Cross Compilation of Linux Kernel Modules: A Comprehensive Guide

Linux kernel modules are a fundamental aspect of the Linux operating system, providing a mechanism to add additional functionality to the kernel. With the advent of embedded systems, cross-compilation of these modules has become increasingly important. Cross-compilation refers to compiling code on one platform for execution on another platform. In this article, we will explain what cross-compilation is, why it’s important, and how to cross-compile a Linux kernel module.

What is Cross-Compilation?

Cross-compilation refers to the process of compiling code on one platform for execution on another platform. This is different from traditional compilation, where code is compiled for the same platform on which it will be executed. Cross-compilation is often used in embedded systems where the development platform may not be the same as the target platform. For example, a developer may use a powerful desktop computer to develop code for an embedded system with limited resources.

Why Cross-Compile Linux Kernel Modules?

Cross-compilation of Linux kernel modules is important in embedded systems because it enables developers to test the modules on their development platform before deploying them on the target platform. This can save time and resources, as it eliminates the need to continually test the modules on the target platform. In addition, cross-compilation can also be used to compile modules for multiple target platforms, making it easier to deploy code on a wide range of devices.

How to Cross-Compile a Linux Kernel Module

To cross-compile a Linux kernel module, you will need the following:

1. A development platform with the Linux kernel source code and the GCC compiler installed.
2. A cross-compiler toolchain for the target platform.
3. The Linux kernel headers and libraries for the target platform.

Step 1: Obtain the Linux kernel source code and GCC compiler

The Linux kernel source code can be obtained from the official Linux kernel website (https://www.kernel.org). The GCC compiler can be obtained from the GNU Compiler Collection website (https://gcc.gnu.org).

Step 2: Obtain the Cross-Compiler Toolchain

The cross-compiler toolchain can be obtained from a variety of sources, including the manufacturer of the target platform or from a trusted third-party provider. The toolchain should be specific to the target platform and architecture.

Step 3: Obtain the Linux Kernel Headers and Libraries for the Target Platform

The Linux kernel headers and libraries for the target platform can be obtained from the manufacturer of the target platform or from a trusted third-party provider.

Step 4: Compile the Linux Kernel Module

Cross compilation of Linux kernel modules can be a complex process, but it is an important tool for developers who need to build kernel modules for platforms that are different from their development environment.

There are two fundamental variables that the kernel uses to select the target hardware architecture. One is ARCH & other is CROSS_COMPILE. The default values for both the variables are found in the top-level make file. The values of both may be overridden on the C command line.

To compile the Linux kernel module, use the following commands:

$ export ARCH=<architecture>
$ export CROSS_COMPILE=<cross-compiler-prefix>
$ make -C <path-to-linux-kernel-source> M=<path-to-module-source> modules

Where <architecture> is the target architecture (e.g. arm, mips, etc.), <cross-compiler-prefix> is the prefix for the cross-compiler toolchain (e.g. arm-linux-gnueabi-), and <path-to-linux-kernel-source> and <path-to-module-source> are the paths to the Linux kernel source and the module source, respectively.

Step 5: Deploy the Linux Kernel Module on the Target Platform

Once the Linux kernel module has been compiled, it can be deployed on the target platform by copying the module to the appropriate location and loading it into the kernel. The exact method for deploying the module will vary depending on the target platform, but it typically involves the following steps:

1. Copy the compiled module to the target platform.
2. Connect to the target platform and load the module into the kernel using the insmod or modprobe command.
3. Verify that the module has been loaded into the kernel by using the lsmod command.
4. Test the module to ensure that it is working as expected.

It is important to note that the Linux kernel version on the target platform must match the version of the Linux kernel source code used to compile the module. If the kernel version does not match, the module may not load or may produce unexpected results.

Notes To Cross Compile any Source Code for any Target

To build a kernel module for a target platform using a cross compiler, the developer will typically follow these steps:

1. Set up the Cross Compiler: This involves installing the cross compiler on the development machine and configuring it to generate code for the target platform.
2. Configure the Kernel: The developer will need to configure the kernel source code to build a kernel module for the target platform. This typically involves specifying the target platform’s architecture and other relevant details.
3. Build the Kernel Module: Once the kernel is configured, the developer can use the cross compiler to build the kernel module. This involves running the appropriate make command, specifying the cross compiler and any necessary build options.
4. Test and Debug the Kernel Module: After the kernel module is built, the developer will typically test and debug it to ensure that it is working correctly on the target platform.