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實(shí)例解析linux內(nèi)核I2C體系結(jié)構(gòu)（2）

四、在內(nèi)核里寫i2c設(shè)備驅(qū)動(dòng)的兩種方式

在《實(shí)例解析linux內(nèi)核I2C體系結(jié)構(gòu)（1）》一文介紹了利用/dev/i2c-0在應(yīng)用層完成對(duì)i2c設(shè)備的操作，但很多時(shí)候我們還是習(xí)慣為i2c設(shè)備在內(nèi)核層編寫驅(qū)動(dòng)程序。目前內(nèi)核支持兩種編寫i2c驅(qū)動(dòng)程序的方式。下面分別介紹這兩種方式的實(shí)現(xiàn)。這里分別稱這兩種方式為“Adapter方式（LEGACY）”和“Probe方式（new style）”。

（1） Adapter方式（LEGACY）

（下面的實(shí)例代碼是在2.6.27內(nèi)核的pca953x.c基礎(chǔ)上修改的，原始代碼采用的是本文將要討論的第2種方式，即Probe方式）

●    構(gòu)建i2c_driver

static struct i2c_driver pca953x_driver = {
                .driver = {
                                    .name= "pca953x", //名稱
                                },
                .id= ID_PCA9555,//id號(hào)
                .attach_adapter= pca953x_attach_adapter, //調(diào)用適配器連接設(shè)備
                .detach_client= pca953x_detach_client,//讓設(shè)備脫離適配器
        };

●    注冊(cè)i2c_driver

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

●    attach_adapter動(dòng)作

執(zhí)行i2c_add_driver(&pca953x_driver)后會(huì)，如果內(nèi)核中已經(jīng)注冊(cè)了i2c適配器，則順序調(diào)用這些適配器來連接我們的i2c設(shè)備。此過程是通過調(diào)用i2c_driver中的attach_adapter方法完成的。具體實(shí)現(xiàn)形式如下：

static int pca953x_attach_adapter(struct i2c_adapter *adapter)
        {
                return i2c_probe(adapter, &addr_data, pca953x_detect);
                /*
                adapter:適配器
                addr_data:地址信息
                pca953x_detect：探測(cè)到設(shè)備后調(diào)用的函數(shù)
                */
        }

地址信息addr_data是由下面代碼指定的。
        /* 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里的地址必須是你i2c芯片的地址。否則將無法正確探測(cè)到設(shè)備。而I2C_ CLIENT_INSMOD是一個(gè)宏，它會(huì)利用normal_i2c構(gòu)建addr_data。

●    構(gòu)建i2c_client，并注冊(cè)字符設(shè)備驅(qū)動(dòng)

i2c_probe在探測(cè)到目標(biāo)設(shè)備后，后調(diào)用pca953x_detect，并把當(dāng)時(shí)的探測(cè)地址address作為參數(shù)傳入。

static int pca953x_detect(struct i2c_adapter *adapter, int address, int kind)
        {
                struct i2c_client *new_client;
                struct pca953x_chip *chip; //設(shè)備結(jié)構(gòu)體
                int err = 0,result;
                dev_t pca953x_dev=MKDEV(pca953x_major,0);//構(gòu)建設(shè)備號(hào)，根據(jù)具體情況設(shè)定，這里我只考慮了normal_i2c中只有一個(gè)地址匹配的情況。
                if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA| I2C_FUNC_SMBUS_WORD_DATA))//判定適配器能力
                goto exit;
                if (!(chip = kzalloc(sizeof(struct pca953x_chip), GFP_KERNEL))) {
                        err = -ENOMEM;
                        goto exit;
                }
                /****構(gòu)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)))//注冊(cè)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); //注冊(cè)字符設(shè)備，此處不詳解
                return 0;
                exit_detach:
                i2c_detach_client(new_client);
        exit_kfree:
                kfree(chip);
        exit:
                return err;
        }

i2c_check_functionality用來判定設(shè)配器的能力，這一點(diǎn)非常重要。你也可以直接查看對(duì)應(yīng)設(shè)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);
        }

●    字符驅(qū)動(dòng)的具體實(shí)現(xiàn)

