如何實(shí)例化i2c設(shè)備？4種方式如下：

• 方法1:靜態(tài)聲明I2C設(shè)備
  • 通過devicetree聲明I2C設(shè)備
  • 在板級(jí)文件中聲明I2C設(shè)備
• 方法2:顯式實(shí)例化設(shè)備
• 方法3:對(duì)某些設(shè)備的I2C總線進(jìn)行探測(cè)
• 方法4:從用戶空間實(shí)例化

在本篇中，主要看一下方法1，3和方法4。對(duì)于方法2，在下一篇的一個(gè)I2C ADC/DAC的misc設(shè)備驅(qū)動(dòng)實(shí)例中再加以詳述。

通過devicetree聲明I2C設(shè)備

/dts-v1/;
/plugin/;
/ {
       compatible = "brcm,bcm2835", "brcm,bcm2708", "brcm,bcm2709";




       fragment@0 {
          target = < &i2c1 >;
          __overlay__ {
                #address-cells = < 1 >;
                #size-cells = < 0 >;
                rtc@68 {
                        compatible = "maxim,ds3231";
                        reg = < 0x68 >;
                        #address-cells = < 2 >;
                        #size-cells = < 1 >;
                };      
          };
       };
    };

首先，寫一個(gè)dts。

編譯，并將生成的.dtbo到/boot/overlays

重啟后，查看/dev可以看到rtc

通過板級(jí)文件中聲明I2C設(shè)備

在架構(gòu)板級(jí)文件中添加i2c設(shè)備信息，并注冊(cè)到特定位置。看一個(gè)nxp imx開發(fā)板的電源芯片的實(shí)例，arch/arm/mach-imx/mach-mx35_3ds.c

static struct mc13xxx_platform_data mx35_3ds_mc13892_data = {
    .flags = MC13XXX_USE_RTC | MC13XXX_USE_TOUCHSCREEN,
    .regulators = {
        .num_regulators = ARRAY_SIZE(mx35_3ds_regulators),
        .regulators = mx35_3ds_regulators,
    },
};




#define GPIO_PMIC_INT IMX_GPIO_NR(2, 0)




static struct i2c_board_info mx35_3ds_i2c_mc13892 = {




    I2C_BOARD_INFO("mc13892", 0x08),
    .platform_data = &mx35_3ds_mc13892_data,
    /* irq number is run-time assigned */
};




static void __init imx35_3ds_init_mc13892(void)
{
    int ret = gpio_request_one(GPIO_PMIC_INT, GPIOF_DIR_IN, "pmic irq");




    if (ret) {
        pr_err("failed to get pmic irq: %dn", ret);
        return;
    }




    mx35_3ds_i2c_mc13892.irq = gpio_to_irq(GPIO_PMIC_INT);
    i2c_register_board_info(0, &mx35_3ds_i2c_mc13892, 1);
}

MC13892是面向i.MX51、i.MX37、i.MX3和i.MX27應(yīng)用處理器的PMIC。在板級(jí)文件中定義了一個(gè) i2c_board_info 結(jié)構(gòu)體，使用i2c_register_board_info函數(shù)將i2c設(shè)備信息添加到特定鏈表。

i2c_register_board_info在/drivers/i2c/i2c-boardinfo.c中：

/**
* i2c_register_board_info - statically declare I2C devices
* @busnum: identifies the bus to which these devices belong
* @info: vector of i2c device descriptors
* @len: how many descriptors in the vector; may be zero to reserve
*    the specified bus number.
*
* Systems using the Linux I2C driver stack can declare tables of board info
* while they initialize.  This should be done in board-specific init code
* near arch_initcall() time, or equivalent, before any I2C adapter driver is
* registered.  For example, mainboard init code could define several devices,
* as could the init code for each daughtercard in a board stack.
*
* The I2C devices will be created later, after the adapter for the relevant
* bus has been registered.  After that moment, standard driver model tools
* are used to bind "new style" I2C drivers to the devices.  The bus number
* for any device declared using this routine is not available for dynamic
* allocation.
*
* The board info passed can safely be __initdata, but be careful of embedded
* pointers (for platform_data, functions, etc) since that won't be copied.
* Device properties are deep-copied though.
*/
int i2c_register_board_info(int busnum, struct i2c_board_info const *info, unsigned len)

對(duì)某些設(shè)備的I2C總線進(jìn)行探測(cè)

有時(shí)沒有關(guān)于I2C設(shè)備的足夠信息，甚至無法調(diào)用i2c_new_scanned_device()。典型的例子是PC主板上的硬件監(jiān)控芯片。有幾十個(gè)模型，可能被分配在25個(gè)不同的地址。考慮到主板的龐大數(shù)量，幾乎不可能建立一個(gè)正在使用的硬件監(jiān)控芯片的詳盡列表。幸運(yùn)的是，大多數(shù)這些芯片都有制造商和設(shè)備ID寄存器，因此可以通過探測(cè)來識(shí)別它們。

