A deep dive into AOSP Passthrough HAL

Android’s Hardware Abstraction Layer (HAL) acts as a bridge between the operating system and hardware components. In our previous blog, we discussed Binderized HAL, which utilizes the Binder IPC for inter-process communication. Now, we will look into Passthrough HAL, another approach within Android Open-Source Project (AOSP) to interact with hardware components. While Binderized HAL is more common in modern Android devices, Passthrough HAL remains relevant for certain performance-critical applications and legacy implementations.

This blog explores the fundamentals of Passthrough HAL, its architecture, communication mechanisms, advantages, and an example to help developers understand its implementation better.

Understanding Passthrough HAL

Passthrough HAL is a way for Android framework components to directly interact with hardware drivers without requiring an IPC mechanism. Unlike Binderized HAL, which runs as a separate process and communicates via Binder, Passthrough HAL operates in the same process as the calling application, reducing overhead and improving performance.

Key Characteristics

• Direct Function Calls: Unlike Binderized HAL, which relies on Binder IPC, Passthrough HAL allows direct function calls, reducing latency.
• Same Process Execution: Both the framework and the HAL implementation run within the same process, eliminating inter-process communication delays.
• Legacy and Performance-Critical Use Cases: Used in older Android versions or scenarios where IPC overhead must be minimized.
• Less Security Isolation: Since there is no separate process boundary, it may introduce security risks compared to Binderized HAL.
• Simpler Debugging: Since it does not involve IPC, debugging is easier compared to Binderized HAL implementations.

Passthrough HAL Architecture

Understanding the architecture of Passthrough HAL is crucial for developers who work on HAL implementations. The key components include:

1. Framework Layer: This includes the Android system services and APIs that interact with HAL. The framework layer calls the HAL interface to access hardware functionality.
2. Hardware Interface Definition Language (HIDL): HIDL defines the interface for the HAL implementation. Whether the HAL is Binderized or Passthrough, it must conform to the HIDL interface definition.
3. Passthrough HAL Implementation: Passthrough HAL implementation is directly linked with the framework at compile time. The framework loads the HAL module dynamically using the hw_get_module() API.
4. HAL Library and Hardware Driver: Passthrough HAL is compiled as a shared library (so file) and is dynamically loaded by system components that need to communicate with the hardware. This library interacts directly with the hardware driver.

Communication in Passthrough HAL

Communication in Passthrough HAL follows a simpler path compared to Binderized HAL:

1. The application or system service calls an API exposed by the framework.
2. The framework invokes the corresponding function in the HAL interface.
3. The HAL library executes the function call directly, interacting with the hardware driver.
4. The hardware driver processes the request and returns the result to the HAL.
5. The HAL implementation returns the result to the framework, which provides it to the application.

Since everything happens in a single process, there is no need for marshalling and unmarshalling data, making it efficient but limiting security benefits.

Understanding Passthrough HAL with a usecase

let’s consider an example of a simple LED control HAL. While we will not include actual code, here is a step-by-step conceptual breakdown:

1. Define the HIDL Interface: The HIDL interface (ILedControl.hal) specifies the functions to turn the LED on and off.
2. Implement the Passthrough HAL: The HAL implementation (LedControl.cpp) directly controls the LED hardware by accessing the driver.
3. Compile as a Shared Library: The implementation is compiled into a shared object file (libledcontrol.so).
4. Load the HAL in the Framework: The Android framework loads libledcontrol.so at runtime using hw_get_module().
5. Call the HAL from an Application: A system service or an app calls the LED control API, and the request is processed directly through the HAL implementation.

Advantages of Passthrough HAL

While modern Android platforms prefer Binderized HAL, Passthrough HAL still offers advantages in specific scenarios:

1. Lower Overhead and Latency: Since function calls happen in the same process without IPC, performance is improved.
2. Simpler Implementation: Passthrough HAL is straightforward to implement compared to Binderized HAL, which requires additional IPC mechanisms.
3. Efficient for Performance-Critical Applications: Applications requiring real-time processing, such as audio and certain sensor operations, benefit from Passthrough HAL.
4. Useful for Legacy Codebases: Older Android devices that still use legacy HAL implementations rely on Passthrough HAL for hardware interaction.

Conclusion

Passthrough HAL remains an essential concept in AOSP for hardware interaction, particularly in performance-sensitive applications. While Binderized HAL is the preferred approach in modern Android devices due to its security and modularity benefits, Passthrough HAL offers lower latency and simpler implementation.

For developers working on AOSP, understanding both HAL mechanisms is crucial for optimizing hardware interactions based on use case requirements. By carefully evaluating performance, security, and maintenance factors, developers can make informed decisions on whether to use Passthrough or Binderized HAL for their Android hardware implementations.

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