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

字符設(shè)備驅(qū)動(dòng)本身沒有什么好說的，這里主要想說一下，如何在驅(qū)動(dòng)中調(diào)用i2c設(shè)配器幫我們完成數(shù)據(jù)傳輸。

目前設(shè)配器主要支持兩種傳輸方法：smbus_xfer和master_xfer。一般來說，如果設(shè)配器支持了master_xfer那么它也可以模擬支持smbus的傳輸。但如果只實(shí)現(xiàn)smbus_xfer，則不支持一些i2c的傳輸。

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中的參數(shù)設(shè)置，和前面的用戶空間編程一致。現(xiàn)在只是要在驅(qū)動(dòng)中構(gòu)建相關(guān)的參數(shù)然后調(diào)用i2c_transfer來完成傳輸既可。

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

smbus_xfer中的參數(shù)設(shè)置及調(diào)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;
        }

上面函數(shù)完成向芯片的地址為reg的寄存器寫一個(gè)16bit的數(shù)據(jù)。i2c_smbus_write_word_data的實(shí)現(xiàn)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);
        }

從中可以看出smbus傳輸一個(gè)16位數(shù)據(jù)的方法。其它操作如：字符寫、字符讀、字讀、塊操作等，可以參考內(nèi)核的i2c-core.c中提供的方法。

●    注銷i2c_driver

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

●    detach_client動(dòng)作

順序調(diào)用內(nèi)核中注冊(cè)的適配器來斷開我們注冊(cè)過的i2c設(shè)備。此過程通過調(diào)用i2c_driver中的attach_adapter方法完成的。具體實(shí)現(xiàn)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)))//斷開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）

●    構(gòu)建i2c_driver

和LEGACY方式一樣，也需要構(gòu)建i2c_driver，但是內(nèi)容有所不同。

static struct i2c_driver pca953x_driver = {
                .driver = {
                        .name= "pca953x",
                        },
                        .probe= pca953x_probe, //當(dāng)有i2c_client和i2c_driver匹配時(shí)調(diào)用
                        .remove= pca953x_remove,//注銷時(shí)調(diào)用
                        .id_table= pca953x_id,//匹配規(guī)則
        };

●    注冊(cè)i2c_driver

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

在注冊(cè)i2c_driver的過程中，是將driver注冊(cè)到了i2c_bus_type的總線上。此總線的匹配規(guī)則是：

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的名稱和id_table中的名稱做匹配的。本驅(qū)動(dòng)中的id_table為

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, },
                { }
        };

看到現(xiàn)在我們應(yīng)該會(huì)有這樣的疑問，在Adapter模式中，i2c_client是我們自己構(gòu)造出來的，而現(xiàn)在的i2c_client是從哪來的呢？看看下面的解釋

●    注冊(cè)i2c_board_info

對(duì)于Probe模式，通常在平臺(tái)代碼中要完成i2c_board_info的注冊(cè)。方法如下：

static struct i2c_board_info __initdata test_i2c_devices[] = {
                {
                        I2C_BOARD_INFO("pca9555", 0x27),//pca9555為芯片名稱，0x27為芯片地址
                        .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)); //注冊(cè)

i2c_client就是在注冊(cè)過程中構(gòu)建的。但有一點(diǎn)需要注意的是i2c_register_board_info并沒有EXPORT_SYMBOL給模塊使用。

●    字符驅(qū)動(dòng)注冊(cè)

在Probe方式下，添加字符驅(qū)動(dòng)的位置在pca953x_probe中。

static int __devinit pca953x_probe(struct i2c_client *client,const struct i2c_device_id *id)
        {
                        ……
                        /****字符設(shè)備驅(qū)動(dòng)注冊(cè)位置****/
                        ……
                        return 0;
        }

●    注銷i2c_driver

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

●    注銷字符設(shè)備驅(qū)動(dòng)

在Probe方式下，注銷字符驅(qū)動(dòng)的位置在pca953x_remove中。

static int __devinit pca953x_remove (struct i2c_client *client)
        {
                ……
                /****字符設(shè)備驅(qū)動(dòng)注銷的位置****/
                ……
                return 0;
        }

●    I2C設(shè)備的數(shù)據(jù)交互方法(即：調(diào)用適配器操作設(shè)備的方法)和Adapter方式下相同。