在這種情況下，I2C設(shè)備既沒有顯式聲明也沒有實(shí)例化。相反，一旦這些設(shè)備的驅(qū)動(dòng)程序被加載，I2C -core就會(huì)探測(cè)它們，如果找到了，就會(huì)自動(dòng)實(shí)例化一個(gè)I2C設(shè)備。為了防止此機(jī)制的任何不當(dāng)行為，適用以下限制:

• I2C設(shè)備驅(qū)動(dòng)程序必須實(shí)現(xiàn)detect()方法，該方法通過從任意寄存器讀取來識(shí)別受支持的設(shè)備。
• 只有那些可能有支持設(shè)備并同意被探測(cè)的總線才會(huì)被探測(cè)。例如，這避免了探測(cè)電視適配器上的硬件監(jiān)控芯片。

detect何時(shí)調(diào)用？

i2c_driver注冊(cè)的時(shí)候，i2c_core會(huì)在所有已經(jīng)注冊(cè)的i2c_adapter上探測(cè)address_list中的所有地址，硬件探測(cè)成功之后后調(diào)用i2c_driver的detect成員，然后根據(jù)detect填充的info建立一個(gè)i2c_client。

例如，MAX1668溫度傳感器的驅(qū)動(dòng)實(shí)現(xiàn)，/drivers/hwmon/max1668.c

static struct i2c_driver max1668_driver = {
    .class = I2C_CLASS_HWMON,
    .driver = {
          .name    = "max1668",
          },
    .probe = max1668_probe,
    .id_table = max1668_id,
    .detect    = max1668_detect,
    .address_list = max1668_addr_list,
};




module_i2c_driver(max1668_driver);

其中，detect函數(shù)

/* Return 0 if detection is successful, -ENODEV otherwise */
static int max1668_detect(struct i2c_client *client,
              struct i2c_board_info *info)
{
    struct i2c_adapter *adapter = client- >adapter;
    const char *type_name;
    int man_id, dev_id;




    if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA))
        return -ENODEV;




    /* Check for unsupported part */
    man_id = i2c_smbus_read_byte_data(client, MAX1668_REG_MAN_ID);
    if (man_id != MAN_ID_MAXIM)
        return -ENODEV;




    dev_id = i2c_smbus_read_byte_data(client, MAX1668_REG_DEV_ID);
    if (dev_id < 0)
        return -ENODEV;




    type_name = NULL;
    if (dev_id == DEV_ID_MAX1668)
        type_name = "max1668";
    else if (dev_id == DEV_ID_MAX1805)
        type_name = "max1805";
    else if (dev_id == DEV_ID_MAX1989)
        type_name = "max1989";




    if (!type_name)
        return -ENODEV;




    strlcpy(info- >type, type_name, I2C_NAME_SIZE);




    return 0;
}

以樹莓派為例，i2c_add_adapter會(huì)在bcm2835_i2c_probe調(diào)用

static int bcm2835_i2c_probe(struct platform_device *pdev)

在i2c_register_adapter 中會(huì)調(diào)用，bus_for_each_drv 來來通知所有總線類型是i2c_bus_type的driver。

/* Notify drivers */
    mutex_lock(&core_lock);
    bus_for_each_drv(&i2c_bus_type, NULL, adap, __process_new_adapter);
    mutex_unlock(&core_lock);

static int __process_new_adapter(struct device_driver *d, void *data)
{
    return i2c_do_add_adapter(to_i2c_driver(d), data);
}

在i2c_do_add_adapter->i2c_detect->i2c_detect_address 中有一段關(guān)鍵code

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;
    }

從用戶空間實(shí)例化

用戶空間通過兩個(gè)sysfs屬性文件來建立和刪除i2c_client：new_device和delete_device。

• new_device有兩個(gè)參數(shù)：i2c設(shè)備的名字（字符串）和地址（以0x開頭的16進(jìn)制數(shù)）。
• delete_device只有一個(gè)參數(shù)：設(shè)備的地址。

pi@raspberrypi:/sys/class/i2c-adapter/i2c-1 $ ls
delete_device  device  i2c-dev  name  new_device  of_node  power  subsystem  uevent

添加rtc設(shè)備

pi@raspberrypi:/sys/class/i2c-adapter/i2c-1 $ echo ds3231 0x68 | sudo tee new_device 
ds3231 0x68

方法2:顯式實(shí)例化設(shè)備，在下一篇"在Linux系統(tǒng)里如何編寫一個(gè)PCF8591的驅(qū)動(dòng)，完成ADC數(shù)據(jù)采集，DAC數(shù)據(jù)輸出"中再詳述。

PCF8591 是一個(gè)單片集成、單獨(dú)供電、低功耗、8-bit CMOS數(shù)據(jù)獲取器件。PCF8591 具有 4 個(gè)模擬輸入、1 個(gè)模擬輸出和 1個(gè)串行 I2C 總線接口。

PCF8591 的 3 個(gè)地址引腳 A0, A1 和 A2 可用于硬件地址編程，允許在同個(gè) I2C 總線上接入 8 個(gè) PCF8591 器件，而無需額外的硬件。在 PCF8591 器件上輸入輸出的地址、控制和數(shù)據(jù)信號(hào)都是通過雙線雙向 I2C 總線以串行的方式進(jìn)行傳輸。