The Makefiles

Overview

The build system defines a set of conventions for the correct use of Makefiles in the kernel source directories. The correct use of Makefiles is driven by the concept of recursion.

In the recursion model, each Makefile within a directory includes the source code and any subdirectories to the build process. Each subdirectory follows the same principle. Developers can focus exclusively in their own work. They integrate their module with the build system by adding a very simple Makefile following the recursive model.

Makefile Conventions

The following conventions restrict how to add modules and Makefiles to the build system. These conventions guard the correct implementation of the recursive model.

• Each source code directory must contain a single Makefile. Directories without a Makefile are not considered source code directories.
• The scope of every Makefile is restricted to the contents of that directory. A Makefile can only make a direct reference to its own files and subdirectories. Any file outside the directory has to be referenced in its home directory Makefile.
• Makefiles list the object files that are included in the link process. The build system finds the source file that generates the object file by matching the file name.
• Parent directories add their child directories into the recursion model.
• The root Makefile adds the directories in the kernel base directory into the recursion model.

Adding Source Files

A source file is added to the build system through its home directory Makefile. The Makefile must refer the source build indirectly, specifying the object file that results from the source file using the obj-y variable. For example, if the file that we want to add is a C file named <file>.c the following line should be added in the Makefile:

obj-y += <file>.o

Source files can be added conditionally using configuration options. For example, if the option CONFIG_VAR is set and it implies that a source file must be added in the compilation process, then the following line adds the source code conditionally:

obj-$(CONFIG_VAR) += <file>.o

Adding Directories

Add a subdirectory to the build system by editing the Makefile in its directory. The subdirectory is added using the obj-y variable. The correct syntax to add a subdirectory into the recursion is:

obj-y += <directory_name>/

The backslash at the end of the directory name signals the build system that a directory, and not a file, is being added to the recursion.

The conventions require us to add only one directory per line and never to mix source code with directory recursion in a single obj-y line. This helps keep the readability of the Makefile by making it clear when an item adds an additional lever of recursion.

Directories can also be conditionally added:

oby-$(CONFIG_VAR) += <directory_name>/

The subdirectory must contain its own Makefile following the rules described in Makefile Conventions.