droidconfr2013-discover-embedded-linux-with-android

Talk given at droidcon fr 2013
http://fr.droidcon.com/2013/sessions/genymobile/

Code sample are available:
https://github.com/CedricCabessa/droidconfr2013

Video:
http://www.youtube.com/watch?v=BgP-QjjWCfA

slide 1 to 37 have speaker notes
slide 38 to the end are the same without speaker notes (download pdf to have correct ratio, speakerdeck do not support slide rotation)

1. Discover embedded Linux with Android Cédric Cabessa ccabessa@genymobile.com
2. Introduction Android is for mass market. Ecosystem is oriented toward multimedia content and consumer. Money is made selling games, music and so on.
3. Introduction On the other hand, embedded linux is used to create everykind of project, you have drivers for everything, you can plug any devices you want. The "hdtv blueray microwave washing machine" come from the Yocto-project
4. Introduction Can we keep Android with it's UI, its fully integrated API and yet use it to build our own embedded system ?
5. Introduction Android IS Linux
6. The Question • How to modify Android to support a new hardware / new features without asking Google. • Android Style You have a dev board (like a pandaboard) with Android. You want to plug a new device (gpio, i2c, rs232, ...). How to use it with an Android like API
7. Case study • http://source.android.com • Nexus • Pandaboard • Manufacturer support Android is opensource (AOSP), it is fairly easy to grab the source and build your own ROM (if you have hardware support: nexus or dev board with manufacturer support).
8. Case study • Let's play with hardware !
9. Case study • Character device: /dev/droidcon • Convert upper case to lower case, in kernel land! In linux, most of the drivers are exposed through the "/dev" fiilesystem. To send data to the driver, you simply open the file and write to it. In real life, we assume you already have a driver for your device
10. Case study We assume we have a support for linux (driver) for our new hardware, we want to create applications that use it. We have to patch every layer from kernel to framework
11. Kernel / Driver • Not part of AOSP • Driver can be built with the kernel • Or as a seperate module (.ko) • GPLv2 Driver is provided by your board manufacturer (kernel) or as a module from your device provider. Of course you can write your own if you have a custom device.
12. Kernel / Driver • init.rc on boot insmod /system/lib/modules/droidcon.ko chown system system /dev/droidcon chmod 0600 /dev/droidcon • device/<vendor>/<product>/device.mk PRODUCT_COPY_FILES := \ device/<vendor>/<product>/droidcon.ko:system/lib/modules/droidcon.ko
13. Kernel / Driver • Try it (connect as root) adb shell # ls -l /dev/droidcon crw------- system system 249, 0 droidcon # echo Hello > /dev/droidcon # cat /dev/droidcon hELLO # with real world devices, it is hard to check everything works just after loading the module
14. Kernel / Driver • Our device is only usable by 'system' • Run our app as system? => NO • system_server use the device • App communicate with system_server We will have to create permissions
15. HAL We simply load the driver and check permission. It's time to use it. Hardware Abstraction Layer
16. HAL • C library • Talk to driver • Expose hardware feature • Often closed source :-( expose hardware feature: driver can only support read / write, HAL should expose more advanced semantics that are implemented using read/write/... ex: usb missile launcher: HAL: missile_launch() Driver: write(/dev/foo, 42)
17. HAL • hardware/droidcon/lib • libdroidcon.h ssize_t droidcon_getdata(void *buf, size_t count); ssize_t droidcon_putdata(const void *buf, size_t count); void droidcon_clear(void); • Do complicated thing here! And only here The driver only support open / read / write / close. We create a "droidcon_clear" function, this semantic make sense from a driver user point of view. In fact it is implemented as write(fd, 0, 0) ==> intelligent stuff go here
18. JNI Hal is C, System Service is java => jni
19. JNI • frameworks/base/core/java/android/droidcon/ LibDroidcon.java package android.droidcon; public class LibDroidcon { public native static void clear(); public native static String getData() throws DroidconException; public native static void putData(String in) throws DroidconException; static { System.loadLibrary("droidcon_jni"); } } Rem: we hack framework! May be we could separate our change from the standard android, but we will have to modify SystemServer so can't stay really separated.
20. JNI • hardware/droidcon/jni/ android_droidcon_LibDroidcon.c JNIEXPORT void JNICALL Java_android_droidcon_LibDroidcon_putData (JNIEnv *env, jclass cls, jstring string) { int ret; const char *buff = (*env)->GetStringUTFChars(env, string, NULL); int length = (*env)->GetStringLength(env, string); ret = droidcon_putdata(buff, length); if (ret < 0) { ThrowDroidconException(env, "fail to put data"); } (*env)->ReleaseStringUTFChars(env, string, buff); }
