Embedded Systems Series Part 10: Android HAL & Native Development

Introduction to Android HAL

The Hardware Abstraction Layer (HAL) bridges Android framework and kernel drivers. HALs define standardized interfaces—OEMs implement vendor-specific code while Android remains hardware-agnostic.

HAL Evolution (Legacy to Treble)

graph TD
    subgraph Legacy["Legacy HAL (Pre-Treble)"]
        A1[Application] --> F1[Framework]
        F1 --> H1["HAL (.so in /system)"]
        H1 --> K1[Kernel Driver]
    end

    subgraph Treble["Project Treble (Android 8+)"]
        A2[Application] --> F2[Framework]
        F2 --> HIDL["HIDL Interface"]
        HIDL --> H2["HAL (.so in /vendor)"]
        H2 --> K2[Kernel Driver]
    end

    subgraph Modern["Modern (Android 11+)"]
        A3[Application] --> F3[Framework]
        F3 --> AIDL["AIDL Interface"]
        AIDL --> H3["HAL (in /vendor)"]
        H3 --> K3[Kernel Driver]
    end

    Legacy -.->|"Tight Coupling"| Treble
    Treble -.->|"Stable Interface"| Modern

    style Legacy fill:#fff5f5,stroke:#BF092F
    style Treble fill:#f0f4f8,stroke:#16476A
    style Modern fill:#e8f4f4,stroke:#3B9797
                        

# HAL locations
/vendor/lib/hw/           # Legacy HAL .so files
/vendor/bin/hw/           # HIDL HAL services
/vendor/lib64/            # 64-bit HAL libraries

# HAL manifest (device capabilities)
/vendor/etc/vintf/manifest.xml

HIDL Interface Definition

HIDL (HAL Interface Definition Language) defines versioned, stable interfaces.

// hardware/interfaces/sensors/2.0/ISensors.hal
package android.hardware.sensors@2.0;

interface ISensors {
    getSensorsList() generates (vec<SensorInfo> list);
    activate(int32_t sensorHandle, bool enabled) generates (Result result);
    poll(int32_t maxCount) generates (vec<Event> events);
};

// Generate C++ from HIDL
hidl-gen -o output -L c++-impl \
    android.hardware.sensors@2.0

AIDL for HAL (Android 11+)

// hardware/interfaces/vibrator/aidl/android/hardware/vibrator/IVibrator.aidl
package android.hardware.vibrator;

@VintfStability
interface IVibrator {
    void on(int timeoutMs);
    void off();
    boolean supportsAmplitudeControl();
    void setAmplitude(int amplitude);
}

HAL Implementation

// Vibrator HAL implementation
#include <aidl/android/hardware/vibrator/BnVibrator.h>

using aidl::android::hardware::vibrator::BnVibrator;

class Vibrator : public BnVibrator {
public:
    ndk::ScopedAStatus on(int32_t timeoutMs) override {
        // Write to kernel driver
        write_to_sysfs("/sys/class/leds/vibrator/state", "1");
        write_to_sysfs("/sys/class/leds/vibrator/duration", timeoutMs);
        return ndk::ScopedAStatus::ok();
    }

    ndk::ScopedAStatus off() override {
        write_to_sysfs("/sys/class/leds/vibrator/state", "0");
        return ndk::ScopedAStatus::ok();
    }
};

NDK Programming

The NDK (Native Development Kit) enables C/C++ code in Android apps for performance-critical operations.

// native-lib.cpp
#include <jni.h>
#include <string>

extern "C" JNIEXPORT jstring JNICALL
Java_com_example_app_MainActivity_stringFromJNI(
    JNIEnv* env, jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

# CMakeLists.txt
cmake_minimum_required(VERSION 3.10)
add_library(native-lib SHARED native-lib.cpp)
find_library(log-lib log)
target_link_libraries(native-lib ${log-lib})

JNI Bindings

// Java side
public class MainActivity extends Activity {
    static {
        System.loadLibrary("native-lib");
    }

    public native String stringFromJNI();
    public native int calculate(int a, int b);
    public native void processArray(int[] data);
}

// C++ JNI implementation
extern "C" JNIEXPORT jint JNICALL
Java_com_example_app_MainActivity_calculate(
    JNIEnv* env, jobject obj, jint a, jint b) {
    return a + b;
}

extern "C" JNIEXPORT void JNICALL
Java_com_example_app_MainActivity_processArray(
    JNIEnv* env, jobject obj, jintArray arr) {
    jint* data = env->GetIntArrayElements(arr, nullptr);
    jsize len = env->GetArrayLength(arr);
    
    for (int i = 0; i < len; i++) {
        data[i] *= 2;  // Process array
    }
    
    env->ReleaseIntArrayElements(arr, data, 0);
}

Native Service Development

// Native service main
#include <binder/ProcessState.h>
#include <binder/IServiceManager.h>

int main() {
    android::ProcessState::self()->startThreadPool();
    
    sp<MyService> service = new MyService();
    defaultServiceManager()->addService(
        String16("my_native_service"), service);
    
    android::IPCThreadState::self()->joinThreadPool();
    return 0;
}

Debugging HAL & Native Code

# List HAL services
lshal                    # List all HAL services
dumpsys hwservicemanager

# Native debugging
adb shell gdbserver64 :5039 /vendor/bin/hw/my_hal
adb forward tcp:5039 tcp:5039
arm-linux-gnueabihf-gdb
(gdb) target remote :5039

# Logcat for native
adb logcat -s "MY_HAL"
__android_log_print(ANDROID_LOG_INFO, "MY_HAL", "msg");

Conclusion & What's Next

You've learned Android HAL development—HIDL/AIDL interfaces, HAL implementation, NDK native programming, and JNI bindings. These skills enable custom hardware integration in Android.

In Part 11, we'll explore Android BSP & Kernel—building board support packages and integrating custom kernels.