Linux Driver : i2c-gpio

# Linux Driver : i2c-gpio

https://www.cnblogs.com/haoxing990/p/4718834.html

https://blog.csdn.net/jianwushuang/article/details/42490119

背景

在学习高通平台的有关知识，看到一篇博客中介绍了GPIO模拟I2C设备，觉得挺有意思的。

跟了一下代码，发现这种模拟方式已经有了一套内核框架。刚好，学习这个驱动有助于加深理解：

1、i2c 子系统框架：

• i2c_adapter对象实现了一组通过一个i2c控制器发送消息的所有信息，包括时序，地址等等， 即封装了i2c控制器的"控制信息"。它被i2c主机驱动创建，通过clien域和i2c_client和i2c_driver相连， 这样设备端驱动就可以通过其中的方法以及i2c物理控制器来和一个i2c总线的物理设备进行交互
• i2c_algorithm描述一个i2c主机的发送时序的信息，该类的对象algo是i2c_adapter的一个域，其中的master_xfer()注册的函数最终被设备驱动端的i2c_transfer()回调，以完成i2c的读写。
• i2c_client描述一个挂接在硬件i2c总线上的设备的设备信息，即i2c设备的设备对象，与i2c_driver对象匹配成功后通过detected和i2c_driver以及i2c_adapter相连，在控制器驱动与控制器设备匹配成功后被控制器驱动通过i2c_new_device()创建。
• i2c_driver描述一个挂接在硬件i2c总线上的设备的驱动方法，即i2c设备的驱动对象，通过i2c_bus_type和设备信息i2c_client匹配，匹配成功后通过clients和i2c_client对象以及i2c_adapter对象相连
• i2c_msg描述一个在设备端和主机端之间进行流动的数据，在设备驱动中打包并通过i2c_transfer()发送。相当于skbuf之于网络设备，urb之于USB设备。

2、gpio子系统api的使用

3、Linux 平台设备驱动

内核：4.9

介绍

在drivers/i2c/busses下包含各种I2C总线驱动，其中就包括了使用GPIO模拟I2C总线的驱动i2c-gpio.c。

驱动分析

i2c-gpio.c实现了gpio模拟I2C总线的驱动。总线也是个设备，在这里将总线当作平台设备处理，那驱动当然是平台设备驱动。

注册与注销

没有什么好说的，它的初始化和注销函数就是注册和注销一个平台设备驱动。

static int __init i2c_gpio_init(void)  
{  
    int ret;  

    ret = platform_driver_register(&i2c_gpio_driver);  
    if (ret)  
        printk(KERN_ERR "i2c-gpio: probe failed: %d
", ret);  

    return ret;  
}  
subsys_initcall(i2c_gpio_init);

static void __exit i2c_gpio_exit(void)  
{  
    platform_driver_unregister(&i2c_gpio_driver);  
}  
module_exit(i2c_gpio_exit);

platform_driver

直接看它的platform_driver结构i2c_gpio_driver

#if defined(CONFIG_OF)
static const struct of_device_id i2c_gpio_dt_ids[] = {
    { .compatible = "i2c-gpio", },
    { /* sentinel */ }
};

MODULE_DEVICE_TABLE(of, i2c_gpio_dt_ids);
#endif

static struct platform_driver i2c_gpio_driver = {
    .driver     = {
        .name   = "i2c-gpio",
        .of_match_table = of_match_ptr(i2c_gpio_dt_ids),
    },
    .probe      = i2c_gpio_probe,
    .remove     = i2c_gpio_remove,
};

平台驱动设备放在arch/arm/mach-xxxx/board-xxx.c中，例如：

 static struct i2c_gpio_platform_data i2c_gpio_adapter_data = {
     .sda_pin = PINID_GPMI_D05,
     .scl_pin = PINID_GPMI_D04,
     .udelay = 5, //100KHz
     .timeout = 100,
     .sda_is_open_drain = 1,
     .scl_is_open_drain = 1,
 };   
   
 static struct platform_device i2c_gpio = {
     .name = "i2c-gpio",
     .id = 0,
     .dev = {
         .platform_data = &i2c_gpio_adapter_data,
         .release = mxs_nop_release,
         },
 };

在这里 struct platform_device结构中的 name字段要和 struct platform_driver中 driver字段中 name字段要相同，因为平台总线就是通过这个来判断设备和驱动是否匹配的。

注意这里的 id将它赋值了 0，后面会提到这个id是用来软件上的一个总线号（nr）。

i2c_gpio_adapter_data

platform_device这个结构里面还包含一个最重要的数据 i2c_gpio_adapter_data，它的原型为是struct i2c_gpio_platform_data类型：

定义在 include/linux/i2c-gpio.h中。

/**
 * struct i2c_gpio_platform_data - Platform-dependent data for i2c-gpio
 * @sda_pin: GPIO pin ID to use for SDA
 * @scl_pin: GPIO pin ID to use for SCL
 * @udelay: signal toggle delay. SCL frequency is (500 / udelay) kHz
 * @timeout: clock stretching timeout in jiffies. If the slave keeps
 *  SCL low for longer than this, the transfer will time out.
 * @sda_is_open_drain: SDA is configured as open drain, i.e. the pin
 *  isn't actively driven high when setting the output value high.
 *  gpio_get_value() must return the actual pin state even if the
 *  pin is configured as an output.
 * @scl_is_open_drain: SCL is set up as open drain. Same requirements
 *  as for sda_is_open_drain apply.
 * @scl_is_output_only: SCL output drivers cannot be turned off.
 */
struct i2c_gpio_platform_data {
    unsigned int    sda_pin;
    unsigned int    scl_pin;
    
    int     udelay;
    int     timeout;
    unsigned int    sda_is_open_drain:1;
    unsigned int    scl_is_open_drain:1;
    unsigned int    scl_is_output_only:1;
};

这个结构体主要描述gpio模拟i2c总线时的一些必要信息：

• sda_pin和scl_pin表示使用哪两个IO管脚来模拟I2C总线
• udelay和timeout分别为它的时钟频率和超时时间
• sda_is_open_drain和scl_is_open_drain表示sda、scl这两个管脚是否是开漏(opendrain)电路，如果是则为1
• scl_is_output_only表示scl这个管脚是否只是作为输出，如果是设置为1。

probe

回到驱动中，看最重要的i2c_gpio_probe。

static int i2c_gpio_probe(struct platform_device *pdev)
{
    struct i2c_gpio_private_data *priv;
    // 
    struct i2c_gpio_platform_data *pdata;
    struct i2c_algo_bit_data *bit_data;
    // i2c适配器
    struct i2c_adapter *adap;
    unsigned int sda_pin, scl_pin;
    int ret;

