Linux-2-0:Introduction to Kconfig and Menuconfig in Linux Kernel Builds

When building a custom Linux kernel, particularly for embedded systems, Kconfig and menuconfig are essential tools for configuration. They allow developers to select, enable, or disable various kernel features, drivers, and hardware support in an interactive way. This article explains how Kconfig and menuconfig work together to streamline kernel configuration.

What is Kconfig?

Kconfig is a configuration system used by the Linux kernel to define build-time options. Each feature or driver in the kernel is described in a Kconfig file, which specifies the type of the option (e.g., boolean or integer), its default value, and any dependencies.

For example:

config USB_SUPPORT
    bool "Enable USB support"
    default y
    help
      Enable support for USB devices.        

This defines a simple option for enabling USB support. When configuring the kernel, Kconfig helps ensure that all dependencies are satisfied and only relevant options are presented to the user.

What is Menuconfig?

menuconfig is a user-friendly interface that presents Kconfig options as a menu. You can launch it by running:

make menuconfig        

Menuconfig displays kernel options in a text-based menu structure, allowing you to navigate through categories like hardware drivers and filesystem support. You can toggle options between built-in, modules, or disabled. Once you make your selections, menuconfig updates the .config file, which stores your choices.

For example:

[*] USB support
< > I2C support        

Here, USB support is selected as a built-in feature, while I2C support is disabled. The resulting .config file will reflect these choices.

How Kconfig and Menuconfig Work Together

1. Kconfig files define options. Kconfig files are found throughout the kernel source and describe every configurable option, from drivers to architecture-specific features.
2. Menuconfig presents options. Running make menuconfig opens an interactive menu that displays the options defined by Kconfig.
3. Selections are written to .config. After making your choices in menuconfig, the tool updates the .config file with your selections.
4. The kernel build uses .config. The .config file drives the kernel build process, ensuring that only the selected features are included in the kernel image.

Why It Matters for Embedded Linux

For embedded systems, the kernel must be customized to fit the hardware. Kconfig and menuconfig help you select only the features necessary for your device, optimizing the kernel’s size and performance. Whether you’re enabling support for a specific sensor, disabling unused drivers, or configuring networking features, these tools provide an easy way to tailor the kernel.

Example: Configuring a Driver

Suppose you want to enable SPI support for a custom sensor. In menuconfig, you would navigate to:

Device Drivers --->
    [*] SPI support
    SPI Master Controller Drivers --->
        [*] MyBoard SPI Controller        

This would generate lines like:

CONFIG_SPI=y
CONFIG_MYBOARD_SPI=y        

in the .config file, enabling SPI support and your specific hardware driver.

Tips for Using Menuconfig

• Search for options using / to quickly find what you need.
• View help by pressing ? for descriptions of each option.
• Review changes by comparing old and new .config files or using make listnewconfig to see what’s changed.

Summary

Kconfig and menuconfig work together to provide an easy and flexible way to configure the Linux kernel. Kconfig defines the available options, and menuconfig provides an interactive interface for selecting and enabling features. For embedded systems, this allows you to tailor the kernel to your hardware needs, optimizing performance and saving memory.

Next Steps

• Explore the kernel documentation on Kconfig and menuconfig.
• Practice configuring a kernel using make menuconfig and examine the resulting .config file.
• Dive into real-world examples from embedded Linux projects to see how these tools are used in production.

By mastering Kconfig and menuconfig, you'll be able to efficiently build and customize the Linux kernel for your embedded development projects.