21. JNI • Tip for testing: ◦ Add a Main.java in android.droidcon public class Main { public static void main (String[] args) { try { LibDroidcon.putData("Hello"); String out = LibDroidcon.getData(); System.out.println(out); LibDroidcon.clear(); } catch (DroidconException e) { Log.d("droidcon", e.toString()); } } } ◦ Run it with the shell # dalvikvm android.droidcon.Main hELLO
22. System Service Now we can use our HAL from java. SystemService run as system. When you use context.getSystemService(...) you only get a proxy that send command through binder IPC to the SystemService. System Service do the real work and check permission.
23. System Service • Must implement a Binder interface • framework/base/droidcon/java/android/droidcon/ IDroidconManager.aidl package android.droidcon; /** {@hide} */ interface IDroidconManager { void clear(); String getData(); void putData(String data); } Declare every methods the client (framework) can call on the server (System Service) Can't throw exception here. Use return code or something else.
24. System Service • frameworks/base/services/java/com/android/server/ DroidconService.java
25. System Service package com.android.server; /** @hide */ class DroidconService extends IDroidconManager.Stub { public String getData() { enforceAccessPermission(); try { return LibDroidcon.getData(); } catch(DroidconException e) { Slog.d(TAG, "cannot getdata"); } return null; } private void enforceAccessPermission() { mContext.enforceCallingOrSelfPermission( android.Manifest.permission.DROIDCON_ACCESS, "DroidconService"); } IDroidconManager.Stub is automatically generated from IDroidconManager.aidl Check for exception here and return Enforce permission (framework/base/core/res/AndroidManifest.xml)
26. System Service • Hack SystemServer.java public void run() { [...] try { Slog.i(TAG, "Droidcon Service"); droidconService = new DroidconService(context); ServiceManager.addService(Context.DROIDCON_SERVICE, droidconService); } catch (Throwable e) { reportWtf("starting Droidcon Service", e); }
27. Framework DroidconService is launched at startup. It listen to a Binder interface. We have to write the client side: Framework
28. Framework • framework/base/droidcon/java/android/droidcon DroidconManager.java public class DroidconManager { IDroidconManager mService; /** @hide */ public DroidconManager(IDroidconManager service) { mService = service; } public String getData() { try { return mService.getData(); } catch (RemoteException e) { e.printStackTrace(); } return null; } Do nothing but calling methods through the binder interface (and checking return value)
29. Framework • Hack ContextImpl.java class ContextImpl extends Context { static { [...] registerService(DROIDCON_SERVICE, new ServiceFetcher() { public Object createService(ContextImpl ctx) { IBinder b = ServiceManager.getService(DROIDCON_SERVICE); IDroidconManager service = IDroidconManager.Stub.asInterface (b); return new DroidconManager(service); }});
30. Framework • Create a new SDK ◦ make update-api ◦ make sdk • Configure eclipse
31. Framework <?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.example.droidcon" android:versionCode="1" android:versionName="1.0" > <uses-permission android:name="android.permission.DROIDCON_ACCESS"/> • AndroidManifest.xml
32. Framework
33. Framework
34. Framework
35. Conclusion • Hardware support is not an excuse to not choose Android • Kernel Driver and HAL should be re-usable • Upper layers are mostly "glue code" • Fragmentation?
36. References • Remixing Android http://marakana.com/s/post/1044/remixing_android • Adding System Service http://processors.wiki.ti.com/index.php/Android- Adding_SystemService
37. Q&A Genymobile SAS - 41, rue Meslay 75003 PARIS - 01 83 64 25 40 www.genymobile.com QUESTIONS ? Cédric Cabessa ccabessa@genymobile.com https://speakerdeck.com/cedriccabessa/ droidconfr2013-discover-embedded-linux-with-android https://github.com/CedricCabessa/droidconfr2013
38. Discover embedded Linux with Android Cédric Cabessa ccabessa@genymobile.com
39. Introduction
40. Introduction
41. Introduction
42. Introduction
43. The Question • How to modify Android to support a new hardware / new features without asking Google. • Android Style
44. Case study • http://source.android.com • Nexus • Pandaboard • Manufacturer support
45. Case study • Let's play with hardware !
46. Case study • Character device: /dev/droidcon • Convert upper case to lower case, in kernel land!
47. Case study
48. Kernel / Driver • Not part of AOSP • Driver can be built with the kernel • Or as a seperate module (.ko) • GPLv2