    /* First get the GPIO pins; if it fails, we'll defer the probe. */
    if (pdev->dev.of_node) {
        ret = of_i2c_gpio_get_pins(pdev->dev.of_node,
                       &sda_pin, &scl_pin);
        if (ret)
            return ret;
    } else {
        if (!dev_get_platdata(&pdev->dev))
            return -ENXIO;
        pdata = dev_get_platdata(&pdev->dev);
        sda_pin = pdata->sda_pin;
        scl_pin = pdata->scl_pin;
    }

    ret = devm_gpio_request(&pdev->dev, sda_pin, "sda");
    if (ret) {
        if (ret == -EINVAL)
            ret = -EPROBE_DEFER;    /* Try again later */
        return ret;
    }
    ret = devm_gpio_request(&pdev->dev, scl_pin, "scl");
    if (ret) {
        if (ret == -EINVAL)
            ret = -EPROBE_DEFER;    /* Try again later */
        return ret;
    }

    priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
    if (!priv)
        return -ENOMEM;
    adap = &priv->adap;
    bit_data = &priv->bit_data;
    pdata = &priv->pdata;

    if (pdev->dev.of_node) {
        pdata->sda_pin = sda_pin;
        pdata->scl_pin = scl_pin;
        of_i2c_gpio_get_props(pdev->dev.of_node, pdata);
    } else {
        memcpy(pdata, dev_get_platdata(&pdev->dev), sizeof(*pdata));
    }

    if (pdata->sda_is_open_drain) {
        gpio_direction_output(pdata->sda_pin, 1);
        bit_data->setsda = i2c_gpio_setsda_val;
    } else {
        gpio_direction_input(pdata->sda_pin);
        bit_data->setsda = i2c_gpio_setsda_dir;
    }

    if (pdata->scl_is_open_drain || pdata->scl_is_output_only) {
        gpio_direction_output(pdata->scl_pin, 1);
        bit_data->setscl = i2c_gpio_setscl_val;
    } else {
        gpio_direction_input(pdata->scl_pin);
        bit_data->setscl = i2c_gpio_setscl_dir;
    }

    if (!pdata->scl_is_output_only)
        bit_data->getscl = i2c_gpio_getscl;
    bit_data->getsda = i2c_gpio_getsda;

    if (pdata->udelay)
        bit_data->udelay = pdata->udelay;
    else if (pdata->scl_is_output_only)
        bit_data->udelay = 50;          /* 10 kHz */
    else
        bit_data->udelay = 5;           /* 100 kHz */

    if (pdata->timeout)
        bit_data->timeout = pdata->timeout;
    else
        bit_data->timeout = HZ / 10;        /* 100 ms */

    bit_data->data = pdata;

    adap->owner = THIS_MODULE;
    if (pdev->dev.of_node)
        strlcpy(adap->name, dev_name(&pdev->dev), sizeof(adap->name));
    else
        snprintf(adap->name, sizeof(adap->name), "i2c-gpio%d", pdev->id);

    adap->algo_data = bit_data;
    adap->class = I2C_CLASS_HWMON | I2C_CLASS_SPD;
    adap->dev.parent = &pdev->dev;
    adap->dev.of_node = pdev->dev.of_node;

    adap->nr = pdev->id;
    ret = i2c_bit_add_numbered_bus(adap);
    if (ret)
        return ret;

    platform_set_drvdata(pdev, priv);

    dev_info(&pdev->dev, "using pins %u (SDA) and %u (SCL%s)
",
         pdata->sda_pin, pdata->scl_pin,
         pdata->scl_is_output_only
         ? ", no clock stretching" : "");

    return 0;
}

获取platform数据

1、pdata = dev_get_platdata(&pdev->dev);正是我们在平台设备结构中定义的数据：我们能够从其中获取到gpio模拟i2c总线时的一些必要信息。

申请有关的资源

接下来使用gpio_request去申请这个两个GPIO管脚，申请的目的是为了防止管脚冲突。

    ret = devm_gpio_request(&pdev->dev, sda_pin, "sda");
    if (ret) {
        if (ret == -EINVAL)
            ret = -EPROBE_DEFER;    /* Try again later */
        return ret;
    }

    ret = devm_gpio_request(&pdev->dev, scl_pin, "scl");
    if (ret) {
        if (ret == -EINVAL)
            ret = -EPROBE_DEFER;    /* Try again later */
        return ret;
    }

2、注意到这里使用kzalloc申请了一个struct i2c_gpio_private_data对象

static int i2c_gpio_probe(struct platform_device *pdev)
{
    struct i2c_gpio_private_data *priv;
    struct i2c_gpio_platform_data *pdata;
    struct i2c_algo_bit_data *bit_data;
    //
    unsigned int sda_pin, scl_pin;
    int ret;

    // ...
    
    // 申请 i2c_gpio_private_data，其中包含了 i2c_adapter i2c_algo_bit_data 这两个成员
    priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
    adap = &priv->adap;
    bit_data = &priv->bit_data;
    pdata = &priv->pdata;

主要是为了struct i2c_adapter、struct i2c_algo_bit_data这2个成员。

// drivers/i2c/busses/i2c-gpio.c

struct i2c_gpio_private_data {
    struct i2c_adapter adap;
    struct i2c_algo_bit_data bit_data;
    struct i2c_gpio_platform_data pdata;
};

• struct i2c_adapter：在I2C子系统中，I2C适配器使用结构struct i2c_adapter描述，代表一条实际的I2C总线。

定义在include/linux/i2c.h中

struct i2c_adapter {  
    struct module *owner;  
    unsigned int id;  
    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   */  
    u8 level;           /* nesting level for lockdep */  
    struct 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 i2c_algo_bit_data：用来定义对I2C-引脚的操作方式

