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    实例解析linux内核I2C体系结构(2)
    时间:2018-08-16作者:华清远见

    四、在内核里写i2c设备驱动的两种方式

    《实例解析linux内核I2C体系结构(1)》一文介绍了利用/dev/i2c-0在应用层完成对i2c设备的操作,但很多时候我们还是习惯为i2c设备在内核层编写驱动程序。目前内核支持两种编写i2c驱动程序的方式。下面分别介绍这两种方式的实现。这里分别称这两种方式为“Adapter方式(LEGACY)”和“Probe方式(new style)”。

    (1) Adapter方式(LEGACY)

    (下面的实例代码是在2.6.27内核的pca953x.c基础上修改的,原始代码采用的是本文将要讨论的第2种方式,即Probe方式)

    ●    构建i2c_driver

    static struct i2c_driver pca953x_driver = {
                    .driver = {
                                        .name= "pca953x", //名称
                                    },
                    .id= ID_PCA9555,//id号
                    .attach_adapter= pca953x_attach_adapter, //调用适配器连接设备
                    .detach_client= pca953x_detach_client,//让设备脱离适配器
            };

    ●    注册i2c_driver

    static int __init pca953x_init(void)
            {
                    return i2c_add_driver(&pca953x_driver);
            }
            module_init(pca953x_init);

    ●    attach_adapter动作

    执行i2c_add_driver(&pca953x_driver)后会,如果内核中已经注册了i2c适配器,则顺序调用这些适配器来连接我们的i2c设备。此过程是通过调用i2c_driver中的attach_adapter方法完成的。具体实现形式如下:

    static int pca953x_attach_adapter(struct i2c_adapter *adapter)
            {
                    return i2c_probe(adapter, &addr_data, pca953x_detect);
                    /*
                    adapter:适配器
                    addr_data:地址信息
                    pca953x_detect:探测到设备后调用的函数
                    */
            }

    地址信息addr_data是由下面代码指定的。
            /* Addresses to scan */
            static unsigned short normal_i2c[] = {0x20,0x21,0x22,0x23,0x24,0x25,0x26,0x27,I2C_CLIENT_END};
            I2C_CLIENT_INSMOD;

    注意:normal_i2c里的地址必须是你i2c芯片的地址。否则将无法正确探测到设备。而I2C_ CLIENT_INSMOD是一个宏,它会利用normal_i2c构建addr_data。

    ●    构建i2c_client,并注册字符设备驱动

    i2c_probe在探测到目标设备后,后调用pca953x_detect,并把当时的探测地址address作为参数传入。

    static int pca953x_detect(struct i2c_adapter *adapter, int address, int kind)
            {
                    struct i2c_client *new_client;
                    struct pca953x_chip *chip; //设备结构体
                    int err = 0,result;
                    dev_t pca953x_dev=MKDEV(pca953x_major,0);//构建设备号,根据具体情况设定,这里我只考虑了normal_i2c中只有一个地址匹配的情况。
                    if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA| I2C_FUNC_SMBUS_WORD_DATA))//判定适配器能力
                    goto exit;
                    if (!(chip = kzalloc(sizeof(struct pca953x_chip), GFP_KERNEL))) {
                            err = -ENOMEM;
                            goto exit;
                    }
                    /****构建i2c-client****/
                    chip->client=kzalloc(sizeof(struct i2c_client),GFP_KERNEL);
                    new_client = chip->client;
                    i2c_set_clientdata(new_client, chip);
                    new_client->addr = address;
                    new_client->adapter = adapter;
                    new_client->driver = &pca953x_driver;
                    new_client->flags = 0;
                    strlcpy(new_client->name, "pca953x", I2C_NAME_SIZE);
                    if ((err = i2c_attach_client(new_client)))//注册i2c_client
                    goto exit_kfree;
                    if (err)
                    goto exit_detach;
                    if(pca953x_major)
                    {
                            result=register_chrdev_region(pca953x_dev,1,"pca953x");
                    }
                    else{
                            result=alloc_chrdev_region(&pca953x_dev,0,1,"pca953x");
                            pca953x_major=MAJOR(pca953x_dev);
                    }
                    if (result < 0) {
                            printk(KERN_NOTICE "Unable to get pca953x region, error %d\n", result);
                            return result;
                    }
                    pca953x_setup_cdev(chip,0); //注册字符设备,此处不详解
                    return 0;
                    exit_detach:
                    i2c_detach_client(new_client);
            exit_kfree:
                    kfree(chip);
            exit:
                    return err;
            }

    i2c_check_functionality用来判定设配器的能力,这一点非常重要。你也可以直接查看对应设配器的能力,如

    static const struct i2c_algorithm smbus_algorithm = {
                    .smbus_xfer= i801_access,
                    .functionality= i801_func,
            };
            static u32 i801_func(struct i2c_adapter *adapter)
            {
                            return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE |
                        I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA |
                    I2C_FUNC_SMBUS_BLOCK_DATA | I2C_FUNC_SMBUS_WRITE_I2C_BLOCK
                                    | (isich4 ? I2C_FUNC_SMBUS_HWPEC_CALC : 0);
            }

