Passing Chrome Flags on Chromium OS: A Comprehensive Guide

Chrome OS is a versatile platform, adapting to various hardware configurations. Sometimes, Chrome needs specific instructions to behave optimally on different devices. This article delves into how to pass Chrome flags, ensuring proper functionality and customization within the Chromium OS environment.

Understanding the Need for Chrome Flags

Chrome flags are command-line arguments that modify Chrome's behavior. They're essential when:

• Hardware features require specific handling: For instance, enabling or disabling features depending on the presence of an accelerometer or a specific GPU.
• Testing experimental features: Flags can activate functionalities still under development.
• Customizing Chrome's behavior: Adjusting settings beyond the standard user interface.

Two Primary Approaches: Runtime vs. Build-Time Configuration

Chromium OS provides two main ways to pass Chrome flags:

• Runtime Configuration: This method applies flags during each session, allowing dynamism based on currently available hardware or system state.
• Build-Time Configuration: These configuration applies flags when creating a Chrome OS system image.

Runtime Configuration: Leveraging session_manager

The session_manager plays a crucial role in runtime configuration. It's responsible for constructing Chrome's command line using chrome_setup.cc.

• chrome_setup.cc: This file uses the ChromiumCommandBuilder class from libchromeos-ui to create necessary directories, configure the environment, and assemble the command line.
• ChromiumCommandBuilder: This class reads a subset of Portage USE flags from /etc/ui_use_flags.txt. These flags, set during the system build, determine which flags should be passed to Chrome.

Adding a New USE Flag:

To introduce a new USE flag, follow these steps:

1. Add the new flag to the libchromeos-use-flags ebuild file. This file is strategically placed in a dedicated package, enabling the use of prebuilt chromeos-chrome and chromeos-login packages across devices with varying USE flag sets.

Configuration Locations: Browser vs. Chromium Codebase

Deciding where to place configuration settings is crucial:

• ChromiumCommandBuilder (in libchromeos-ui): Use this for configurations applicable to both the Chrome browser and other Chromium-based products (e.g., a dedicated web app shell). This primarily includes compositor- and audio-related flags.
• chrome_setup.cc: Reserve this for configurations specific to the Chrome browser, particularly flags implemented within Chrome's //ash and //chrome directories.

Best Practices: Feature-Based USE Flags

Prioritize creating new USE flags named after the specific feature you are enabling. Instead of directly examining board USE flags like samus or eve, use :

builder -> AddArgs ( "--enable-my-feature" );

This method reduces the amount of change needed to enable the feature for a new board, you just need to set USE flag in the new board’s overlay.

Unibuild and chromeos-config

The Unibuild project allows multiple device types to share the same system image. Device-specific configurations are managed through additional files read by chromeos-config. For per-device configurations, it's recommended to follow the example in the SetUpWallpaperFlags function within chrome_setup.cc.

Quick Changes for Development: /etc/chrome_dev.conf

For development environments, /etc/chrome_dev.conf offers a rapid way to modify Chrome's command line or environment. By enabling root access, developers can directly add or remove flags. The file itself contains detailed instructions on its format.

Conclusion

Passing Chrome flags effectively is essential for tailoring Chromium OS to diverse hardware and usage scenarios. By understanding the distinctions between runtime and build-time configurations, adhering to best practices for USE flags, and utilizing the appropriate configuration files, developers can ensure optimal Chrome behavior across all devices. Understanding these processes also help in Chrome OS development.