device_create分析

在2.6比较新的内核里面，这个函数在设备驱动中比较重要，拿出来单独分析下

源码如下：

/**

 * device_create - creates a device and registers it with sysfs

 * @class: pointer to the struct class that this device should be registered to

 * @parent: pointer to the parent struct device of this new device, if any

 * @devt: the dev_t for the char device to be added

 * @drvdata: the data to be added to the device for callbacks

 * @fmt: string for the device's name

 *

 * This function can be used by char device classes.  A struct device

 * will be created in sysfs, registered to the specified class.

 *

 * A "dev" file will be created, showing the dev_t for the device, if

 * the dev_t is not 0,0.

 * If a pointer to a parent struct device is passed in, the newly created

 * struct device will be a child of that device in sysfs.

 * The pointer to the struct device will be returned from the call.

 * Any further sysfs files that might be required can be created using this

 * pointer.

 *

 * Note: the struct class passed to this function must have previously

 * been created with a call to class_create().

 */

struct device *device_create(struct class *class, struct device *parent,

                          dev_t devt, void *drvdata, const char *fmt, ...)

{

       va_list vargs;

       struct device *dev;

 

       va_start(vargs, fmt);

       dev = device_create_vargs(class, parent, devt, drvdata, fmt, vargs);

       va_end(vargs);

       return dev;

}

先看参数

@class: pointer to the struct class that this device should be registered to

the struct class 指针，必须在本函数调用之前先被创建

parent: pointer to the parent struct device of this new device

该设备的parent指针。

devt: the dev_t for the char device to be added

字符设备的设备号，如果dev_t不是0,0的话，1个”dev”文件将被创建。

drvdata: the data to be added to the device for callbacks

被添加到该设备回调的数据。

fmt: string for the device's name

设备名字。

 

可以看下面的函数调用例子

例子1

/* register your own device in sysfs, and this will cause udev to create corresponding device node */

  device_create( my_class, NULL, MKDEV(hello_major, 0), "hello" "%d", 0 );

如果成功，它将会在/dev目录下产生/dev/hello0设备。

 

例子2

led_cdev->dev = device_create_drvdata(leds_class, parent, 0, led_cdev,

                                         "%s", led_cdev->name);

这是前文platform之gpio-leds里面的源码

由于dev_t是0，所以它不会在/dev下产生设备文件。

led_cdev为传递给class的私有数据。

会把第6个参数的内容复制到第5个参数 “%s”，就像printf一样。

 

通过这2个例子，应该对它的用法有所了解了，下面看下源码。

dev = device_create_vargs(class, parent, devt, drvdata, fmt, vargs);

深入device_create_vargs

/**

 * device_create_vargs - creates a device and registers it with sysfs

 * @class: pointer to the struct class that this device should be registered to

 * @parent: pointer to the parent struct device of this new device, if any

 * @devt: the dev_t for the char device to be added

 * @drvdata: the data to be added to the device for callbacks

 * @fmt: string for the device's name

 * @args: va_list for the device's name

 *

 * This function can be used by char device classes.  A struct device

 * will be created in sysfs, registered to the specified class.

 *

 * A "dev" file will be created, showing the dev_t for the device, if

 * the dev_t is not 0,0.

 * If a pointer to a parent struct device is passed in, the newly created

 * struct device will be a child of that device in sysfs.

 * The pointer to the struct device will be returned from the call.

 * Any further sysfs files that might be required can be created using this

 * pointer.

 *

 * Note: the struct class passed to this function must have previously

 * been created with a call to class_create().

 */

struct device *device_create_vargs(struct class *class, struct device *parent,

                               dev_t devt, void *drvdata, const char *fmt,

                               va_list args)

{

       struct device *dev = NULL;

       int retval = -ENODEV;

 

       if (class == NULL || IS_ERR(class))

              goto error;

 

       dev = kzalloc(sizeof(*dev), GFP_KERNEL);

       if (!dev) {

              retval = -ENOMEM;

              goto error;

       }

 

       dev->devt = devt;

       dev->class = class;

       dev->parent = parent;

       dev->release = device_create_release;

       dev_set_drvdata(dev, drvdata);

 

       vsnprintf(dev->bus_id, BUS_ID_SIZE, fmt, args);

       retval = device_register(dev);

       if (retval)

              goto error;

 

       return dev;

 

error:

       kfree(dev);

       return ERR_PTR(retval);

}

代码比较容易，最灵活的在这个地方

vsnprintf(dev->bus_id, BUS_ID_SIZE, fmt, args);

vsnprintf()会根据参数fmt字符串来转换并格式化数据，然后将结果复制到数组dev->bus_id所指的字符串数组，知道出现字符串结束符或达到参数BUS_ID_SIZE为止。