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)

Transcript

1. Discover embedded Linux
   with Android
   Cédric Cabessa
   ccabessa@genymobile.com
   

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2. Introduction
   Android is for mass market. Ecosystem is oriented toward multimedia content and
   consumer.
   Money is made selling games, music and so on.
   

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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
   

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4. Introduction
   Can we keep Android with it's UI, its fully integrated API and yet use it to build our
   own embedded system ?
   

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5. Introduction
   Android IS Linux
   

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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
   

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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).
   

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8. Case study
   ● Let's play with hardware !
   

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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
   

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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
    

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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.
    

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12. Kernel / Driver
    ●
    init.rc
    on boot
    insmod /system/lib/modules/droidcon.ko
    chown system system /dev/droidcon
    chmod 0600 /dev/droidcon
    ●
    device///device.mk
    PRODUCT_COPY_FILES := \
    device///droidcon.ko:system/lib/modules/droidcon.ko
    

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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
    

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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
    

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15. HAL
    We simply load the driver and check permission. It's time to use it.
    Hardware Abstraction Layer
    

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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)
    

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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
    

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18. JNI
    Hal is C, System Service is java
    => jni
    

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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.
    

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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);
    }
    

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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
    

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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.
    

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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.
    

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24. System Service
    ●
    frameworks/base/services/java/com/android/server/
    DroidconService.java
    

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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)
    

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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);
    }
    

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27. Framework
    DroidconService is launched at startup. It listen to a Binder interface.
    We have to write the client side: Framework
    

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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)
    

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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);
    }});
    

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30. Framework
    ●
    Create a new SDK
    ○
    make update-api
    ○
    make sdk
    ●
    Configure eclipse
    

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31. Framework
    
    package="com.example.droidcon"
    android:versionCode="1"
    android:versionName="1.0" >
    
    ● AndroidManifest.xml
    

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32. Framework
    

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33. Framework
    

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34. Framework
    

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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?
    

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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
    

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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
    

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38. Discover embedded Linux
    with Android
    Cédric Cabessa
    ccabessa@genymobile.com
    

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39. Introduction
    

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40. Introduction
    

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41. Introduction
    

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42. Introduction
    

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43. The Question
    ● How to modify Android to support a new hardware /
    new features without asking Google.
    ● Android Style
    

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44. Case study
    ●
    http://source.android.com
    ●
    Nexus
    ●
    Pandaboard
    ●
    Manufacturer support
    

    View Slide

45. Case study
    ● Let's play with hardware !
    

    View Slide

46. Case study
    ● Character device: /dev/droidcon
    ● Convert upper case to lower case, in kernel land!
    

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47. Case study
    

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48. Kernel / Driver
    ●
    Not part of AOSP
    ●
    Driver can be built with the kernel
    ●
    Or as a seperate module (.ko)
    ●
    GPLv2
    

    View Slide

49. Kernel / Driver
    ●
    init.rc
    on boot
    insmod /system/lib/modules/droidcon.ko
    chown system system /dev/droidcon
    chmod 0600 /dev/droidcon
    ●
    device///device.mk
    PRODUCT_COPY_FILES := \
    device///droidcon.ko:system/lib/modules/droidcon.ko
    

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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
    #
    

    View Slide

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
    

    View Slide

52. HAL
    

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53. HAL
    ●
    C library
    ●
    Talk to driver
    ●
    Expose hardware feature
    ●
    Often closed source :-(
    

    View Slide

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
    

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55. JNI
    

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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");
    }
    }
    

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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);
    }
    

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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
    

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59. System Service
    

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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);
    }
    

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61. System Service
    ●
    frameworks/base/services/java/com/android/server/
    DroidconService.java
    

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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");
    }
    

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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);
    }
    

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64. Framework
    

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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;
    }
    

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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);
    }});
    

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67. Framework
    ●
    Create a new SDK
    ○
    make update-api
    ○
    make sdk
    ●
    Configure eclipse
    

    View Slide

68. Framework
    
    package="com.example.droidcon"
    android:versionCode="1"
    android:versionName="1.0" >
    
    ● AndroidManifest.xml
    

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69. Framework
    

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70. Framework
    

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71. Framework
    

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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?
    

    View Slide

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
    

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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
    

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