8.4 Kernel Drivers, Subsystems, and Modules

Before building a custom kernel, consider the reasons for doing so. If there is a need for specific hardware support, it may already exist as a module.

Kernel modules exist in the /boot/kernel directory and may be dynamically loaded into the running kernel using kldload(8). Most, if not all kernel drivers have a specific module and manual page. For example, the last section noted the ath wireless Ethernet driver. This device has the following information in its manual page:

Alternatively, to load the driver as a module at boot time, place the
following line in loader.conf(5):


As instructed, adding the if_ath_load="YES" line to the /boot/loader.conf file will enable loading this module dynamically at boot time.

In some cases; however, there is no associated module. This is mostly true for certain subsystems and very important drivers, for instance, the fast file system (FFS) is a required option in the kernel. As is network support (INET). Unfortunately the only way to tell if a driver is required is to check for the module itself.

Warning: It is considerably easy to remove built in support for a device or option and have a broken kernel. For example, if the ata(4) driver is pulled from the kernel configuration file, a system using ATA disk drivers may not boot without the line added to loader.conf. When in doubt, check for the module and then just leave support in the kernel.

This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.

For questions about FreeBSD, read the documentation before contacting <questions@FreeBSD.org>.
For questions about this documentation, e-mail <doc@FreeBSD.org>.