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# Dynamic Kernel Module Support

From ArchWiki

Dynamic Kernel Module Support allows one to compile and install kernel modules without recompiling the entire kernel.

## Installation

Install package dkms in the Official Repositories.

After installing the DKMS, you still need to install a DKMS package such as nvidia-dkms, otherwise having DKMS installed is just wasting your disk space.

## Configuration

To get modules rebuilt automatically after a kernel upgrade, enable the `dkms` service. This service will build DKMS modules if they were not already available and then exit. For systemd:

    # systemctl enable dkms.service

Sometimes, a program depends on the module. If that program is started before the DKMS module is built, you often need to reboot (or restart that program). An example are the proprietary graphics drivers such as nvidia and catalyst.

## Usage

If you have just upgraded your kernel and want to avoid a reboot, you can trigger a rebuild for all modules by executing `dkms autoinstall -k NEW_KERNEL_VERSION` as root. This will build and install DKMS modules for all available kernel versions if they have not been built before.

Forcing a build of a specific module for a specific kernel version is also possible, for example:

    # dkms install -m nvidia -v 304.51 -k 3.6.2-1-ARCH

Hint: use tab completion to get the module and kernel version.

## Guidelines

Here are some guidelines to follow.

### Package name

DKMS packages are named by appending `-dkms` to their non-DKMS counterpart (or the module name if no counterpart can be found). Packages beginning with `dkms-` should be renamed to follow this format.

### Where should source go?

DKMS by default uses /usr/src/, but [namcap] complains that is a non-standard directory. (Even though the FHS reference namcap uses states that /usr/src is "optional". One could place the source in /opt/<package> and then sym-link it (which is what some non-DKMS packages do.)

Sources should go into /usr/src/<package>-<package version> which is the default directory that DKMS commands use. <package> is the "true package name" (which is usually the PKGBUILD $_pkgname variable, which is the $pkgname minus the "dkms-" prefix). <package version> refers to the PACKAGE_VERSION field in dkms.conf. By convention, PKGBUILDs $pkgver should have the same value.

### Where should patches be applied?

One could patch the source either in the PKGBUILD or through DKMS. Since non-DKMS packages patch from the PKGBUILD, and to keep the DKMS PKGBUILD as close to the non-DKMS version, I am patching in the PKGBUILD.

### Should the .install file attempt to modprobe/rmmod the module?

No, it should not. Consider a module that crashes when loaded. That could halt a pacman upgrade or installation which has more severe consequences.

Loading/ removing modules is a task for the user.

### How to create .install / dkms.conf files?

Try to avoid updating things like version numbers in multiple files. Try to avoid cluttering up PKGBUILDs/.install files with DKMS-specific stuff. (This keeps them closer to the non-DKMS files).

I've just started using a simple bash script to create the dkms.conf file and to replace text in an install.template file. This leads to much cleaner and easier to understand files.

You should not run `depmod` in your .install script, this is done automatically by `dkms install`. Running `dkms install` depends on `dkms build` which depends on `dkms add` and are executed automatically. Thus, you only need to put the sources in `/usr/src/MODULE-VERSION` and run `dkms install` in your .install script.

Example for a module put in `/usr/src/MODULE-VERSION` (substitute `MODULE` and `PACKAGE_NAME` accordingly):

    post_install() {
        dkms install -m MODULE -v ${1%%-*}
    }
    pre_upgrade() {
        local curver=${2%%-*}
        # $2 is unset due to a bug. See, https://bugs.archlinux.org/task/32278
        # Query current version using pacman as fallback
        [ -n "$curver" ] || curver=$(pacman -Q PACKAGE_NAME | cut -d' ' -f2)
        pre_remove $curver
    }
    post_upgrade() {
        post_install ${1%%-*}
    }
    pre_remove() {
        dkms remove -m MODULE -v ${1%%-*} --all
    }

### DKMS breaks the ABS

Sort of, yes. The problem is that the resulting modules don't belong to the package, so pacman can't track or report on them.

I don't think that's a show-stopper, but it probably puts DKMS out on the fringe a bit. There is some talk about pacman adding similar support through pacman hooks. [1]. I think you could "fake" the ownership, by installing the module as normal (or even a "dummy" file) and just letting DKMS overwrite it.

### namcap issues

As mentioned earlier, namcap complains if you put source in /usr/src. Also, namcap can not detect that dkms is a dependency. (I guess maybe it's not technically a dependency, but...)

Basically, take the dependencies of the non-DKMS version, add 'dkms' and remove 'linux-headers' (since dkms optdepends on that)

## See Also

- Dell's DKMS man page Official site.
- Ubuntu's DKMS man page (Documents some officially un-documented bits.)
- Exploring Dynamic Kernel Module Support

Retrieved from "https://wiki.archlinux.org/index.php?title=Dynamic_Kernel_Module_Support&oldid=268348"
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