    ●    字符驱动的具体实现

    struct file_operations pca953x_fops = {
                    .owner = THIS_MODULE,
                    .ioctl= pca953x_ioctl, 
                    .open= pca953x_open, 
                    .release =pca953x_release, 
            };

    字符设备驱动本身没有什么好说的,这里主要想说一下,如何在驱动中调用i2c设配器帮我们完成数据传输。

    目前设配器主要支持两种传输方法:smbus_xfer和master_xfer。一般来说,如果设配器支持了master_xfer那么它也可以模拟支持smbus的传输。但如果只实现smbus_xfer,则不支持一些i2c的传输。

    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);

    master_xfer中的参数设置,和前面的用户空间编程一致。现在只是要在驱动中构建相关的参数然后调用i2c_transfer来完成传输既可。

    int i2c_transfer(struct i2c_adapter * adap, struct i2c_msg *msgs, int num)

    smbus_xfer中的参数设置及调用方法如下:

    static int pca953x_write_reg(struct pca953x_chip *chip, int reg, uint16_t val)
            {
                    int ret;
                    ret = i2c_smbus_write_word_data(chip->client, reg << 1, val);
                    if (ret < 0) {
                                    dev_err(&chip->client->dev, "failed writing register\n");
                                            return -EIO;
                                    }
                    return 0;
            }

    上面函数完成向芯片的地址为reg的寄存器写一个16bit的数据。i2c_smbus_write_word_data的实现如下:

    s32 i2c_smbus_write_word_data(struct i2c_client *client, u8 command, u16 value)
            {
                    union i2c_smbus_data data;
                    data.word = value;
                    return i2c_smbus_xfer(client->adapter,client->addr,client->flags,
                                                                            I2C_SMBUS_WRITE,command,
                                                                            I2C_SMBUS_WORD_DATA,&data);
            }

    从中可以看出smbus传输一个16位数据的方法。其它操作如:字符写、字符读、字读、块操作等,可以参考内核的i2c-core.c中提供的方法。

    ●    注销i2c_driver

    static void __exit pca953x_exit(void)
            {
                    i2c_del_driver(&pca953x_driver);
            }
            module_exit(pca953x_exit);

    ●    detach_client动作

    顺序调用内核中注册的适配器来断开我们注册过的i2c设备。此过程通过调用i2c_driver中的attach_adapter方法完成的。具体实现形式如下:

    static int pca953x_detach_client(struct i2c_client *client)
            {
                    int err;
                    struct pca953x_chip *data;
                    if ((err = i2c_detach_client(client)))//断开i2c_client
                    return err;
                    data=i2c_get_clientdata(client);
                    cdev_del(&(data->cdev));
                    unregister_chrdev_region(MKDEV(pca953x_major, 0), 1);
                    kfree(data->client);
                    kfree(data);
                    return 0;
            }

    (2) Probe方式(new style)

    ●    构建i2c_driver

    和LEGACY方式一样,也需要构建i2c_driver,但是内容有所不同。

    static struct i2c_driver pca953x_driver = {
                    .driver = {
                            .name= "pca953x",
                            },
                            .probe= pca953x_probe, //当有i2c_client和i2c_driver匹配时调用
                            .remove= pca953x_remove,//注销时调用
                            .id_table= pca953x_id,//匹配规则
            };

    ●    注册i2c_driver

    static int __init pca953x_init(void)
            {
                    return i2c_add_driver(&pca953x_driver);
            }
            module_init(pca953x_init);

    在注册i2c_driver的过程中,是将driver注册到了i2c_bus_type的总线上。此总线的匹配规则是:

    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;
            }

    可以看出是利用i2c_client的名称和id_table中的名称做匹配的。本驱动中的id_table为

    static const struct i2c_device_id pca953x_id[] = {
                    { "pca9534", 8, },
                    { "pca9535", 16, },
                    { "pca9536", 4, },
                    { "pca9537", 4, },
                    { "pca9538", 8, },
                    { "pca9539", 16, },
                    { "pca9554", 8, },
                    { "pca9555", 16, },
                    { "pca9557", 8, },
                    { "max7310", 8, },
                    { }
            };

