    int         status;

  /*
   * 为需要实例化的设备申请内存
   */
    client = kzalloc(sizeof *client, GFP_KERNEL);
    if (!client)
        return NULL;

  /*
   * 指定适配器以及platform_data
   */
    client->adapter = adap;
    client->dev.platform_data = info->platform_data;

  /*
   * info->archdata存在将其赋值给client
   */
    if (info->archdata)
        client->dev.archdata = *info->archdata;
  /*
   * 标志、地址、中断号、名字
   */
    client->flags = info->flags;
    client->addr = info->addr;
    client->irq = info->irq;
    strlcpy(client->name, info->type, sizeof(client->name));
    /* Check for address validity */

  /*
   * 检查地址有效性
   */
    status = i2c_check_client_addr_validity(client);
    if (status) {
        dev_err(&adap->dev, "Invalid %d-bit I2C address 0x%02hx/n",
            client->flags & I2C_CLIENT_TEN ? 10 : 7, client->addr);
        goto out_err_silent;
    }
    /* Check for address business */

  /*
   * 检查地址是否被使用
   */
    status = i2c_check_addr_busy(adap, client->addr);
    if (status)
        goto out_err;
  /*
   * 内嵌标准device的赋值
   */
    client->dev.parent = &client->adapter->dev;
    client->dev.bus = &i2c_bus_type;
    client->dev.type = &i2c_client_type;
#ifdef CONFIG_OF
    client->dev.of_node = info->of_node;
#endif
    dev_set_name(&client->dev, "%d-%04x", i2c_adapter_id(adap),
             client->addr);
  /*
   * 注册内嵌的标准device
   */
    status = device_register(&client->dev);
    if (status)
        goto out_err;
    dev_dbg(&adap->dev, "client [%s] registered with bus id %s/n",
        client->name, dev_name(&client->dev));
    return client;
out_err:
    dev_err(&adap->dev, "Failed to register i2c client %s at 0x%02x "
        "(%d)/n", client->name, client->addr, status);
out_err_silent:
    kfree(client);
    return NULL;
}

     以上就是i2c通用driver添加的流程，下面看一下设备端，适配器的流程
四、i2c_add_adapter分析

int i2c_add_adapter(struct i2c_adapter *adapter)
{
    int id, res = 0;
retry:
  /*
   * 得到bus号并将其插入搜索树，便于高效查找
   * 此处不做深入分析
   */
    if (idr_pre_get(&i2c_adapter_idr, GFP_KERNEL) == 0)
        return -ENOMEM;
    mutex_lock(&core_lock);
    /* "above" here means "above or equal to", sigh */
    res = idr_get_new_above(&i2c_adapter_idr, adapter,
                __i2c_first_dynamic_bus_num, &id);
    mutex_unlock(&core_lock);
    if (res < 0) {
        if (res == -EAGAIN)
            goto retry;
        return res;
    }

  /*
   * 适配器号赋值，代表i2c的编号
   */
    adapter->nr = id;
    return i2c_register_adapter(adapter);
}
/***********************************************
* 来看一下适配器的注册函数i2c_register_adapter*
***********************************************/
static int i2c_register_adapter(struct i2c_adapter *adap)
{
    int res = 0;
    /* Can't register until after driver model init */

  /*
   * bus私有属性结构不能为NULL
   */
    if (unlikely(WARN_ON(!i2c_bus_type.p))) {
        res = -EAGAIN;
        goto out_list;
    }
    rt_mutex_init(&adap->bus_lock);
    mutex_init(&adap->userspace_clients_lock);
    INIT_LIST_HEAD(&adap->userspace_clients);
    /* Set default timeout to 1 second if not already set */

  /*
   * 超时时间设置为1s
   */
    if (adap->timeout == 0)
        adap->timeout = HZ;