49. Kernel / Driver • init.rc on boot insmod /system/lib/modules/droidcon.ko chown system system /dev/droidcon chmod 0600 /dev/droidcon • device/<vendor>/<product>/device.mk PRODUCT_COPY_FILES := \ device/<vendor>/<product>/droidcon.ko:system/lib/modules/droidcon.ko
50. Kernel / Driver • Try it (connect as root) adb shell # ls -l /dev/droidcon crw------- system system 249, 0 droidcon # echo Hello > /dev/droidcon # cat /dev/droidcon hELLO #
51. Kernel / Driver • Our device is only usable by 'system' • Run our app as system? => NO • system_server use the device • App communicate with system_server
52. HAL
53. HAL • C library • Talk to driver • Expose hardware feature • Often closed source :-(
54. HAL • hardware/droidcon/lib • libdroidcon.h ssize_t droidcon_getdata(void *buf, size_t count); ssize_t droidcon_putdata(const void *buf, size_t count); void droidcon_clear(void); • Do complicated thing here! And only here
55. JNI
56. JNI • frameworks/base/core/java/android/droidcon/ LibDroidcon.java package android.droidcon; public class LibDroidcon { public native static void clear(); public native static String getData() throws DroidconException; public native static void putData(String in) throws DroidconException; static { System.loadLibrary("droidcon_jni"); } }
57. JNI • hardware/droidcon/jni/ android_droidcon_LibDroidcon.c JNIEXPORT void JNICALL Java_android_droidcon_LibDroidcon_putData (JNIEnv *env, jclass cls, jstring string) { int ret; const char *buff = (*env)->GetStringUTFChars(env, string, NULL); int length = (*env)->GetStringLength(env, string); ret = droidcon_putdata(buff, length); if (ret < 0) { ThrowDroidconException(env, "fail to put data"); } (*env)->ReleaseStringUTFChars(env, string, buff); }
58. JNI • Tip for testing: ◦ Add a Main.java in android.droidcon public class Main { public static void main (String[] args) { try { LibDroidcon.putData("Hello"); String out = LibDroidcon.getData(); System.out.println(out); LibDroidcon.clear(); } catch (DroidconException e) { Log.d("droidcon", e.toString()); } } } ◦ Run it with the shell # dalvikvm android.droidcon.Main hELLO
59. System Service
60. System Service • Must implement a Binder interface • framework/base/droidcon/java/android/droidcon/ IDroidconManager.aidl package android.droidcon; /** {@hide} */ interface IDroidconManager { void clear(); String getData(); void putData(String data); }
61. System Service • frameworks/base/services/java/com/android/server/ DroidconService.java
62. System Service package com.android.server; /** @hide */ class DroidconService extends IDroidconManager.Stub { public String getData() { enforceAccessPermission(); try { return LibDroidcon.getData(); } catch(DroidconException e) { Slog.d(TAG, "cannot getdata"); } return null; } private void enforceAccessPermission() { mContext.enforceCallingOrSelfPermission( android.Manifest.permission.DROIDCON_ACCESS, "DroidconService"); }
63. System Service • Hack SystemServer.java public void run() { [...] try { Slog.i(TAG, "Droidcon Service"); droidconService = new DroidconService(context); ServiceManager.addService(Context.DROIDCON_SERVICE, droidconService); } catch (Throwable e) { reportWtf("starting Droidcon Service", e); }
64. Framework
65. Framework • framework/base/droidcon/java/android/droidcon DroidconManager.java public class DroidconManager { IDroidconManager mService; /** @hide */ public DroidconManager(IDroidconManager service) { mService = service; } public String getData() { try { return mService.getData(); } catch (RemoteException e) { e.printStackTrace(); } return null; }
66. Framework • Hack ContextImpl.java class ContextImpl extends Context { static { [...] registerService(DROIDCON_SERVICE, new ServiceFetcher() { public Object createService(ContextImpl ctx) { IBinder b = ServiceManager.getService(DROIDCON_SERVICE); IDroidconManager service = IDroidconManager.Stub.asInterface (b); return new DroidconManager(service); }});
67. Framework • Create a new SDK ◦ make update-api ◦ make sdk • Configure eclipse
68. Framework <?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.example.droidcon" android:versionCode="1" android:versionName="1.0" > <uses-permission android:name="android.permission.DROIDCON_ACCESS"/> • AndroidManifest.xml
69. Framework
70. Framework
71. Framework
72. Conclusion • Hardware support is not an excuse to not choose Android • Kernel Driver and HAL should be re-usable • Upper layers are mostly "glue code" • Fragmentation?
73. References • Remixing Android http://marakana.com/s/post/1044/remixing_android • Adding System Service http://processors.wiki.ti.com/index.php/Android- Adding_SystemService
74. Q&A Genymobile SAS - 41, rue Meslay 75003 PARIS - 01 83 64 25 40 www.genymobile.com QUESTIONS ? Cédric Cabessa ccabessa@genymobile.com https://speakerdeck.com/cedriccabessa/ droidconfr2013-discover-embedded-linux-with-android https://github.com/CedricCabessa/droidconfr2013