定义在include/linux/i2c-algo-bit.h中

struct i2c_algo_bit_data {  
    void *data;     /* private data for lowlevel routines */  
    void (*setsda) (void *data, int state);  
    void (*setscl) (void *data, int state);  
    int  (*getsda) (void *data);  
    int  (*getscl) (void *data);  
    int  (*pre_xfer)  (struct i2c_adapter *);
    void (*post_xfer) (struct i2c_adapter *);
    
    /* local settings */  
    int udelay;     /* half clock cycle time in us, 
                   minimum 2 us for fast-mode I2C, 
                   minimum 5 us for standard-mode I2C and SMBus, 
                   maximum 50 us for SMBus */  
    int timeout;        /* in jiffies */  
};  

3、将所需数据中保存到设备中

    pdata = &priv->pdata;

    if (pdev->dev.of_node) {
        pdata->sda_pin = sda_pin;
        pdata->scl_pin = scl_pin;
        of_i2c_gpio_get_props(pdev->dev.of_node, pdata);
    } else {
        memcpy(pdata, dev_get_platdata(&pdev->dev), sizeof(*pdata));
    }

4、然后是根据struct i2c_gpio_platform_data结构中定义的数据对struct i2c_algo_bit_data规定具体操作I2C引脚的方法。

    if (pdata->sda_is_open_drain) {
        gpio_direction_output(pdata->sda_pin, 1);
        bit_data->setsda = i2c_gpio_setsda_val;
    } else {
        gpio_direction_input(pdata->sda_pin);
        bit_data->setsda = i2c_gpio_setsda_dir;
    }

    if (pdata->scl_is_open_drain || pdata->scl_is_output_only) {
        gpio_direction_output(pdata->scl_pin, 1);
        bit_data->setscl = i2c_gpio_setscl_val;
    } else {
        gpio_direction_input(pdata->scl_pin);
        bit_data->setscl = i2c_gpio_setscl_dir;
    }

    if (!pdata->scl_is_output_only)
        bit_data->getscl = i2c_gpio_getscl;
    bit_data->getsda = i2c_gpio_getsda;

配置I2C时钟属性

接下来是I2C时钟频率和超时设置：如果在struct i2c_gpio_platform_data结构中定义了值，那么就采用定义的值，否则就采用默认的值。

    if (pdata->udelay)
        bit_data->udelay = pdata->udelay;
    else if (pdata->scl_is_output_only)
        bit_data->udelay = 50;          /* 10 kHz */
    else
        bit_data->udelay = 5;           /* 100 kHz */

    if (pdata->timeout)
        bit_data->timeout = pdata->timeout;
    else
        bit_data->timeout = HZ / 10;        /* 100 ms */

然后是对struct i2c_adapter结构的一些赋值操作：

指定它的父设备为这里的平台设备，前面在平台设备中定义的一个id（值为0），这里用到了，赋给了struct i2c_adapter中的nr成员，这个值表示总线号。

这里的总线号和硬件无关，只是在软件上的区分。

    bit_data->data = pdata;

    adap->owner = THIS_MODULE;
    if (pdev->dev.of_node)
        strlcpy(adap->name, dev_name(&pdev->dev), sizeof(adap->name));
    else
        snprintf(adap->name, sizeof(adap->name), "i2c-gpio%d", pdev->id);

    adap->algo_data = bit_data;
    adap->class = I2C_CLASS_HWMON | I2C_CLASS_SPD;
    adap->dev.parent = &pdev->dev;
    adap->dev.of_node = pdev->dev.of_node;

    adap->nr = pdev->id;

i2c_bit_add_numbered_bus

到了最后的主角：i2c_bit_add_numbered_bus

定义在drivers/i2c/algos/i2c-algo-bit.c

int i2c_bit_add_numbered_bus(struct i2c_adapter *adap)
{
    return __i2c_bit_add_bus(adap, i2c_add_numbered_adapter);
}
EXPORT_SYMBOL(i2c_bit_add_numbered_bus);

i2c_bit_add_numbered_bus实际上是调用了__i2c_bit_add_bus，同时传入了i2c_add_numbered_adapter作为参数。

__i2c_bit_add_bus

函数原型：

/*
 * registering functions to load algorithms at runtime
 */
static int __i2c_bit_add_bus(struct i2c_adapter *adap,
                 int (*add_adapter)(struct i2c_adapter *))
{
    struct i2c_algo_bit_data *bit_adap = adap->algo_data;
    int ret;
    
	// ...

    /* register new adapter to i2c module... */
    adap->algo = &i2c_bit_algo; // 添加这个设备的读写实现方法
    adap->retries = 3;
    if (bit_adap->getscl == NULL)
        adap->quirks = &i2c_bit_quirk_no_clk_stretch;
    
	// 添加适配器，以i2c 总线的方式进行添加
    ret = add_adapter(adap);
    if (ret < 0)
        return ret;

    // ...
    
    return 0;
}

1、添加：指定 该类的对象algo是i2c_adapter的一个域，其中的

adap->algo= &i2c_bit_algo;

来看这个结构定义

// 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) // 从设备相关，先不纠结
    int (*reg_slave)(struct i2c_client *client);
    int (*unreg_slave)(struct i2c_client *client);
#endif
};

/* -----exported algorithm data: -------------------------------------  */
const struct i2c_algorithm i2c_bit_algo = {
    .master_xfer    = bit_xfer,
    .functionality  = bit_func,
};
EXPORT_SYMBOL(i2c_bit_algo);

除了注释以外，只是指定了adapter的行为：

• master_xfer指定为主机的数据传输方式（包括读与写）：master_xfer()注册的函数最终被设备驱动端的i2c_transfer()回调，以完成i2c的读写
• functionality：告知这个adapter能够支持的操作。

i2c数据传输

具体来看bit_xfer这个函数，这个函数和I2C协议相关，因此理解的前提是搞懂I2C协议。

根据I2C协议规定，要先发送起始信号，才能开始进行数据的传输，最后数据传输完成后发送停止信号。

static int bit_xfer(struct i2c_adapter *i2c_adap,
            struct i2c_msg msgs[], int num)
{
    struct i2c_msg *pmsg;
    struct i2c_algo_bit_data *adap = i2c_adap->algo_data;
    int i, ret;
    unsigned short nak_ok;

    // ...
    