  /*
   * 设置内嵌device的名字，指定bus，指定自身类型为适配器
   */
    dev_set_name(&adap->dev, "i2c-%d", adap->nr);
    adap->dev.bus = &i2c_bus_type;
    adap->dev.type = &i2c_adapter_type;

  /*
   * 注册内嵌的标准device，此时将会出现在i2c_bus目录下
   */
    res = device_register(&adap->dev);
    if (res)
        goto out_list;
    dev_dbg(&adap->dev, "adapter [%s] registered/n", adap->name);
#ifdef CONFIG_I2C_COMPAT
    res = class_compat_create_link(i2c_adapter_compat_class, &adap->dev,
                       adap->dev.parent);
    if (res)
        dev_warn(&adap->dev,
             "Failed to create compatibility class link/n");
#endif
    /* create pre-declared device nodes */

  /*
   * client可以静态的添加，如果发现适配器号也就是i2c号
   * 小于动态bus号，说明设备静态添加，则进行扫描
   */
    if (adap->nr < __i2c_first_dynamic_bus_num)
        i2c_scan_static_board_info(adap);
    /* Notify drivers */
    mutex_lock(&core_lock);

  /*
   * 遍历bus的驱动端，对于每一个driver都调用__process_new_adapter
   */
    bus_for_each_drv(&i2c_bus_type, NULL, adap, __process_new_adapter);
    mutex_unlock(&core_lock);
    return 0;
out_list:
    mutex_lock(&core_lock);
    idr_remove(&i2c_adapter_idr, adap->nr);
    mutex_unlock(&core_lock);
    return res;
}
/*******************************************
* 先来看一下i2c_scan_static_board_info函数*
*******************************************/
static void i2c_scan_static_board_info(struct i2c_adapter *adapter)
{
    struct i2c_devinfo  *devinfo;
    down_read(&__i2c_board_lock);
  /*
   * 遍历全局的i2c的client链表，找到该适配器器所代表
   * 总线上挂接的设备，用i2c_new_device实例化它，
   * i2c_new_device在前面已经分析过了，不在赘述
   */
    list_for_each_entry(devinfo, &__i2c_board_list, list) {
        if (devinfo->busnum == adapter->nr
                && !i2c_new_device(adapter,
                        &devinfo->board_info))
            dev_err(&adapter->dev,
                "Can't create device at 0x%02x/n",
                devinfo->board_info.addr);
    }
    up_read(&__i2c_board_lock);
}
/**************************************
* 再来看一下__process_new_adapter函数*
**************************************/
static int __process_new_adapter(struct device_driver *d, void *data)
{
  /*
   * 同样的归宿到了i2c_do_add_adapter下面，与前面分析的
   * __process_new_driver相似，只是driver是针对适配器，
   * 而这次没有这个限制
   */
    return i2c_do_add_adapter(to_i2c_driver(d), data);
}

  依据以上的分析画出流程图如下:

 

      i2c_driver依据内部成员的设定，会走不同的分支，产生不同的作用，下面根据流程前后顺序总结一下：
当发现的是i2c_bus上的一个client时（发生在标准driver注册的匹配）：
 1、首先会进入到bus定义的匹配函数i2c_device_match如果定义了CONFIG_OF_DEVICE宏并且内部的标准
    driver结构定义了of_match_table成员，则利用其进行匹配；
 2、否则如果driver->id_table成员设定，则利用其进行匹配，否则匹配失败。
 3、如果匹配成功会调用i2c_bus的i2c_device_probe函数，该函数会判断，如果该i2c_driver的probe成员
    或者id_table成员为NULL，则返回，否则利用i2c_driver->probe初始化这个client。
当发现的是代表该i2c_bus的上的adapter时（发生在bus的遍历）:
 1、如果该driver的driver->detect或者address_list为NULL退出
 2、如果该adapter->class和driver->class不匹配也退出
 3、如果以上都成立最终会调用driver->detect函数，实例化支持的client
 4、如果driver->attach_adapter也被设定，含会走旧式的路线，直接利用driver->attach_adapter进行探测
      不过，一般不会让3&&4这种结果出现
  可见，i2c_driver的设置非常灵活，抓住关键成员就不难掌握其流程。