    看到现在我们应该会有这样的疑问,在Adapter模式中,i2c_client是我们自己构造出来的,而现在的i2c_client是从哪来的呢?看看下面的解释

    ●    注册i2c_board_info

    对于Probe模式,通常在平台代码中要完成i2c_board_info的注册。方法如下:

    static struct i2c_board_info __initdata test_i2c_devices[] = {
                    {
                            I2C_BOARD_INFO("pca9555", 0x27),//pca9555为芯片名称,0x27为芯片地址
                            .platform_data = &pca9555_data,
                    }, {
                            I2C_BOARD_INFO("mt9v022", 0x48),
                            .platform_data = &iclink[0], /* With extender */
                    }, {
                            I2C_BOARD_INFO("mt9m001", 0x5d),
                            .platform_data = &iclink[0], /* With extender */
                    },
            };
            i2c_register_board_info(0, test_i2c_devices,ARRAY_SIZE(test_i2c_devices)); //注册

    i2c_client就是在注册过程中构建的。但有一点需要注意的是i2c_register_board_info并没有EXPORT_SYMBOL给?槭褂。

    ●    字符驱动注册

    在Probe方式下,添加字符驱动的位置在pca953x_probe中。

    static int __devinit pca953x_probe(struct i2c_client *client,const struct i2c_device_id *id)
            {
                            ……
                            /****字符设备驱动注册位置****/
                            ……
                            return 0;
            }

    ●    注销i2c_driver

    static void __exit pca953x_exit(void)
            {
                    i2c_del_driver(&pca953x_driver);
            }
            module_exit(pca953x_exit);

    ●    注销字符设备驱动

    在Probe方式下,注销字符驱动的位置在pca953x_remove中。

    static int __devinit pca953x_remove (struct i2c_client *client)
            {
                    ……
                    /****字符设备驱动注销的位置****/
                    ……
                    return 0;
            }

    ●    I2C设备的数据交互方法(即:调用适配器操作设备的方法)和Adapter方式下相同。


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    (下面的实例代码是在2.6.27内核的pca953x.c基础上修改的,原始代码采用的是本文将要讨论的第2种方式,即Probe方式)

    ●    构建i2c_driver

    static struct i2c_driver pca953x_driver = {
                    .driver = {
                                        .name= "pca953x", //名称
                                    },
                    .id= ID_PCA9555,//id号
                    .attach_adapter= pca953x_attach_adapter, //调用适配器连接设备
                    .detach_client= pca953x_detach_client,//让设备脱离适配器
            };

    ●    注册i2c_driver

    static int __init pca953x_init(void)
            {
                    return i2c_add_driver(&pca953x_driver);
            }
            module_init(pca953x_init);

    ●    attach_adapter动作

    执行i2c_add_driver(&pca953x_driver)后会,如果内核中已经注册了i2c适配器,则顺序调用这些适配器来连接我们的i2c设备。此过程是通过调用i2c_driver中的attach_adapter方法完成的。具体实现形式如下:

    static int pca953x_attach_adapter(struct i2c_adapter *adapter)
            {
                    return i2c_probe(adapter, &addr_data, pca953x_detect);
                    /*
                    adapter:适配器
                    addr_data:地址信息
                    pca953x_detect:探测到设备后调用的函数
                    */
            }

    地址信息addr_data是由下面代码指定的。
            /* Addresses to scan */
            static unsigned short normal_i2c[] = {0x20,0x21,0x22,0x23,0x24,0x25,0x26,0x27,I2C_CLIENT_END};
            I2C_CLIENT_INSMOD;

    注意:normal_i2c里的地址必须是你i2c芯片的地址。否则将无法正确探测到设备。而I2C_ CLIENT_INSMOD是一个宏,它会利用normal_i2c构建addr_data。

    ●    构建i2c_client,并注册字符设备驱动

    i2c_probe在探测到目标设备后,后调用pca953x_detect,并把当时的探测地址address作为参数传入。

    static int pca953x_detect(struct i2c_adapter *adapter, int address, int kind)
            {
                    struct i2c_client *new_client;
                    struct pca953x_chip *chip; //设备结构体
                    int err = 0,result;
                    dev_t pca953x_dev=MKDEV(pca953x_major,0);//构建设备号,根据具体情况设定,这里我只考虑了normal_i2c中只有一个地址匹配的情况。
                    if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA| I2C_FUNC_SMBUS_WORD_DATA))//判定适配器能力
                    goto exit;
                    if (!(chip = kzalloc(sizeof(struct pca953x_chip), GFP_KERNEL))) {
                            err = -ENOMEM;
                            goto exit;
                    }
                    /****构建i2c-client****/
                    chip->client=kzalloc(sizeof(struct i2c_client),GFP_KERNEL);
                    new_client = chip->client;
                    i2c_set_clientdata(new_client, chip);
                    new_client->addr = address;
                    new_client->adapter = adapter;
                    new_client->driver = &pca953x_driver;
                    new_client->flags = 0;