hwservicemanager

Introduction

With the introduction of Project Treble in Android 8.0, the operating system architectural boundary was split between the framework (System) and the hardware implementation (Vendor). To facilitate this, a new IPC mechanism called HwBinder was created specifically for HALs, along with a dedicated service manager to handle hardware services.

hwservicemanager vs servicemanager

Android runs two distinct binder domains:

1. /dev/binder: Used by standard framework services (like ActivityManager). Managed by the servicemanager daemon.
2. /dev/hwbinder: Used by HIDL (Hardware Interface Definition Language) HALs. Managed by the hwservicemanager daemon.

hwservicemanager serves the same fundamental purpose as servicemanager: it acts as a central registry where hardware services announce their presence, and clients (like the system server) look them up to establish a connection.

Registering and Retrieving HAL services

A vendor HAL process registers itself with hwservicemanager using the HIDL C++ API registerAsService().

// Vendor HAL process
#include <android/hardware/light/2.0/ILight.h>

using android::hardware::light::V2_0::ILight;
using android::hardware::light::V2_0::implementation::Light;

int main() {
    android::sp<ILight> service = new Light();
    // Registers with hwservicemanager under the name "default"
    android::status_t status = service->registerAsService("default");
    
    android::hardware::joinRpcThreadpool();
    return 0;
}

On the framework side, a client retrieves the service using getService():

// Framework client
android::sp<ILight> light = ILight::getService("default");
if (light != nullptr) {
    light->setLight(...);
}

HIDL Service Manifest

Unlike the standard servicemanager which accepts any service registration dynamically, hwservicemanager strictly enforces access control via the VINTF (Vendor Interface Object) manifest.

Before a HAL can be registered or retrieved, it must be declared in the device's manifest.xml. If a vendor daemon attempts to register a HIDL service that is not declared in the manifest, hwservicemanager will reject the registration.

<!-- device/vendor/manifest.xml -->
<hal format="hidl">
    <name>android.hardware.light</name>
    <transport>hwbinder</transport>
    <version>2.0</version>
    <interface>
        <name>ILight</name>
        <instance>default</instance>
    </interface>
</hal>

This strict manifesto ensures that the framework knows exactly which HAL versions the vendor partition provides at boot time, enforcing Treble compatibility requirements.