五、i2c关于dev下节点的产生及其操作

  该部分的代码位于rivers/i2c/i2c-dev.c下，我们从头看起：

 

static int __init i2c_dev_init(void)
{
    int res;
    printk(KERN_INFO "i2c /dev entries driver/n");
  /*
   * 注册名称为i2c主设备号为89的一个字符设备
   */
    res = register_chrdev(I2C_MAJOR, "i2c", &i2cdev_fops);
    if (res)
        goto out;
  /*
   * 在class下产生i2c-dev节点，用于自动产生设备文件
   */
    i2c_dev_class = class_create(THIS_MODULE, "i2c-dev");
    if (IS_ERR(i2c_dev_class)) {
        res = PTR_ERR(i2c_dev_class);
        goto out_unreg_chrdev;
    }

  /*
   * i2c_add_driver在上面已经分析过了
   */
    res = i2c_add_driver(&i2cdev_driver);
    if (res)
        goto out_unreg_class;
    return 0;
out_unreg_class:
    class_destroy(i2c_dev_class);
out_unreg_chrdev:
    unregister_chrdev(I2C_MAJOR, "i2c");
out:
    printk(KERN_ERR "%s: Driver Initialisation failed/n", __FILE__);
    return res;
}
//其中i2cdev_driver结构为：
static struct i2c_driver i2cdev_driver = {
    .driver = {
        .name   = "dev_driver",
    },
    .attach_adapter = i2cdev_attach_adapter,
    .detach_adapter = i2cdev_detach_adapter,
};

     由于没有关于client的支持表的定义，因此匹配client时就会直接返回， 由于存在成员attach_adapter，因此当匹配adapter时会进入该函数。

/************************************
* 来看一下i2cdev_attach_adapter函数*
************************************/
static int i2cdev_attach_adapter(struct i2c_adapter *adap)
{
    struct i2c_dev *i2c_dev;
    int res;
    i2c_dev = get_free_i2c_dev(adap);
    if (IS_ERR(i2c_dev))
        return PTR_ERR(i2c_dev);
    /* register this i2c device with the driver core */

    /*
   * 以上面注册的i2c_dev_class为父节点在目录class/i2c-dev下
   * 产生i2c-0之类的节点，这样上层udev会根据该节点在dev目录下
   * 自动创建对应的设备文件
   */
    i2c_dev->dev = device_create(i2c_dev_class, &adap->dev,
                     MKDEV(I2C_MAJOR, adap->nr), NULL,
                     "i2c-%d", adap->nr);
    if (IS_ERR(i2c_dev->dev)) {
        res = PTR_ERR(i2c_dev->dev);
        goto error;
    }

  /*
   * 产生相关属性文件
   */
    res = device_create_file(i2c_dev->dev, &dev_attr_name);
    if (res)
        goto error_destroy;
    pr_debug("i2c-dev: adapter [%s] registered as minor %d/n",
         adap->name, adap->nr);
    return 0;
error_destroy:
    device_destroy(i2c_dev_class, MKDEV(I2C_MAJOR, adap->nr));
error:
    return_i2c_dev(i2c_dev);
    return res;
}

    通过以上分析可以看到i2c-dev层，找到一个adapter就会自动为其创建设备节点，形式类似于i2c-*,那么当应用层open对应的设备节点的时候，内核会自动调用刚才注册的字符设备的操作函数， 我们先来看一下刚才注册的字符设备的操作集：

 static const struct file_operations i2cdev_fops = {
    .owner      = THIS_MODULE,
    .llseek     = no_llseek,
    .read       = i2cdev_read,
    .write      = i2cdev_write,
    .unlocked_ioctl = i2cdev_ioctl,
    .open       = i2cdev_open,
    .release    = i2cdev_release,
};