    i2c_start(adap);
    
    for (i = 0; i < num; i++) {
        pmsg = &msgs[i];
        nak_ok = pmsg->flags & I2C_M_IGNORE_NAK;
        if (!(pmsg->flags & I2C_M_NOSTART)) {
            if (i) {
                bit_dbg(3, &i2c_adap->dev, "emitting "
                    "repeated start condition
");
                i2c_repstart(adap);
            }
            ret = bit_doAddress(i2c_adap, pmsg);
            if ((ret != 0) && !nak_ok) {
                bit_dbg(1, &i2c_adap->dev, "NAK from "
                    "device addr 0x%02x msg #%d
",
                    msgs[i].addr, i);
                goto bailout;
            }
        }
        if (pmsg->flags & I2C_M_RD) {
            /* read bytes into buffer*/
            ret = readbytes(i2c_adap, pmsg);
            if (ret >= 1)
                bit_dbg(2, &i2c_adap->dev, "read %d byte%s
",
                    ret, ret == 1 ? "" : "s");
            if (ret < pmsg->len) {
                if (ret >= 0)
                    ret = -EIO;
                goto bailout;
            }
        } else {
            /* write bytes from buffer */
            ret = sendbytes(i2c_adap, pmsg);
            if (ret >= 1)
                bit_dbg(2, &i2c_adap->dev, "wrote %d byte%s
",
                    ret, ret == 1 ? "" : "s");
            if (ret < pmsg->len) {
                if (ret >= 0)
                    ret = -EIO;
                goto bailout;
            }
        }
    }
    ret = i;
    
bailout:
    bit_dbg(3, &i2c_adap->dev, "emitting stop condition
");
    i2c_stop(adap);

    if (adap->post_xfer)
        adap->post_xfer(i2c_adap);
    return ret;
}

发送起始信号

i2c_start(adap);

看这个函数前，先看I2C协议怎么定义起始信号的：起始信号就是在SCL为高电平期间，SDA从高到低的跳变。

再来看代码是怎么实现的

/* --- setting states on the bus with the right timing: --------------- */

#define setsda(adap, val)   adap->setsda(adap->data, val)
#define setscl(adap, val)   adap->setscl(adap->data, val)
#define getsda(adap)        adap->getsda(adap->data)
#define getscl(adap)        adap->getscl(adap->data)

static void i2c_start(struct i2c_algo_bit_data *adap)  
{  
    /* assert: scl, sda are high */  
    setsda(adap, 0);  
    udelay(adap->udelay);  
    scllo(adap);  
}  

注意，这些 setsda和 setscl这些都是使用的总线的函数，在这里是使用的 i2c-gpio.c中定义的函数，例如：

// i2c-gpio.c

/*
 * Toggle SDA by changing the output value of the pin. This is only
 * valid for pins configured as open drain (i.e. setting the value
 * high effectively turns off the output driver.)
 */
static void i2c_gpio_setsda_val(void *data, int state)
{
    struct i2c_gpio_platform_data *pdata = data;

    gpio_set_value(pdata->sda_pin, state);
}

static int i2c_gpio_probe(struct platform_device *pdev) 
{
    // ...
    
    if (pdata->sda_is_open_drain) {
        gpio_direction_output(pdata->sda_pin, 1);
        bit_data->setsda = i2c_gpio_setsda_val;
    } else {
        gpio_direction_input(pdata->sda_pin);
        bit_data->setsda = i2c_gpio_setsda_dir;
    }
    // ...
}

循环处理

往下是个大的for循环：判断pmsg->flags设置而采取不同的行动。

回顾一下i2c_msg原型

// include/linux/i2c.h

#define I2C_M_TEN           0x0010  /* this is a ten bit chip address */  //表示10位设备地址
#define I2C_M_RD            0x0001  /* read data, from slave to master */  // 读标志
#define I2C_M_NOSTART       0x4000  /* if I2C_FUNC_PROTOCOL_MANGLING */  // 无起始信号标志
#define I2C_M_REV_DIR_ADDR  0x2000  /* if I2C_FUNC_PROTOCOL_MANGLING */  
#define I2C_M_IGNORE_NAK    0x1000  /* if I2C_FUNC_PROTOCOL_MANGLING */  //忽略应答信号标志
#define I2C_M_NO_RD_ACK     0x0800  /* if I2C_FUNC_PROTOCOL_MANGLING */  
#define I2C_M_RECV_LEN      0x0400  /* length will be first received byte */  

struct i2c_msg {  
    __u16 addr;   /* I2C设备地址  */  
    __u16 flags;  /* 标志位       */
    __u16 len;    /* 数据的长度    */  
    __u8 *buf;    /* 数据         */  
};  

有了上面的基础，再看看下面的for就简单了：

	// num代表有几个 struct i2c_msg
	for (i = 0; i < num; i++) { 
        pmsg = &msgs[i];
        nak_ok = pmsg->flags & I2C_M_IGNORE_NAK;
        if (!(pmsg->flags & I2C_M_NOSTART)) {
            if (i) {
                bit_dbg(3, &i2c_adap->dev, "emitting "
                    "repeated start condition
");
                i2c_repstart(adap);
            }
            ret = bit_doAddress(i2c_adap, pmsg);
            if ((ret != 0) && !nak_ok) {
                bit_dbg(1, &i2c_adap->dev, "NAK from "
                    "device addr 0x%02x msg #%d
",
                    msgs[i].addr, i);
                goto bailout;
            }
        }
        if (pmsg->flags & I2C_M_RD) {
            /* read bytes into buffer*/
            ret = readbytes(i2c_adap, pmsg);
            if (ret >= 1)
                bit_dbg(2, &i2c_adap->dev, "read %d byte%s
",
                    ret, ret == 1 ? "" : "s");
            if (ret < pmsg->len) {
                if (ret >= 0)
                    ret = -EIO;
                goto bailout;
            }
        } else {
            /* write bytes from buffer */
            ret = sendbytes(i2c_adap, pmsg);
            if (ret >= 1)
                bit_dbg(2, &i2c_adap->dev, "wrote %d byte%s
",
                    ret, ret == 1 ? "" : "s");
            if (ret < pmsg->len) {
                if (ret >= 0)
                    ret = -EIO;
                goto bailout;
            }
        }
    }
    ret = i;

