Building and installing modules

Modules can be built separately using the modules target. You can install them using the modules_install target. Modules are built in the same directory as their corresponding source. Thus, the resulting kernel objects are spread over the kernel source tree:

• For a native build and installation, you can use the following commands:

  make modules
  sudo make modules_install

The resulting modules will be installed in /lib/modules/$(uname -r)/kernel/, in the same directory structure as their corresponding source. A custom install path can be specified using the INSTALL_MOD_PATH environment variable.

• When you're cross-compiling for embedded systems, as with all make commands, ARCH and CROSS_COMPILE must be specified. As it is not possible to install a directory in the target device filesystem, embedded Linux build systems (such as Yocto or Buildroot) set INSTALL_MOD_PATH to a path that corresponds to the target root filesystem so that the final root filesystem image contains the modules that have been built; otherwise, the modules will be installed on the host. The following is an example of a 32-bit ARM architecture:

  ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make modules
  ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- INSTALL_MOD_PATH=<dir> make modules_install

In addition to the kernel directory that is shipped with modules, the following files are installed in /lib/modules/<version> as well:

• modules.builtin: This lists all the kernel objects (.ko) that are built into the kernel. It is used by the module loading utility (modprobe, for example) so that it does not fail when it's trying to load something that's already built in. modules.builtin.bin is its binary counterpart.
• modules.alias: This contains the aliases for module loading utilities, which are used to match drivers and devices. This concept of module aliases will be explained in Chapter 6, Introduction to Devices, Drivers, and Platform Abstraction. modules.alias.bin is its binary equivalent.
• modules.dep: This lists modules, along with their dependencies. modules.dep.bin is its binary counterpart.
• modules.symbols: This tells us which module a given symbol belongs to. They are in the form of alias symbol:<symbol> <modulename>. An example is alias symbol:v4l2_async_notifier_register videodev. modules.symbols.bin is the binary counterpart of this file.

With that, we have installed the necessary modules. We've finished learning how to build and install Linux kernels and modules. We've also finished learning how to configure the Linux kernel and add the features we need.