Make Magic

Here's a concise breakdown of make modules_prepare, make modules, and their role in the kernel build process:

1. make modules_prepare

What it does: Prepares the kernel source tree for building loadable kernel modules (LKMs). This includes:

• Generating headers needed for module compilation (e.g., include/generated/).
• Creating symbolic links and configuration files (e.g., .config, Module.symvers).
• Setting up the build environment for external modules (out-of-tree modules).

When to use it:

• After configuring the kernel (make menuconfig, etc.) but before building modules.
• If you’re compiling out-of-tree modules (e.g., proprietary drivers) and need the kernel headers/symbols prepped.

Example:

bash

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# Configure the kernel first (e.g., via make menuconfig)
make modules_prepare  # Prepares headers and symbols for modules        

2. make modules

What it does: Compiles in-tree kernel modules (drivers/filesystems/etc. marked as =m in .config).

• Outputs .ko (Kernel Object) files in their respective subdirectories.
• Does not build the kernel image (vmlinux, Image, etc.).

When to use it:

• To rebuild modules after modifying kernel configuration or module code.
• To avoid rebuilding the entire kernel when only modules need updating.

Example:

bash

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make modules  # Compiles modules (*.ko files)        

3. Full Kernel Build (make or make all)

What it does: Builds both:

• The kernel image (e.g., vmlinux, Image.gz, or bzImage).
• All in-tree modules (equivalent to make modules).

When to use it:

• For a fresh kernel build.
• After major configuration changes affecting the kernel image.

Example:

bash

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make  # Builds kernel image + modules        

Why They’re Necessary

1. Efficiency:
2. Out-of-Tree Modules:
3. Separation of Concerns:

Typical Workflow

1. Configure the kernel:
2. Build the kernel image and modules:
3. OR, for incremental module work:

Key Takeaways

• Use make for full kernel builds.
• Use make modules to save time when only modules need rebuilding.
• Use make modules_prepare to set up headers for external modules.

This separation streamlines development, especially when working with drivers or embedded systems! ??