我们来看看这3个if，分别实现了：发送地址、发送或读取字节数据

发送地址

        if (!(pmsg->flags & I2C_M_NOSTART)) {
            if (i) {
                bit_dbg(3, &i2c_adap->dev, "emitting "
                    "repeated start condition
");
                i2c_repstart(adap);
            }
            ret = bit_doAddress(i2c_adap, pmsg);
            if ((ret != 0) && !nak_ok) {
                bit_dbg(1, &i2c_adap->dev, "NAK from "
                    "device addr 0x%02x msg #%d
",
                    msgs[i].addr, i);
                goto bailout;
            }
        }

判断这个设备是否定义了I2C_M_NOSTART标志，这个标志主要用于读写操作

• 写：不必重新发送起始信号和设备地址。
• 读：要调用i2c_repstart这个函数去重新发送起始信号，调用bit_doAddress函数去重新构造设备地址字节。

重新构造设备地址字节

/* doAddress initiates the transfer by generating the start condition (in
 * try_address) and transmits the address in the necessary format to handle
 * reads, writes as well as 10bit-addresses.
 * returns:
 *  0 everything went okay, the chip ack'ed, or IGNORE_NAK flag was set
 * -x an error occurred (like: -ENXIO if the device did not answer, or
 *  -ETIMEDOUT, for example if the lines are stuck...)
 */
static int bit_doAddress(struct i2c_adapter *i2c_adap, struct i2c_msg *msg)
{
    unsigned short flags = msg->flags;
    unsigned short nak_ok = msg->flags & I2C_M_IGNORE_NAK;
    struct i2c_algo_bit_data *adap = i2c_adap->algo_data;

    unsigned char addr;
    int ret, retries;

    retries = nak_ok ? 0 : i2c_adap->retries;

    if (flags & I2C_M_TEN) { // 判断是否是10位地址
        // ...
    } else {        /* normal 7bit address  */
        addr = msg->addr << 1;
        if (flags & I2C_M_RD)
            addr |= 1;
        if (flags & I2C_M_REV_DIR_ADDR)
            addr ^= 1;
        ret = try_address(i2c_adap, addr, retries);
        if ((ret != 1) && !nak_ok)
            return -ENXIO;
    }

    return 0;
}

1、这里先做了一个判断， 10位设备地址和 7位设备地址分别做不同的处理，通常一条 I2C总线上不会挂那么多 I2C设备，所以 10位地址不常用，直接看对 7位地址的处理。

2、 struct i2c_msg中 addr中是真正的设备地址，而这里发送的 addr高 7位才是设备地址，最低位为读写位。

• 如果为读，最低位为 1
• 如果为写，最低位为 0。

3、所以要将 struct i2c_msg中 addr向左移 1位，再根据读写情况，对最低位进行置位/清零。

4、最后调用 try_address函数将这个地址字节发送出去。

地址的发送

这里负责发送，以及超时重传

/* try_address tries to contact a chip for a number of
 * times before it gives up.
 * return values:
 * 1 chip answered
 * 0 chip did not answer
 * -x transmission error
 */
static int try_address(struct i2c_adapter *i2c_adap,
                       unsigned char addr, int retries)
{
    struct i2c_algo_bit_data *adap = i2c_adap->algo_data;
    int i, ret = 0;

    for (i = 0; i <= retries; i++) {
        // 输出字节
        ret = i2c_outb(i2c_adap, addr);
        if (ret == 1 || i == retries)
            break;
        bit_dbg(3, &i2c_adap->dev, "emitting stop condition
");
        i2c_stop(adap);
        udelay(adap->udelay);
        yield();
        bit_dbg(3, &i2c_adap->dev, "emitting start condition
");
        i2c_start(adap);
    }
    if (i && ret)
        bit_dbg(1, &i2c_adap->dev, "Used %d tries to %s client at "
                "0x%02x: %s
", i + 1,
                addr & 1 ? "read from" : "write to", addr >> 1,
                ret == 1 ? "success" : "failed, timeout?");
    return ret;
}

最主要的就是调用i2c_outb发送一个字节，retries为重复次数，看前面adap->retries= 3;

如果发送失败，也就是设备没有给出应答信号，那就发送停止信号，发送起始信号，再发送这个地址字节，这就叫retries。

来看这个具体的i2c_outb函数

/* send a byte without start cond., look for arbitration,
   check ackn. from slave */
/* returns:
 * 1 if the device acknowledged
 * 0 if the device did not ack
 * -ETIMEDOUT if an error occurred (while raising the scl line)
 */
static int i2c_outb(struct i2c_adapter *i2c_adap, unsigned char c)
{
    int i;
    int sb;
    int ack;
    struct i2c_algo_bit_data *adap = i2c_adap->algo_data;

    /* assert: scl is low */
    for (i = 7; i >= 0; i--) {
        sb = (c >> i) & 1;
        setsda(adap, sb);
        udelay((adap->udelay + 1) / 2);
        if (sclhi(adap) < 0) { /* timed out */
            bit_dbg(1, &i2c_adap->dev, "i2c_outb: 0x%02x, "
                "timeout at bit #%d
", (int)c, i);
            return -ETIMEDOUT;
        }

        scllo(adap);
    }
    sdahi(adap);
    if (sclhi(adap) < 0) { /* timeout */
        bit_dbg(1, &i2c_adap->dev, "i2c_outb: 0x%02x, "
            "timeout at ack
", (int)c);
        return -ETIMEDOUT;
    }

    /* read ack: SDA should be pulled down by slave, or it may
     * NAK (usually to report problems with the data we wrote).
     */
    ack = !getsda(adap);    /* ack: sda is pulled low -> success */
    bit_dbg(2, &i2c_adap->dev, "i2c_outb: 0x%02x %s
", (int)c,
        ack ? "A" : "NA");

    scllo(adap);
    return ack;
    /* assert: scl is low (sda undef) */
}

这个函数有两个参数，一个是structi2c_adapter代表I2C主机，一个是发送的字节数据。那么I2C是怎样将一个字节数据发送出去的呢，那再来看看协议。

首先是发送字节数据的最高位，在时钟为高电平期间将一位数据发送出去，最后是发送字节数据的最低位。

发送完成之后，我们需要一个ACK信号，要不然我怎么知道发送成功没有：

ACK信号就是在第九个时钟周期时数据线为低，所以在一个字节数据传送完成后，还要将数据线拉高，我们看程序中就是这一句sdahi(adap);等待这个ACK信号的到来，这样一个字节数据就发送完成。

