Kernel Korner - Exploring Dynamic Kernel Module Support (DKMS)

would add qla2x00/v6.04.00 to the extant /var/dkms tree. This command includes creating the directory /var/dkms/qla2x00/v6.04.00/, creating a symlink from /var/dkms/qla2x00/v6.04.00/source to → /usr/src/qla2x00-v6.04.00/ and copying the dkms.conf file from its original location to /var/dkms/qla2x00/v6.04.00/dkms.conf.

Once this add is complete, the module is ready to be built. The dkms build command requires that the proper kernel sources are located on the system from the /lib/module/kernel-version/build symlink. The make command used to compile the module is specified in the dkms.conf configuration file. Continuing with the qla2x00/v6.04.00 example:

dkms build -m qla2x00 -v v6.04.00 -k 2.4.20-8smp

compiles the module but stops short of installing it. Although build expects a kernel-version parameter, if this kernel name is left out, it assumes the currently running kernel. However, building modules for kernels not currently running also is a viable option. This functionality is assured through the use of a kernel preparation subroutine that runs before any module build is performed. This paranoid kernel preparation involves running a make mrproper, copying the proper kernel .config file to the kernel source directory, running a make oldconfig and, finally, running a make dep. These steps ensure that the module being built is built against the proper kernel symbols. By default, DKMS looks for the kernel .config file in the /lib/modules/kernel-version/build/configs/ directory, utilizing Red Hat's naming structure for those config files. If the kernel .config file is not located in this directory, you must specify a --config option with your build command and tell DKMS where the .config file can be found.

Successful completion of a build creates, for this example, the /var/dkms/qla2x00/v6.04.00/2.4.20-8smp/ directory as well as the log and module subdirectories within this directory. The log directory holds a log file of the module make, and the module directory holds copies of the compiled .o binaries.

With the completion of a build, the module now can be installed on the kernel for which it was built. Installation copies the compiled module binary to the correct location in the /lib/modules/ tree, as specified in the dkms.conf file. If a module by that name is already found in that location, DKMS saves it in its tree as an original module, so it can be put back into place at a later time if the newer module is uninstalled. The example install command:

dkms install -m qla2x00 -v v6.04.00 -k 2.4.20-8smp

creates the symlink /var/dkms/qla2x00/v6.04.00/kernel-2.4.20-8smp → /var/dkms/qla2x00/v6.04.00/2.4.20-8smp. This symlink is how DKMS keeps tabs on which driver version is installed on which kernel. As stated earlier, if a module by the same name is installed already, DKMS saves a copy in its tree in the /var/dkms/module-name/original_module/ directory. In this case, it would be saved to /var/dkms/qla2x00/original_module/2.4.20-8smp/.

To complete the DKMS cycle, you also can uninstall or remove your module from the tree. Uninstall removes the module you installed and, if applicable, replaces it with its original module. In scenarios where multiple versions of a module are located within the DKMS tree, when one version is uninstalled, DKMS does not try to understand or assume which of these other versions should be put in its place. Instead, if a true original_module was saved from the original DKMS installation, it is put back into the kernel. All of the other module versions for that module are left in the built state. An example uninstall would be:

dkms uninstall -m qla2x00 -v v6.04.00 -k 2.4.20-8smp

If the kernel version parameter is unset, the currently running kernel is assumed, but the same behavior does not occur with the remove command. Remove and uninstall are similar in that a remove command completes all of the same steps as does an uninstall. However, if the module-version being removed is the last instance of that module-version for all kernels on your system, after the uninstall portion of the remove completes, remove physically removes all traces of that module from the DKMS tree. In other words, when an uninstall command completes, your modules are left in the “built” state. However, when a remove completes, you have to start over from the add command before you can use this module again with DKMS. Here are two sample remove commands:

dkms remove -m qla2x00 -v v6.04.00 -k 2.4.20-8smp
dkms remove -m qla2x00 -v v6.04.00 --all