Shows how a device maker can extend Android to support new devices, while preserving Android compatibility. We demonstrate a joystick & an industrial barcode scanner.

1. Extending Android with New Devices
Android outside the mobile context
Shree Kumar Nitheesh K L
InnoMinds Software PES Institute of Technology

2. Speaker Intro
Shree Kumar
InnoMinds Software
Nitheesh K L
PES Institute of Technology

3. Background
• Android expected to proliferate
– Not just on smart-phones & tablets
• Android built with MID devices in mind
– Newer use cases => other devices
– Changing the API is not permitted
• New devices, Additional APIs !
Extending Android with New Devices | DroidCon India 2011

4. Overview
• Device support in Linux
• Layered approach for Android
• Two examples
– Joystick
– Industrial Barcode Scanner
• Device Maker’s Perspective
– Standardized interfaces may not happen due to
schedules
– Market compatibility provided
Extending Android with New Devices | DroidCon India 2011

5. Linux Device Support
User App
Device API
setuid()
Privileged Daemon
setgid()
Device
udev
Node
Kernel
SUBSYSTEMS="usb",ATTRS(idVendor)=="0bb4",MODE="0666",OWNER="shree"
SUBSYSTEMS="usb",ATTRS(idVendor)=="8086",MODE="0666",OWNER="shree"
Device
Linux devs : Remember adding these lines to
/etc/udev/rules.d/51-android.rules?
Extending Android with New Devices | DroidCon India 2011

6. Our Android Approach
User App (Java)
uses-permission=USE_DEVICE
Service Wrapper (optional) uses-library=sample_device
Privileged Service (Java) Platform Library
Preserves Android
sharedUserId=android.uid.system JNI Interface Compatibility
ueventd Device Node
Kernel
chown USER:GROUP /dev/XYZ mknod /dev/XYZ
chmod 0ppp /dev/XYZ Device
Extending Android with New Devices | DroidCon India 2011

7. Example 1
• Supporting a USB Joystick
• Why this device ?
– Simplicity
– Map concepts to details
• You need
– Android device with USB host/OTG
Extending Android with New Devices | DroidCon India 2011

8. Joystick : Step 1
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
• Kernel drivers
Kernel
Device
– Defaults to HID raw driver present
– Boot & Check
• Device node
– /dev/input/eventX
$ cat /proc/bus/input/devices | grep ^[NH] | grep –A 1 Gamepad
> | grep event
N: Logitech Precision Gamepad
H: Handlers=event15

9. Joystick : Step 2
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
Kernel
Device
• Interface to the device node
fd = open(“/dev/input/event15”, O_RDONLY);
– Read /dev/input/eventX
• Get single key code per event
– ev.code : scan code
– ev.value : key down/up
struct input_event ev;
rd = read (fd, &ev, sizeof(struct input_event);

10. Joystick : Step 2 (contd)
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
• Wrap native code using JNI
ueventd Device Node
Kernel
Device
static const JNINativeMethod gMethods[] = {
{“open”, “()Z”, (void *)Java_Joystick_open},
{“getKey”, “()I”, (void *)Java_Joystick_getKey},
{“close”, “()V”, (void *)Java_Joystick_close},
};
Jint JNI_OnLoad(JavaVM* vm, void* reserved) {
//register your methods
...
}
• Android.mk – generate lib
LOCAL_MODULE := libsample_joystick

11. Joystick : Step 3
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
• Permission for device node Kernel
Device
– <Android-src>/system/core/init/devices.c
Static struct perms_devperms[] = {
...
{“/dev/input”, 0666, AID_SYSTEM, AID_INPUT, 1},
...
}
$ls –l /dev/input
crw-rw---- system input 13, 79 2011-11-18 14:05 event15
crw-rw---- system input 13, 79 2011-11-18 14:05 event11
crw-rw---- system input 13, 79 2011-11-18 14:05 event4
...
– Ueventd.rc

12. Joystick : Step 4
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
• Service Wrapper around JNI Kernel
Device
|
+---com
| ---sample
| +---hardware
| | ---joystick
| | JoystickAPI.aidl
| | JoystickCallback.aidl
| | Joystick.java
| | Joystick.class
| |
| ---service
| JoystickService.java
|
+---jni
| Android.mk
| Joystick.h
| Joystick.cpp
| Jsfunctions.h
| Jsfunctions.cpp

13. Joystick : Step 4 (contd)
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
• Signed APK provides privileged access Kernel
Device
# we ask for restricted permissions for our service,
# so the apk has to be signed
LOCAL_CERTIFICATE := platform
• Define permission in AndroidManifest.xml
<permission android:name = “com.sample.USE_JOYSTICK” />

14. Joystick : Step 4 (contd)
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
Kernel
• IPC service exposes API to applications
Device
– …/ JoystickAPI.aidl
interface JoystickAPI {
boolean setCallback(in JoystickCallback cb);
boolean clearCallback();
}
– … / JoystickService.java
Private JoystickAPI.Stub api = new JoystickAPI.Stub(){
public boolean setCallback(JoystickCallback cb){
// enable setting callback
}
public boolean clearCallback(){
// enable clearing callback
}
}