回到bit_xfer函数中，前面只是将设备地址字节发送出去了，那么接下来就是该读写数据了。

读取字节

先看读

        if (pmsg->flags & I2C_M_RD) { // 如果是读则调用readbytes函数去读
            /* read bytes into buffer*/
            ret = readbytes(i2c_adap, pmsg);
            if (ret >= 1)
                bit_dbg(2, &i2c_adap->dev, "read %d byte%s
",
                    ret, ret == 1 ? "" : "s");
            if (ret < pmsg->len) {
                if (ret >= 0)
                    ret = -EIO;
                goto bailout;
            }
        } else //... 如果是写则调用sendbytes去写

注意：这里的数据包括操作设备的基地址。

static int readbytes(struct i2c_adapter *i2c_adap, struct i2c_msg *msg)  
{  
    int inval;  
    int rdcount = 0;    /* counts bytes read */  
    unsigned char *temp = msg->buf;  
    int count = msg->len;  
    const unsigned flags = msg->flags;  

    while (count > 0) {  
        // 读取一个字节
        inval = i2c_inb(i2c_adap);  
        if (inval >= 0) {  
            *temp = inval;  
            rdcount++;  
        } else {   /* read timed out */  
            break;  
        }  

        temp++;  
        count--;  

        /* Some SMBus transactions require that we receive the 
           transaction length as the first read byte. */  
        if (rdcount == 1 && (flags & I2C_M_RECV_LEN)) {  
            if (inval <= 0 || inval > I2C_SMBUS_BLOCK_MAX) {  
                if (!(flags & I2C_M_NO_RD_ACK))  
                    acknak(i2c_adap, 0);  
                dev_err(&i2c_adap->dev, "readbytes: invalid "  
                        "block length (%d)
", inval);  
                return -EREMOTEIO;  
            }  
            /* The original count value accounts for the extra 
               bytes, that is, either 1 for a regular transaction, 
               or 2 for a PEC transaction. */  
            count += inval;  
            msg->len += inval;  
        }  

        bit_dbg(2, &i2c_adap->dev, "readbytes: 0x%02x %s
",  
                inval,  
                (flags & I2C_M_NO_RD_ACK)  
                ? "(no ack/nak)"  
                : (count ? "A" : "NA"));  

        if (!(flags & I2C_M_NO_RD_ACK)) {  
            inval = acknak(i2c_adap, count);  
            if (inval < 0)  
                return inval;  
        }  
    }  
    return rdcount;  
}  

其中一个大的while循环，调用i2c_inb去读一个字节，count为数据的长度，单位为多少个字节。

那就来看i2c_inb函数。按位读取，就可以了。

static int i2c_inb(struct i2c_adapter *i2c_adap)  
{  
    /* read byte via i2c port, without start/stop sequence  */  
    /* acknowledge is sent in i2c_read.         */  
    int i;  
    unsigned char indata = 0;  
    struct i2c_algo_bit_data *adap = i2c_adap->algo_data;  
  
    /* assert: scl is low */  
    sdahi(adap);  
    for (i = 0; i < 8; i++) {  
        if (sclhi(adap) < 0) { /* timeout */  
            bit_dbg(1, &i2c_adap->dev, "i2c_inb: timeout at bit "  
                "#%d
", 7 - i);  
            return -ETIMEDOUT;  
        }  
        indata *= 2;  
        if (getsda(adap))  
            indata |= 0x01;  
        setscl(adap, 0);  
        udelay(i == 7 ? adap->udelay / 2 : adap->udelay);  
    }  
    /* assert: scl is low */  
    return indata;  
} 

发送字节

		else {
            /* write bytes from buffer */
            ret = sendbytes(i2c_adap, pmsg);
            if (ret >= 1)
                bit_dbg(2, &i2c_adap->dev, "wrote %d byte%s
",
                    ret, ret == 1 ? "" : "s");
            if (ret < pmsg->len) {
                if (ret >= 0)
                    ret = -EIO;
                goto bailout;
            }

再来看 sendbytes函数

static int sendbytes(struct i2c_adapter *i2c_adap, struct i2c_msg *msg)
{
    const unsigned char *temp = msg->buf;
    int count = msg->len;
    unsigned short nak_ok = msg->flags & I2C_M_IGNORE_NAK;
    int retval;
    int wrcount = 0;

    while (count > 0) {
        retval = i2c_outb(i2c_adap, *temp);

        /* OK/ACK; or ignored NAK */
        if ((retval > 0) || (nak_ok && (retval == 0))) {
            count--;
            temp++;
            wrcount++;

        /* A slave NAKing the master means the slave didn't like
         * something about the data it saw.  For example, maybe
         * the SMBus PEC was wrong.
         */
        } else if (retval == 0) {
            dev_err(&i2c_adap->dev, "sendbytes: NAK bailout.
");
            return -EIO;

        /* Timeout; or (someday) lost arbitration
         *
         * FIXME Lost ARB implies retrying the transaction from
         * the first message, after the "winning" master issues
         * its STOP.  As a rule, upper layer code has no reason
         * to know or care about this ... it is *NOT* an error.
         */
        } else {
            dev_err(&i2c_adap->dev, "sendbytes: error %d
",
                    retval);
            return retval;
        }
    }
    return wrcount;
}

也是一个大的while循环，同发送地址字节一样，也是调用i2c_outb去发送一个字节，count也是数据长度。类似的代码就不再赘述了。

发送停止信号

还是回到bit_xfer函数，数据传输完成后，调用i2c_stop函数发送停止信号。

bailout:
    bit_dbg(3, &i2c_adap->dev, "emitting stop condition
");
    i2c_stop(adap);

    if (adap->post_xfer)
        adap->post_xfer(i2c_adap);
    return ret;

我们看停止信号函数怎么去实现的。

static void i2c_stop(struct i2c_algo_bit_data *adap)
{
    /* assert: scl is low */
    sdalo(adap);
    sclhi(adap);
    setsda(adap, 1);
    udelay(adap->udelay);
}

根据i2c的协议，停止信号就是在时钟为高电平期间，数据线从低到高的跳变。

在i2c_stop中，执行的顺序也是先将数据线拉低，将时钟线拉高，最后将数据拉高，这样就够成了一个停止信号。

i2c_add_numbered_adapter

刚刚说了，i2c_bit_add_numbered_bus实际上是调用了__i2c_bit_add_bus，同时传入了i2c_add_numbered_adapter作为参数。

