Compiling a Custom 'Pristine' Kernel in Debian Linux

Taylor Talkington : ~
Tags:
linux debian kernel

I've really been enjoying Linux since coming back to it late last year. To my surprise, almost everything worked or had prebuilt packages: all of my Razer RGB peripherals, NVidia GPU, sound card etc.

The only thing I hadn't gotten working yet was the ASUS Aura RGB headers on my motherboard. Those could be controlled via OpenRGB, but my particular setup required running a patched kernel. I put this aside for a few months because I figured a custom kernel wouldn't be worth it for something I might use once or twice a year, and I was worried that I'd also have to manually install any dkms built modules I'm using as well, notably my NVidia drivers and OpenRazer.

When I finally did get around to it, I found out that, like many Linux tasks, this is much easier than it used to be, and even dkms packages play nicely with a custom kernel when it's built properly.

Kernerl Sources

One thing that hasn't changed is where to get the sources: kernel.org. Download them, unpack them somewhere convenient and open a new terminal there.

Applying Patches

Patches can be applied directly to the kenerl source. In my case:

$ patch -p1 < ~/src/OpenRGB/OpenRGB.patch

Configuration

The current configuration can be used as a base. This will copy the configuration for the currently running Kernel:

$ cp /boot/config-$(uname -r) .config

Then make a configuration based on that. Any missing/new values will be prompted. This is the time to select the proper values for anything that might have been added with the patches:

$ make oldconfig

Building

This part may take quite a while, but it's easy. Once it's complete there will be several debs in the folder above:

$ make deb-pkg -j6

Installing

Installation is just a matter of installing the generated deb packages. Any existing dkms modules will build themselves as needed, GRUB will automatically be updated, etc.

$ sudo dpkg -i ../*.deb

After that just reboot to use the new kernel!

Uninstalling old kernels

At this point you likely have 3 or more kernels installed, especially if you do this a few times to update to new kernels. The old versions can be uninstalled.

First, see what versions are installed:

$ dpkg --list | grep linux-

Then uninstall the packages for the old kernels:

$ sudo apt purge linux-headers-5.15.0-2-amd64 linux-headers-5.15.0-2-common linux-image-5.15.0-2-amd64

It's a good idea to have the last known good version of the kernel around and probably still the last official Debian built kernel too, so don't uninstall all of them.

Automation

I recently made a script that does everything but installing the deb packages above. It simple and doesn't do any error checking beyond checking to see if the latest version is new:

#!/bin/bash

# this script requires jq: sudo apt install jq

BASEDIR=`dirname "${BASH_SOURCE[0]}"`

echo "Finding latest release version..."

REL_INFO=`curl -s 'https://kernel.org/releases.json' | jq '.releases[] | select(.moniker=="stable")'`

REL_VERSION=`echo "$REL_INFO" | jq -r .version`
RUN_VERSION=`uname -r`

echo "Latest release is $REL_VERSION"
echo "Currently running $RUN_VERSION"

if [ "$REL_VERSION" = "$RUN_VERSION" ]; then
    echo "Already running latest release"
    exit 0
fi

REL_DIR=$BASEDIR/$REL_VERSION
REL_SRC=`echo "$REL_INFO" | jq -r .source`

echo "Building new kernel packages in $REL_DIR"

mkdir -p $REL_DIR
pushd $REL_DIR > /dev/null

echo "Downloading kernel sources: $REL_SRC..."
wget -q $REL_SRC

echo "Extracting sources..."
tar lxf linux-$REL_VERSION.tar.xz

pushd linux-$REL_VERSION > /dev/null

echo "Applying OpenRGB Patch..."
patch -p1 < ~/src/OpenRGB/OpenRGB.patch

echo "Copying currently running kernel configuration..."
cp /boot/config-$(uname -r) .config
make oldconfig

echo "Building..."
make deb-pkg -j6

popd > /dev/null

popd > /dev/null

echo "done!"