Flutter UI Framework Support in Buildroot

Overview

The support for the Flutter UI framework in the Buildroot embedded Linux build system was primarily developed and contributed by Adam Duskett of Amarula Solutions. This significant effort introduced the necessary toolchain components and runtime packages to allow developers to cross-compile and run high-performance Flutter applications on custom embedded Linux distributions.

Official support for Flutter was successfully merged and is available starting with Buildroot version 2023.11.

Key Components and Packages

The Buildroot integration includes several critical packages necessary to handle the complex Flutter build system and provide the runtime environment on the target device:

flutter-engine: The core package containing the compiled Flutter engine runtime library (libflutter_engine.so). This is the main component deployed to the target device.
flutter-sdk-bin: A set of host tools and plugins required during the cross-compilation process to compile Flutter applications.
depot-tools: Contains the gclient Python script, which is essential for downloading the Flutter engine source code in the format expected by the Flutter build system.
flutter-pi: A lightweight Flutter embedder for embedded Linux. It typically uses Direct Rendering Infrastructure (DRI) and Kernel Mode Setting (KMS), supporting OpenGL ES and Vulkan.
flutter-gallery: A demonstration application that serves as a template for developers when porting their own Flutter applications.

Technical Implementation Details

The integration of Flutter posed significant challenges, primarily because Flutter’s upstream source acquisition and build process are non-standard for traditional build systems like Buildroot.

Source Acquisition and Reproducibility

Flutter source code is not released as a standard tarball. It must be downloaded using the gclient tool from the depot-tools package, which checks out the source code and its many third-party dependencies. Furthermore, the build process relies on the presence of local .git directories.

To ensure Buildroot’s requirement for reproducible builds is met, the Flutter packages utilize the following solution:

A custom script, package/flutter-engine/gen-tarball, was created to run gclient and package the necessary source files into a Buildroot-compatible tarball for offline use and checksum verification.

Toolchain Dependencies

The Flutter engine is uniquely dependent on specific versions of its toolchain:

Bundled Clang Compiler: The Flutter source tree includes a patched version of Clang that must be used for compilation. The Buildroot package configuration is set up to use this bundled compiler rather than the default cross-toolchain built by Buildroot to prevent linking errors.
GN Build System: The package also requires the Google Ninja (gn) build tool, which is provided by the dependency chain starting with depot-tools.

Usage and Profiling

Integrating a custom Flutter application into a Buildroot project is typically done by mimicking the structure of the existing flutter-gallery package and placing the new package definitions in an External Tree.

Profiling a Flutter application’s performance on the target device is possible by configuring the engine build and runtime:

Enable Profiling: Enable the profiling option for the flutter-engine package in Buildroot’s menuconfig:
```
Libraries ---> Graphics ---> flutter-engine ---> [*] Enable profiling
```
Run with VM Service: Boot the target device and run the application using flutter-pi, enabling the Dart VM service for remote debugging:
```
flutter-pi --profile /usr/share/flutter/${your_app}/profile/ --vm-service-host=0.0.0.0 --vm-service-port=1234
```
The application output will provide a URL that can be accessed from a remote machine using Dart DevTools for profiling and debugging.