在__i2c_bit_add_bus中的添加适配器的操作是由i2c_add_numbered_adapter完成的。

所以现在看看另外一个函数调用i2c_add_numbered_adapter。

在i2c_add_numbered_adapter中，向i2c总线上添加了一个adapter。

// drivers/i2c/i2c-core.c


/**
 * i2c_add_adapter - declare i2c adapter, use dynamic bus number
 * @adapter: the adapter to add
 * Context: can sleep
 *
 * This routine is used to declare an I2C adapter when its bus number
 * doesn't matter or when its bus number is specified by an dt alias.
 * Examples of bases when the bus number doesn't matter: I2C adapters
 * dynamically added by USB links or PCI plugin cards.
 *
 * When this returns zero, a new bus number was allocated and stored
 * in adap->nr, and the specified adapter became available for clients.
 * Otherwise, a negative errno value is returned.
 */
int i2c_add_adapter(struct i2c_adapter *adapter)
{
    struct device *dev = &adapter->dev;
    int id;

    if (dev->of_node) {
        id = of_alias_get_id(dev->of_node, "i2c");
        if (id >= 0) {
            adapter->nr = id;
            return __i2c_add_numbered_adapter(adapter);
        }
    }

    // ...

    adapter->nr = id;

    return i2c_register_adapter(adapter);
}
EXPORT_SYMBOL(i2c_add_adapter);

/**
 * __i2c_add_numbered_adapter - i2c_add_numbered_adapter where nr is never -1
 * @adap: the adapter to register (with adap->nr initialized)
 * Context: can sleep
 *
 * See i2c_add_numbered_adapter() for details.
 */
static int __i2c_add_numbered_adapter(struct i2c_adapter *adap)
{
    int id;

    // ...

    return i2c_register_adapter(adap);
}

/**
 * i2c_add_numbered_adapter - declare i2c adapter, use static bus number
 * @adap: the adapter to register (with adap->nr initialized)
 * Context: can sleep
 *
 * This routine is used to declare an I2C adapter when its bus number
 * matters.  For example, use it for I2C adapters from system-on-chip CPUs,
 * or otherwise built in to the system's mainboard, and where i2c_board_info
 * is used to properly configure I2C devices.
 *
 * If the requested bus number is set to -1, then this function will behave
 * identically to i2c_add_adapter, and will dynamically assign a bus number.
 *
 * If no devices have pre-been declared for this bus, then be sure to
 * register the adapter before any dynamically allocated ones.  Otherwise
 * the required bus ID may not be available.
 *
 * When this returns zero, the specified adapter became available for
 * clients using the bus number provided in adap->nr.  Also, the table
 * of I2C devices pre-declared using i2c_register_board_info() is scanned,
 * and the appropriate driver model device nodes are created.  Otherwise, a
 * negative errno value is returned.
 */
int i2c_add_numbered_adapter(struct i2c_adapter *adap)
{
    if (adap->nr == -1) /* -1 means dynamically assign bus id */
        return i2c_add_adapter(adap);

    return __i2c_add_numbered_adapter(adap);
}
EXPORT_SYMBOL_GPL(i2c_add_numbered_adapter);

无论adap->nr为多少，__i2c_add_numbered_adapter与``i2c_add_adapter最终都调用了i2c_register_adapter`注册这条 I2C总线：

// i2c-core.c
static int i2c_register_adapter(struct i2c_adapter *adap)
{
    int res = -EINVAL;
    
	// ...
    
    if (!adap->lock_ops)
        adap->lock_ops = &i2c_adapter_lock_ops;
    
	// ...

    /* Set default timeout to 1 second if not already set */
    if (adap->timeout == 0)    adap->timeout = HZ;
    
	// 初始化，并注册这个I2C总线设备
    dev_set_name(&adap->dev, "i2c-%d", adap->nr);
    adap->dev.bus = &i2c_bus_type;
    adap->dev.type = &i2c_adapter_type;
    res = device_register(&adap->dev);

    dev_dbg(&adap->dev, "adapter [%s] registered
", adap->name);

    // ...

#ifdef CONFIG_I2C_COMPAT
    res = class_compat_create_link(i2c_adapter_compat_class, &adap->dev,
                                   adap->dev.parent);
#endif
    
    /* create pre-declared device nodes */
    of_i2c_register_devices(adap);
    i2c_acpi_register_devices(adap);
    i2c_acpi_install_space_handler(adap);

    // 扫描板级信息（由于 nr = 0 ，因此一点会执行）
    if (adap->nr < __i2c_first_dynamic_bus_num)
        i2c_scan_static_board_info(adap);

	// ...

    return 0;
    
out_list:
    // ...
    return res;
}

看内核代码有时就会这样，会陷入内核代码的汪洋大海中，而拔不出来，直接后果是最后都忘记看这段代码的目的，丧失继续看下去的信心（勿忘初心的重要性）。所以为了避免这样情况出现，所以最好在开始看代码的时候要明确目标，我通过这段代码到底要了解什么东西，主干要抓住，其它枝叶就不要看了。

关于i2c注册设备比较关键的有下面3点。

1. 注册这个I2C总线设备

adap->dev.bus = &i2c_bus_type;  
adap->dev.type = &i2c_adapter_type;  
res = device_register(&adap->dev);  

i2c的总线类型 i2c_bus_type的类型为bus_type

struct bus_type i2c_bus_type = {  
    .name       = "i2c",  
    .match      = i2c_device_match,  
    .probe      = i2c_device_probe,  
    .remove     = i2c_device_remove,  
    .shutdown   = i2c_device_shutdown,  
    .suspend    = i2c_device_suspend,  
    .resume     = i2c_device_resume,  
};  

看一下它的 match函数：

static int i2c_device_match(struct device *dev, struct device_driver *drv)  
{  
    struct i2c_client   *client = i2c_verify_client(dev);  
    struct i2c_driver   *driver;  
  
    if (!client)  
        return 0;  
  	// ...
    driver = to_i2c_driver(drv);  
    /* match on an id table if there is one */  
    if (driver->id_table)  
        return i2c_match_id(driver->id_table, client) != NULL;  
  
    return 0;  
}  

这个 match函数主要用来匹配我们的 I2C设备和 I2C驱动的，如果匹配成功，最后会调用驱动的 probe函数，来看它如何匹配的。

static const struct i2c_device_id *i2c_match_id(const struct i2c_device_id *id,  
                        const struct i2c_client *client)  
{  
    while (id->name[0]) {  
        if (strcmp(client->name, id->name) == 0)  
            return id;  
        id++;  
    }  
    return NULL;  
}  

