device_create函数分析
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