User:Flexibeast/guides/A minimal Gentoo kernel for your hardware

The "Configuring the Linux kernel" chapter of the Gentoo Handbook describes a number of ways of configuring and installing a kernel:

manually working through all the kernel configuration options, changing settings as necessary;
using `genkernel` to compile and install a kernel with broad hardware support;
compiling and installing the latest Gentoo-patched kernel ('distribution kernel'); or
directly installing the latest precompiled Gentoo-patched kernel ('distribution kernel').

But what if you just want the minimal kernel and kernel config for a functioning system?

Here are the steps.

root #emerge gentoo-kernel-bin

This will install the latest precompiled Gentoo kernel. Restart the system and boot from that kernel. If all the system's hardware is detected and successfully functioning, proceed to the next step. Otherwise, determine the cause of any issues and resolve them before continuing.

Note that any pluggable devices intended to be used on the system should be plugged in, so that the relevant kernel modules get loaded. The sys-kernel/modprobed-db::guru package in the GURU repository can be used to create a list of modules that were needed over time:

Modprobed-db simply logs every module ever probed on the target system to a text-based database ($XDG_CONFIG_HOME/modprobed-db) which can be read directly by "make localmodconfig"

-- https://github.com/graysky2/modprobed-db

This will install the Gentoo-patched sources for the precompiled kernel. It should also change the /usr/src/linux/ symlink to point to the appropriate sources for that kernel version, e.g. linux-5.16.20-gentoo-dist. Confirm this via the output of eselect:

root #eselect kernel list

If necessary, use eselect to ensure /usr/src/linux/ points to the correct version of the kernel sources.

root #make localmodconfig

The second line will automatically create a modules-based config, compiling only the modules the running system has actually loaded (and/or their dependencies). An alternative make target is localyesconfig, which will compile the necessary hardware support directly into the kernel, rather than into modules.

If the /etc/portage/savedconfig/sys-kernel directory does not already exist, create it, then copy the new config into that directory, with the name gentoo-kernel:

root #mkdir -p /etc/portage/savedconfig/sys-kernel/

root #cp .config /etc/portage/savedconfig/sys-kernel/gentoo-kernel

Be sure to include the leading . in front of config!

The /etc/portage/savedconfig/sys-kernel/gentoo-kernel file will be used when compiling and installing the kernel.

Now add the savedconfig USE flag for sys-kernel/gentoo-kernel in the appropriate place in /etc/portage/. If no USE flags have yet been specified:

root #mkdir /etc/portage/package.use/

root #echo 'sys-kernel/gentoo-kernel savedconfig' > /etc/portage/package.use/kernel

Remove the -bin kernel package:

root #emerge -c gentoo-kernel-bin

Compile and install the kernel:

root #emerge gentoo-kernel

Building a localmodconfig kernel can substantially reduce compile times.

In addition to compiling and installing the kernel, the emerge process will also create an initramfs for the kernel, using dracut.

Finally, assuming GRUB is the system bootloader, and that kernels are stored in /boot/grub/, refresh the GRUB menu to include the new kernel and initramfs:

root #grub-mkconfig -o /boot/grub/grub.cfg

You should now be able to boot with the new kernel.

Using sys-kernel/gentoo-kernel means that kernel updates are handled like any other package, as part of system updates; manually updating the kernel is not required, nor is running dracut after a kernel update. However, users of GRUB (rather than e.g. rEFInd) will still need to run grub-mkconfig after a kernel update. Additionally, this process doesn't provide the user with an interactive session about new kernel configuration options; whether this is an issue depends on your system and use-cases.