就是判断I2C设备的name字段和驱动中id_table中定义的name字段是否相等。

2.往这条总线上添加设备

static void i2c_scan_static_board_info(struct i2c_adapter *adapter)  
{  
    struct i2c_devinfo  *devinfo;  

    down_read(&__i2c_board_lock);  
    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
",  
                    devinfo->board_info.addr);  
    }  
    up_read(&__i2c_board_lock);  
}  

遍历 __i2c_board_list这条链表。判断要将新的i2c适配器添加到哪里。

nr就是 i2c总线的总线号，这里可以理解为是在往这条总线上添加设备。

所以，如果我们要向 I2C注册一个 I2C设备的话，直接向 __i2c_board_list添加一个设备信息就可以了，先来看这个设备信息结构是怎么定义的。

struct i2c_board_info {  
    char        type[I2C_NAME_SIZE];  
    unsigned short  flags;  
    unsigned short  addr;  
    void        *platform_data;  
    struct dev_archdata *archdata;  
    int     irq;  
};  

定义这样一个信息呢一般使用一个宏 I2C_BOARD_INFO

dev_type为设备的名字，前面也说了，这个name一定要和I2C驱动相同。addr为设备的地址。

/**
 * I2C_BOARD_INFO - macro used to list an i2c device and its address
 * @dev_type: identifies the device type
 * @dev_addr: the device's address on the bus.
 *
 * This macro initializes essential fields of a struct i2c_board_info,
 * declaring what has been provided on a particular board.  Optional
 * fields (such as associated irq, or device-specific platform_data)
 * are provided using conventional syntax.
 */
#define I2C_BOARD_INFO(dev_type, dev_addr) 
    .type = dev_type, .addr = (dev_addr)

定义了这样一组信息之后呢，接下来当然是往链表添加这些信息了。

int __init  
i2c_register_board_info(int busnum,  
    struct i2c_board_info const *info, unsigned len)  
{  
    int status;  
  
    down_write(&__i2c_board_lock);  
  
    /* dynamic bus numbers will be assigned after the last static one */  
    if (busnum >= __i2c_first_dynamic_bus_num)  
        __i2c_first_dynamic_bus_num = busnum + 1;  
  
    for (status = 0; len; len--, info++) {  
        struct i2c_devinfo  *devinfo;  
  
        devinfo = kzalloc(sizeof(*devinfo), GFP_KERNEL);  
        if (!devinfo) {  
            pr_debug("i2c-core: can't register boardinfo!
");  
            status = -ENOMEM;  
            break;  
        }  
  
        devinfo->busnum = busnum;  
        devinfo->board_info = *info;  
        list_add_tail(&devinfo->list, &__i2c_board_list);  
    }  
  
    up_write(&__i2c_board_lock);  
  
    return status;  
}  

第一个参数呢需要注意，它是 I2C总线号，一定要和具体的 I2C总线对应。我们看又定义了这样一个结构 struct i2c_devinfo。

最后是调用list_add_tail往__i2c_board_list这条链表添加设备信息。

然后是i2c_new_device

struct i2c_client *  
i2c_new_device(struct i2c_adapter *adap, struct i2c_board_info const *info)  
{  
    struct i2c_client   *client;  
    int         status;  
  
    /*为I2C设备申请内存*/  
    client = kzalloc(sizeof *client, GFP_KERNEL);  
    if (!client)  
        return NULL;  
  
    /*指定I2C设备的总线*/  
    client->adapter = adap;  
  
    client->dev.platform_data = info->platform_data;  
  
    if (info->archdata)  
        client->dev.archdata = *info->archdata;  
  
    client->flags = info->flags;  
    client->addr = info->addr; /*I2C设备地址*/  
    client->irq = info->irq;  
  
    strlcpy(client->name, info->type, sizeof(client->name));  
  
    /*检查这个地址有没有被设备占用*/  
    /* Check for address business */  
    status = i2c_check_addr(adap, client->addr);  
    if (status)  
        goto out_err;  
  
    client->dev.parent = &client->adapter->dev; /*指定设备的父设备*/  
    client->dev.bus = &i2c_bus_type; /*指定设备的总线类型*/  
    client->dev.type = &i2c_client_type;  
  
    dev_set_name(&client->dev, "%d-%04x", i2c_adapter_id(adap),  
             client->addr);  
    status = device_register(&client->dev); /*注册设备*/  
    if (status)  
        goto out_err;  
  
    dev_dbg(&adap->dev, "client [%s] registered with bus id %s
",  
        client->name, dev_name(&client->dev));  
  
    return client;  
  
out_err:  
    dev_err(&adap->dev, "Failed to register i2c client %s at 0x%02x "  
        "(%d)
", client->name, client->addr, status);  
    kfree(client);  
    return NULL; 

这个函数的功能是新建一个I2C设备并注册它，在I2C子系统中，I2C设备使用结构structi2c_client描述，那么首先要申请内存空间，I2C设备的主机是谁，必须知道挂载到哪条总线上的，然后就是一些赋值操作，最后就是注册设备，那么这个设备就实实在在的挂在到这条总线上了，这也是新的I2C设备注册方式。

3.i2c_do_add_adapter

你看说着说着就跑远了

static int i2c_do_add_adapter(struct device_driver *d, void *data)  
{  
    struct i2c_driver *driver = to_i2c_driver(d);  
    struct i2c_adapter *adap = data;  
  
    /* Detect supported devices on that bus, and instantiate them */  
    i2c_detect(adap, driver);  
  
    /* Let legacy drivers scan this bus for matching devices */  
    if (driver->attach_adapter) {  
        /* We ignore the return code; if it fails, too bad */  
        driver->attach_adapter(adap);  
    }  
    return 0;  
}  

前面通过 i2c_scan_static_board_info往 I2C总线上添加设备是新的方式，而这里调用每个 I2C设备驱动的 attach_adapter函数，然后在 attach_adapter函数中去实现设备的注册，这是老的方式， i2c-dev.c中就是采用的这种方式。至此，总线这块就看完了。