Android 获取USB设备的类型

Version info：Linux: 3.10Android: 4.4.2

一、问题

现在有USB设备插入Android系统，那么如何得知插入的设备类型？是USB打印机，U盘，还是USB鼠标？

二、USB类型规定

对于USB类型，USB组织是有规定的，见：http://www.usb.org/

比如：

Base Class

Descriptor Usage

Description

00h

Device

Use class information in the Interface Descriptors

01h

Interface

Audio 

02h

Both

Communications and CDC Control

03h

Interface

HID (Human Interface Device)

05h

Interface

Physical

06h

Interface

Image

07h

Interface

Printer

三、Linux对USB设备类型定义

在kernel中，有两个结构体的相关成员表征USB设备的类型，第一个usb_device_descriptor：

include/uapi/linux/usb/ch9.h/* USB_DT_DEVICE: Device descriptor */struct usb_device_descriptor {__u8  bDeviceClass;__u8  bDeviceSubClass;__u8  bDeviceProtocol;} __attribute__ ((packed));

bDeviceClass成员值区别不同的USB设备，如果该值为0呢？看上边：

Base Class

Descriptor Usage

Description

00h

Device

Use class information in the Interface Descriptors

就在第二个结构体usb_interface_descriptor中：

include/uapi/linux/usb/ch9.hstruct usb_interface_descriptor {__u8  bInterfaceClass;__u8  bInterfaceSubClass;__u8  bInterfaceProtocol;} __attribute__ ((packed));

bInterfaceClass成员即是。

比如，对于USB打印机设备，定义如下：

include/uapi/linux/usb/ch9.h/* * Device and/or Interface Class codes * as found in bDeviceClass or bInterfaceClass * and defined by www.usb.org documents */#define USB_CLASS_PER_INTERFACE0/* for DeviceClass */#define USB_CLASS_AUDIO1#define USB_CLASS_COMM2#define USB_CLASS_HID3#define USB_CLASS_PHYSICAL5#define USB_CLASS_STILL_IMAGE6#define USB_CLASS_PRINTER7#define USB_CLASS_MASS_STORAGE8#define USB_CLASS_HUB9#define USB_CLASS_CDC_DATA0x0a#define USB_CLASS_CSCID0x0b/* chip+ smart card */#define USB_CLASS_CONTENT_SEC0x0d/* content security */#define USB_CLASS_VIDEO0x0e#define USB_CLASS_WIRELESS_CONTROLLER0xe0#define USB_CLASS_MISC0xef#define USB_CLASS_APP_SPEC0xfe#define USB_CLASS_VENDOR_SPEC0xffdrivers/usb/gadget/printer.cstatic struct usb_interface_descriptor intf_desc = {.bLength =sizeof intf_desc,.bDescriptorType =USB_DT_INTERFACE,.bNumEndpoints =2,.bInterfaceClass =USB_CLASS_PRINTER,.bInterfaceSubClass =1,/* Printer Sub-Class */.bInterfaceProtocol =2,/* Bi-Directional */.iInterface =0};

对上了，不是？！

四、JNI和Framework层

JNI层的usbhost.c文件为我们获取Linux driver中USB设备的信息提供了便利，除了USB设备类型，还可以得到USB设备的厂商名称、产品名称、序列号等：

system/core/libusbhost/usbhost.c

usbhost.c该文件中提供的方法在JNI中调用：

frameworks/base/services/jni/com_android_server_UsbHostManager.cpp

上面代码截图信息量很大！很重要！很重要！很重要！很重要！重要的地方说四遍。

1、调用usbhost.c文件中方法获取USB设备信息

2、获取的信息一部分传到UsbInterface.java，APP部分调用UsbInterface类获取信息

frameworks/base/core/java/android/hardware/usb/UsbInterface.javapublic class UsbInterface implements Parcelable {    private final int mId;    private final int mClass;    private final int mSubclass;    private final int mProtocol;    private final Parcelable[] mEndpoints;}

3、获取的信息另一部分通过调用UsbHostManager.java中的usbDeviceAdded()方法，把信息写到UsbDevice.java中，APP部分调用UsbDevice类获取信息

frameworks/base/core/java/android/hardware/usb/UsbDevice.javapublic class UsbDevice implements Parcelable {    private final String mName;    private final int mVendorId;    private final int mProductId;    private final int mClass;    private final int mSubclass;    private final int mProtocol;    private final Parcelable[] mInterfaces;}

接下来，我们就看下如此关键的usbDeviceAdded()方法：

frameworks/base/services/java/com/android/server/usb/UsbHostManager.java/* Called from JNI in monitorUsbHostBus() to report new USB devices */private void usbDeviceAdded(String deviceName, int vendorID, int productID,int deviceClass, int deviceSubclass, int deviceProtocol,/* array of quintuples containing id, class, subclass, protocol   and number of endpoints for each interface */int[] interfaceValues,   /* array of quadruples containing address, attributes, max packet size  and interval for each endpoint */int[] endpointValues) {synchronized (mLock) {for (intf = 0; intf < numInterfaces; intf++) {int interfaceId = interfaceValues[ival++];int interfaceClass = interfaceValues[ival++];int interfaceSubclass = interfaceValues[ival++];int interfaceProtocol = interfaceValues[ival++];int numEndpoints = interfaceValues[ival++];interfaces[intf] = new UsbInterface(interfaceId, interfaceClass,interfaceSubclass, interfaceProtocol, endpoints);}UsbDevice device = new UsbDevice(deviceName, vendorID, productID,deviceClass, deviceSubclass, deviceProtocol, interfaces);mDevices.put(deviceName, device);}}

五、APP层

经过上面分析，获取USB设备类型等信息，已经心有成竹了，使用UsbDevice、UsbInterface两个类即可。其中，UsbInterface类是包含在UsbDevice类中的：

frameworks/base/core/java/android/hardware/usb/UsbDevice.javapublic class UsbDevice implements Parcelable {    private final Parcelable[] mInterfaces;/**     * Returns the {@link UsbInterface} at the given index.     *     * @return the interface     */    public UsbInterface getInterface(int index) {        return (UsbInterface)mInterfaces[index];    }}

之后，就是如何使用UsbDevice类了。

USB设备属于热插拔设备，一旦系统监测到其热插拔时间，会有相应事件上报：

frameworks/base/core/java/android/hardware/usb/UsbManager.java/** * This intent is sent when a USB device is attached to the USB bus when in host mode. */public static final String ACTION_USB_DEVICE_ATTACHED ="android.hardware.usb.action.USB_DEVICE_ATTACHED";/** * This intent is sent when a USB device is detached from the USB bus when in host mode. */public static final String ACTION_USB_DEVICE_DETACHED ="android.hardware.usb.action.USB_DEVICE_DETACHED";

所以，注册监听事件的广播即可，如：

class UsbDeviceEventReceiver extends BroadcastReceiver {@Overridepublic void onReceive(Context context, Intent intent) {UsbDevice device = (UsbDevice)intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);}}mReceiver = new UsbDeviceEventReceiver();IntentFilter filter = new IntentFilter();filter.addAction(UsbManager.ACTION_USB_DEVICE_ATTACHED);filter.addAction(UsbManager.ACTION_USB_DEVICE_DETACHED);registerReceiver(mReceiver, filter);

六、结束

Android流程：driver->JNI->Framework->APP，几乎所有模块都是这个模式，对于一个外设，如果这几个部分都清楚的话心里会特别明亮。