    按照用户层的流程先看一下open函数i2cdev_open:

static int i2cdev_open(struct inode *inode, struct file *file)
{
    unsigned int minor = iminor(inode);           //得到次设备号
    struct i2c_client *client;
    struct i2c_adapter *adap;
    struct i2c_dev *i2c_dev;

  /*
   * 次设备号其实是对应i2c总线号，下面函数遍历由次设备构成的链表
   * i2c_dev_list，找到上面挂接的号码对应的i2c_dev结构
   */
    i2c_dev = i2c_dev_get_by_minor(minor);

    if (!i2c_dev)                                 //没找到，出错
        return -ENODEV;
    adap = i2c_get_adapter(i2c_dev->adap->nr);    //得到绑定的adapter
    if (!adap)
        return -ENODEV;
    /* This creates an anonymous i2c_client, which may later be
     * pointed to some address using I2C_SLAVE or I2C_SLAVE_FORCE.
     *
     * This client is ** NEVER REGISTERED ** with the driver model
     * or I2C core code!!  It just holds private copies of addressing
     * information and maybe a PEC flag.
     */
    client = kzalloc(sizeof(*client), GFP_KERNEL); //申请个client内存
    if (!client) {
        i2c_put_adapter(adap);
        return -ENOMEM;
    }

    //命名，依据adapter
    snprintf(client->name, I2C_NAME_SIZE, "i2c-dev %d", adap->nr);

    //指定driver，代表创建他的driver
    client->driver = &i2cdev_driver;

        //指定适配器
    client->adapter = adap;
    //通过file的私有成员传递创建的这个client
    file->private_data = client;
    return 0;
}

      由open可见，我们要操作i2c下的设备，始终是需要通过adapter，物理上也是如此，操作设备都是通过控制器进行读写的，因此我们打开的始终是 adapter而open过程中会创建client，来表述我们主观上是要操作设备。下面在看一下read函数：

static ssize_t i2cdev_read(struct file *file, char __user *buf, size_t count,
        loff_t *offset)
{
    char *tmp;
    int ret;
  //得到由open传递过来的创建的client
    struct i2c_client *client = file->private_data;

  //大小不能超过8192
    if (count > 8192)
        count = 8192;

  //申请count大小内存
    tmp = kmalloc(count, GFP_KERNEL);
    if (tmp == NULL)
        return -ENOMEM;
    pr_debug("i2c-dev: i2c-%d reading %zu bytes./n",
        iminor(file->f_path.dentry->d_inode), count);

  //调用i2c_master_recv进行进一步传送
    ret = i2c_master_recv(client, tmp, count);
    if (ret >= 0)
        //read的信息反馈给用户
        ret = copy_to_user(buf, tmp, count) ? -EFAULT : ret;
    kfree(tmp);
    return ret;
}
/****************************
* 其中i2c_master_recv函数为*
****************************/
int i2c_master_recv(struct i2c_client *client, char *buf, int count)
{
    struct i2c_adapter *adap = client->adapter;
    struct i2c_msg msg;
    int ret;

  //利用msg组织信息结构
    msg.addr = client->addr;
    msg.flags = client->flags & I2C_M_TEN;
    msg.flags |= I2C_M_RD;
    msg.len = count;
    msg.buf = buf;

  //调用i2c_transfer发送
    ret = i2c_transfer(adap, &msg, 1);
    /* If everything went ok (i.e. 1 msg transmitted), return #bytes
       transmitted, else error code. */
    return (ret == 1) ? count : ret;
}

    i2c_transfer函数上面已经分析过了，其会最终调用client所在adapter的adap->algo->master_xfer函数发送。

六、总结

    分析了linux下i2c子系统模型及其关键点，针对核心的平台无关代码进行了描述，以上为个人观点，如有不妥，还望指正 ^_^