LinuxUSB Gadget软件分为三层：

Gadget功能驱动层: 最主要的结构是structusb_composite_driver，这个结构在这层定义，并且实现结构中的各个函数。

USB设备层: 最主要的数据结构是structusb_composite_dev与usb_gadget_driver。前一个代表一个USB设备，而后一个是Gadget驱动，与UDC层交互。

UDC层: 最主要的数据结构是structusb_gadget，通常包含在其他结构体中。这个结构体代表了一个USB设备控制器的所有关于USB通信的信息。

在android平台gadget的代码是android.c，它实现了ffs,adb,acm,mtp,ptp,rndis,mass_storage_function,accessory_function,audio_source_function，在代码中这些功能的定义如下：

static struct android_usb_function *supported_functions[] = {
#if (USE_FFS == 1)
	&ffs_function,
#endif
	&adb_function,
	&acm_function,
	&mtp_function,
	&ptp_function,
	&rndis_function,
	&mass_storage_function,
	&accessory_function,
	&audio_source_function,
	NULL
};

android.c的代码位于gadget功能驱动层，composite.c位于usb设备层，udc-core.c位于udc层。

在gadget功能驱动层，它的相关数据结构定义是：

struct android_dev {
	struct android_usb_function **functions;
	struct list_head enabled_functions;
	struct usb_composite_dev *cdev;
	struct device *dev;

	bool enabled;
	int disable_depth;
	struct mutex mutex;
	bool connected;
	bool sw_connected;
	struct work_struct work;
	char ffs_aliases[256];
};

(2)enabled_functions是当前使用的功能，应用层可以去设置它;

(3)struct usb_composite_dev是USB设置层的结构，这里与设备层关联;

(4)定义struct work_struct内核线程来监测USB的状态，相关代码是android_work：

static void android_work(struct work_struct *data)
{
	struct android_dev *dev = container_of(data, struct android_dev, work);
	struct usb_composite_dev *cdev = dev->cdev;
	char *disconnected[2] = { "USB_STATE=DISCONNECTED", NULL };
	char *connected[2]    = { "USB_STATE=CONNECTED", NULL };
	char *configured[2]   = { "USB_STATE=CONFIGURED", NULL };
	char **uevent_envp = NULL;
	unsigned long flags;

    /* psw0523 add for dwc otg */
#ifdef CONFIG_USB_DWCOTG
    if (!cdev) return;
#endif

	spin_lock_irqsave(&cdev->lock, flags);
	if (cdev->config)
		uevent_envp = configured;
	else if (dev->connected != dev->sw_connected)
		uevent_envp = dev->connected ? connected : disconnected;
	dev->sw_connected = dev->connected;
	spin_unlock_irqrestore(&cdev->lock, flags);

	if (uevent_envp) {
		kobject_uevent_env(&dev->dev->kobj, KOBJ_CHANGE, uevent_envp);
		pr_info("%s: sent uevent %s\n", __func__, uevent_envp[0]);
	} else {
		pr_info("%s: did not send uevent (%d %d %p)\n", __func__,
			 dev->connected, dev->sw_connected, cdev->config);
	}
}

static int __init init(void)
{
	struct android_dev *dev;
	int err;

	android_class = class_create(THIS_MODULE, "android_usb");   //创建/sys/class/android_usb
	if (IS_ERR(android_class))
		return PTR_ERR(android_class);

	dev = kzalloc(sizeof(*dev), GFP_KERNEL);
	if (!dev)
		return -ENOMEM;

	dev->disable_depth = 1;                   //初始化disable计数
	dev->functions = supported_functions;
	INIT_LIST_HEAD(&dev->enabled_functions);    //初始化enabled_functions链表
	INIT_WORK(&dev->work, android_work);
	mutex_init(&dev->mutex);

	err = android_create_device(dev);               //初始化功能设备
	if (err) {
		class_destroy(android_class);
		kfree(dev);
		dev = NULL;
		return err;
	}

	_android_dev = dev;

	/* Override composite driver functions */
	composite_driver.setup = android_setup;                   //修改compoite的setup函数
	composite_driver.disconnect = android_disconnect;         //修改compoite的disconnect函数

	err = usb_composite_probe(&android_usb_driver, android_bind);   //调用USB设备层代码
	if (err)
		return err;

	/*
	 * Calling usb_gadget_disconnect here. As we will call
	 * usb_gadget_connect only when config is ready.
	 */
	usb_gadget_disconnect(dev->cdev->gadget);
	return err;
}

static int android_create_device(struct android_dev *dev)
{
	struct device_attribute **attrs = android_usb_attributes;
	struct device_attribute *attr;
	int err;

	dev->dev = device_create(android_class, NULL,
					MKDEV(0, 0), NULL, "android0");
	if (IS_ERR(dev->dev))
		return PTR_ERR(dev->dev);

	dev_set_drvdata(dev->dev, dev);

	while ((attr = *attrs++)) {
		err = device_create_file(dev->dev, attr);
		if (err) {
			device_destroy(android_class, dev->dev->devt);
			return err;
		}
	}
	return 0;
}