Ubuntu kernel variants and branches

Ubuntu maintains several kernel variants to balance the needs of stability, hardware enablement, and rapid development. This document outlines the primary kernel trees, their roles in the development lifecycle, and the git branching strategies used by the Canonical Kernel Team.

See also

Ubuntu development kernels

Ubuntu releases every 6 months; therefore, the Canonical Kernel Team constantly works on the active Ubuntu development series to integrate the latest Linux kernel features for the release. The development kernels serve as the testing ground for new features, upstream updates, and partner integration before they reach the stable releases.

The following table summarizes the main Ubuntu development kernels:

Kernel tree

Purpose

Stability

Upstream tracking

Notes

linux-unstable

Early integration and testing

Highly volatile

Tracks upstream RCs (frequent rebases)

Lacks full Ubuntu integration (AppArmor, NVIDIA, ZFS, etc.)

Ubuntu Kernel Next (UKN)

Stable snapshot for integration work

Moderately stable

Snapshot of linux-unstable

Read-only; used for partner / integration testing

linux

Ubuntu kernel

Relatively stable

Tracks upstrean stable releases

Becomes the GA kernel for the Ubuntu release

The linux-unstable tree

The linux-unstable tree represents the bleeding edge of Ubuntu kernel development. It is a fast-moving target designed to track the latest upstream Linux developments.

  • Upstream alignment: It is usually rebased weekly against the latest upstream Release Candidate (-rcX) during the most active development period. New patches are also constantly applied to the upstream branch.

  • Volatility: Patchsets can be removed if problems are found.

  • Purpose: It serves as the initial landing ground for upstream code and feature integration. It may still lack support for core components (e.g., AppArmor, NVIDIA drivers, ZFS, etc.) and is not guaranteed to have all features necessary to fully support an Ubuntu system.

  • Availability: As it usually does not have all core components necessary to run a full Ubuntu user-space, it is available only via development PPAs and not in the Ubuntu archive.

Ubuntu Kernel Next (UKN)

Because the linux-unstable kernel moves too fast for larger integration work, the Canonical Kernel Team maintains the Ubuntu Kernel Next tree. UKN integration tree designed to bridge the gap between the volatility of linux-unstable and the stability required for partner development.

  • Workflow: It is a periodic, read-only snapshot of linux-unstable.

  • Integration: It provides a stable code base for integration work into the next development kernel. Integration issues (e.g., conflicts) can be spotted and fixed earlier.

The linux tree

Once a linux-unstable kernel version includes support for all core components and is deemed relatively stable, it is moved to the linux tree.

  • Upstream alignment: Similar to the linux-unstable tree, it usually follows the latest upstream release. However, rebases are less frequent as some level of stability needs to be maintained. During a development cycle, it is expected to be updated to all major.minor upstream releases until the last version available before the Ubuntu release GA.

  • Purpose: The linux kernel is the Ubuntu generic kernel which will be available as the default choice for most installations, and will be the base for all derivatives and custom kernels based on the same upstream major.minor version.

Optimized kernels

In addition to the generic kernel, Canonical also provides optimized kernels which are derived from the generic kernel.

See also

  • Purpose: Kernels that have their configuration, hardware support, or additional features optimized for a variety of hardware platforms and workloads. Examples include kernels optimized for the Raspberry Pi ARM board (linux-raspi), for running as guests on the major public cloud providers, for IoT devices, etc.

  • Release and update cadence: Optimized kernels are released and receive security updates with the same cadence as the Ubuntu generic kernel upon which they are based.

OEM kernels

OEM kernels are optimized derivatives designed specifically for Original Equipment Manufacturer (OEM) projects to support hardware pre-installed with Ubuntu.

See also

  • Purpose: They address unique timelines and hardware needs that may not align with the generic kernel release cadence and scope.

  • Staging strategy: OEM kernels often serve as a staging area. Modifications made here are intended to be merged into the generic Ubuntu kernel in subsequent releases.

Production and enablement kernels

Once an Ubuntu version is released to General Availability (GA), all kernels available in that release become production-ready and stable. They are maintained for the duration of the Ubuntu release support period and receive security updates and bug fixes via Stable Release Updates (SRU).

HWE (Hardware Enablement) kernels

Hardware Enablement (HWE) kernels are available for Ubuntu Long Term Support (LTS) releases. They provide a way for LTS users to consume newer kernel versions and receive support for newer hardware (such as the graphics stack).

See also

  • Rolling release: HWE kernels are “rolling” in nature; they are typically backported from subsequent interim or LTS Ubuntu releases to the previous LTS.

  • Lifecycle: HWE kernels are supported until the next HWE stack is released, at which point users are encouraged to upgrade to the newer version.

Git branching strategy

When working with Ubuntu kernel repositories, you will primarily interact with two key branches.

master-next/main-next branches

The master-next or main-next branches serve as the staging area for the next Stable Release Update (SRU).

  • Purpose: Commits intended for the next update are applied here first after being reviewed and acknowledged by the Canonical Kernel Team. This branch will also contain commits for the kernel builds that are published for testing in the -proposed pocket of the Ubuntu archive.

  • Flow: Patches land in master-next for integration and testing before being promoted to master after the kernel SRU update is published to -updates.

master/main branches

The master or main branches represent the current state of the kernel source as it exists in the -updates pocket of the Ubuntu archive. It contains the linear history of all the stable releases published for that kernel.