15. Joystick : Step 5
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
Kernel
• Platform Library
Device
• Declare your library to the framework
– /system/etc/permissions
– com.sample.hardware.joystick.xml
<?xml version="1.0" encoding="utf-8"?>
<permissions>
<library name="com.sample.hardware.joystick"
file="/system/framework/com.sample.hardware.joystick.jar"/>
</permissions>
• Output product is raw .jar file, NOT a .apk
adb push
out/target/product/generic_x86/system/framework/com.sample.hardware.joystick.jar
/system/framework

16. Joystick : Step 6
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
• Application interacts with device
Kernel
Device
– Binds to Service
Intent intent = new Intent(“com.sample.android.service.JoystickService”);
bindService(intent, serviceConnection, Service.BIND_AUTO_CREATE);
– Uses API provided by Service
ServiceConnection sc = new ServiceConnection(){
private void onServiceConnected(ComponentName n, Ibinder service){
api = Ijoystick.stub.asInterface(service);
try{
api.setCallback(jsCallback);
}catch (RemoteException e){
}
}
}

17. Joystick : Step 6 (contd)
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
• Run on UI Thread
Kernel
Device
JoystickCallback jsCallback = new JoystickCallback.Stub() {
private void onKeyPress(int arg0) throws RemoteException {
final int key = arg0;
runOnUiThread(new Runnable(){
public void run(){
updateUI(key);
}
});
}
};

18. Joystick : Step 6 (contd)
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
Kernel
Device

19. Joystick : Overall
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
Kernel
Device

20. Example 2
• Barcode Scanner
– Opticon MDI 2300
• 2D scanner
– Available as a module for development
Extending Android with New Devices | DroidCon India 2011

21. Example 2
• Why use a dedicated device ?
– Supports tons of symbologies
– Purpose made
• Fast
• Long cables, Laser illumination
– Would you drop your smart-
phone ?
Opticon symbology support, “Menubook” excerpt
Extending Android with New Devices | DroidCon India 2011 ZXing symbology support

22. Barcode Scanner : Step 1
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
• Setup the device
Kernel
Device
– We’ll use USB VCP (Virtual COM Port) mode
– Scan barcodes! Barcodes vary by device.
Excerpts from Sec 2-5 of Xenon 1900 User Guide
USB VCP configuration for MDI 2300 | Screenshot from opticonfigure.opticon.com
Extending Android with New Devices | DroidCon India 2011

23. Barcode Scanner : Step 2
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
• Change the kernel
Kernel
Device
– Specific steps depend on Android/kernel version
– Include support for CDC ACM devices
• Ensure support for protocol=“None” in cdc-acm.c
/* control interfaces without any protocol set */
{ USB_INTERFACE_INFO(USB_CLASS_COMM, USB_CDC_SUBCLASS_ACM,
USB_CDC_PROTO_NONE) },
• Boot with the new kernel & check
Extending Android with New Devices | DroidCon India 2011

24. Barcode Scanner : Step 3
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
• Interface to the driver
Kernel
Device
– Read /dev/ttyACM0
– Get single barcode per line
• Test… at every step
• Wrap native code using JNI
– Can an app use the wrapper ?
# ls –l /dev/input*
crw-r----- 1 system input 13, 64 2011-11-02 14:51 event0
crw-r----- 1 system input 13, 64 2011-11-02 14:51 event1
Extending Android with New Devices | DroidCon India 2011

25. Barcode Scanner : Step 4
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
• ueventd : fix /dev/ttyACM0 access
Kernel
Device
– owner “system” (android.uid.system)
/dev/ttyACM0 0660 system root
Add this line to : system/core/rootdir/ueventd.rc
– Another method: owner “com.sample.uid.acm”
• Passes Android Compatibility !
mSettings.addSharedUserLP("android.uid.system",
Process.SYSTEM_UID, ApplicationInfo.FLAG_SYSTEM);
…
mSettings.addSharedUserLP("com.sample.uid.acm",
1018, ApplicationInfo.FLAG_SYSTEM);
frameworks/base/services/java/com/android/server/PackageManagerService.java
Extending Android with New Devices | DroidCon India 2011

26. Barcode Scanner : Step 5
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
• Service Wrapper around JNI
Kernel
Device
– AIDL based IPC service exposes API to applications
interface IBarcodeScanner {
boolean setCallback(in BarcodeScannerCallback callback);
boolean scanBarcode(int timeout);
boolean clearCallback();
};
Interface BarcodeScannerCallback {
void handlerBarcode(boolean timedout, String barcode);
}
– Signed APK provides privileged access
android:sharedUsedId=“android.uid.system”
Extending Android with New Devices | DroidCon India 2011

27. Barcode Scanner : Step 6
User App (Java)
Service Wrapper (optional)
Privileged Service (Java)
JNI Interface
ueventd Device Node
• Application interacts with device
Kernel
Device
– Binds to Service
– Uses API provided by Service
• Could be beautified…
– Unbinds when done!
Extending Android with New Devices | DroidCon India 2011

28. Summary
• HOWTO-support-devices-on-android.txt
– Important steps
– Examples
• Kept simple on purpose
– Skipped
• Power Management
• Deeper system integration
– You may want to rethink the interfaces !
Extending Android with New Devices | DroidCon India 2011

29. Questions ?
Shree Nitheesh
shree.shree@gmail.com nitheeshkl@gmail.com