AOSP Passthrough HAL: Architecture, Use Cases & Performance Guide

With over 3 billion Android devices in circulation and a growing need for optimized hardware-software integration, understanding the different types of Hardware Abstraction Layers (HALs) is crucial for modern developers. In our last blog, we explored the Binderized HAL — now the standard in most Android platforms due to its secure and modular design. But did you know that Passthrough HAL still plays a vital role in performance-critical applications and legacy hardware deployments?

Despite its aging architecture, Passthrough HAL remains relevant in scenarios where low-latency hardware access and minimal overhead are priorities. In this blog, we’ll break down how Passthrough HAL works, when it makes sense to use it, and why understanding both HAL types is essential for building efficient, scalable, and backward-compatible Android systems.

Understanding Passthrough HAL

Without the need for an IPC mechanism, Android framework components can communicate directly with hardware drivers through Passthrough HAL. Passthrough HAL reduces overhead and improves performance by operating in the same process as the calling application, in contrast to Binderized HAL, which runs as a separate process and communicates via Binder.

Key Characteristics

Direct Function Calls: Passthrough HAL reduces latency by enabling direct function calls, in contrast to Binderized HAL, which depends on Binder IPC. Same Process Execution: To avoid delays in inter-process communication, the framework and the HAL implementation operate inside the same process. Use cases that are critical to performance and legacy: utilized in situations where IPC overhead needs to be kept to a minimum or in older Android versions. Less Security Isolation: Due to the absence of a separate process boundary, Passthrough HAL carries higher security risks compared to Binderized HAL. Simpler Debugging: Compared to Binderized HAL implementations, debugging is simpler because IPC is not involved.

Passthrough HAL Architecture

Framework Layer: The Android system services and APIs that communicate with HAL are included in this. To access hardware functionality, the framework layer makes a call to the HAL interface. Hardware Interface Definition Language (HIDL): The HAL implementation’s interface is defined by HIDL. The HIDL interface definition must be followed by the HAL regardless of whether it is binderized or passthrough. Passthrough HAL Implementation: At compile time, the implementation of Passthrough HAL is directly connected to the framework. The hw_get_module() API is used by the framework to dynamically load the HAL module. HAL Library and Hardware Driver: Passthrough System components that require communication with the hardware dynamically load HAL, which is compiled as a shared library (so file). The hardware driver and this library communicate directly.

Communication in Passthrough HAL

Compared to Binderized HAL, communication in Passthrough HAL takes a more straightforward route:

• The framework exposes an API that is called by the application or system service.
• The relevant HAL interface function is called by the framework.
• The function call is executed directly within the HAL library, which interacts with the hardware driver without IPC.
• After processing the request, the hardware driver sends the outcome back to the HAL.
• The framework gives the result to the application after receiving it from the HAL implementation.

Understanding Passthrough HAL with a use case

Define the HIDL Interface The functions to turn the LED on and off are specified by the HIDL interface (ILedControl.hal). Implement the Passthrough HAL By gaining access to the driver, the HAL implementation (LedControl.cpp) directly controls the LED hardware. Compile as a Shared Library A shared object file called libledcontrol.so contains the implementation’s compilation. Load the HAL in the Framework The Android framework uses hw_get_module() to load libledcontrol.so at runtime. Call the HAL from an Application When an application or system service makes a request to the LED control API, the HAL implementation handles it directly.

Advantages of Passthrough HAL

Lower Overhead and Latency :Performance is enhanced because function calls take place in the same process without IPC. Easier Implementation: Unlike Binderized HAL, which necessitates extra IPC mechanisms, Passthrough HAL is simple to implement. Efficient for Performance-Critical Applications: Passthrough HAL is useful for applications that need real-time processing, like audio and some sensor functions. Useful for Legacy Codebases: For hardware interaction, older Android devices that continue to use legacy HAL implementations depend on Passthrough HAL.

Conclusion While Binderized HAL dominates modern Android development for its modularity and security, Passthrough HAL still offers undeniable advantages in low-latency, performance-critical, or legacy applications. For AOSP developers, understanding both models is key to making hardware interactions faster, safer, and more efficient.

At Silicon Signals, we bring deep expertise in AOSP HAL implementations, whether you’re building for the future or supporting legacy systems.

Need help with HAL architecture or AOSP customization? Reach out to us at info@siliconsignals.io to collaborate with our embedded Android experts.