docs(exp/about-sru): migrate from kteam-docs

docs/explanation/index.md

@@ -0,0 +1,12 @@
+# Explanation
+
+The explanatory guides in this section talk about different aspects of the
+kernel and kernel development process at Canonical.
+
+```{toctree}
+:titlesonly:
+:maxdepth: 1
+:glob:
+
+*
+```

docs/explanation/stable-release-updates.md

@@ -0,0 +1,131 @@
+# About kernel stable release updates (SRU)
+
+Every supported kernel for an Ubuntu release is part of a Stable Release Updates
+(SRU) cycle. The Ubuntu Kernel {term}`SRU` is a structured procedure to ensure
+that kernel updates in Ubuntu's stable releases are both reliable and
+non-disruptive to users.
+
+This document aims to provide an overview about the various aspects of the
+Ubuntu kernel SRU process.
+
+## SRU purpose
+
+Kernel SRU focuses on delivering necessary updates without changing core
+functionalities with low potential of introducing regressions in stable Ubuntu
+releases. This typically covers:
+
+- Upstream stable updates
+- Bug fixes that address relevant issues or improve system stability
+- Common Vulnerabilities and Exposures (CVE) security updates
+- Hardware enablement (HWE) patches
+
+## SRU cycle cadence
+
+Since August 2023, the Ubuntu Kernel team has adopted a 4/2 Kernel SRU cycle to
+improve predictability and responsiveness. It involves a 4-week ("4/") stable
+update cycle for regular fixes and features, combined with an additional
+mid-cycle 2-week ("/2") update focused on urgent CVE security patches and
+critical fixes. This approach enables more timely updates for critical issues
+while maintaining stability, and continues to support mid-cycle
+{ref}`respins <exp-sru-kernel-respins>` for regression fixes as needed.
+
+```{figure} /_images/exp-kernel-sru-lifecycle-stages.svg
+:alt: Ubuntu kernel lifecycle stages and ideal SRU 4/2 cadence timeline
+
+Ubuntu kernel lifecycle stages and ideal SRU 4/2 cadence timeline
+```
+
+See the [Ubuntu Kernel Team] home page for details on SRU cycle dates.
+
+```{important}
+While the Ubuntu kernel team strives to meet the SRU cycle 4/2 cadence, please
+note that SRU cycle dates are tentative. As such, they cannot be guaranteed and
+may be subject to change.
+```
+
+## SRU patch submission and review process
+
+All updates that are applied to stable kernels go through the following patch
+submission and review process. More details can be found at [Ubuntu Wiki -
+Stable Patch Format].
+
+### Patch creation
+
+The first step for every SRU is to create a patch containing all the necessary
+information, including a link to the associated public Launchpad bug report that
+contains the SRU justification.
+The only exception to this are CVE fixes, where only the CVE number is required.
+
+See the [Ubuntu Wiki - Kernel Updates] for more information on the SRU
+requirements and justification.
+
+### Patch submission
+
+Next, contributors send the stable patches to the Ubuntu Kernel Team mailing
+list ([email protected]) for review.
+Where appropriate, the patch should also be submitted to upstream stable in
+parallel.
+
+### Mailing list review
+
+Stable patch sets on the mailing list (ML) are then carefully reviewed by the
+Ubuntu Kernel Team.
+This review process involves validating that the patch fixes the intended
+issues, ensuring no regressions are introduced to the kernel, evaluating the
+risk and relevance of including the patch into a stable release, and
+reconciling mainline and Ubuntu-specific changes.
+
+### Patch acceptance
+
+Once a mailing list patch has been vetted and has at least two `ACK`s from
+senior members of the Ubuntu Kernel Team, the commit will then be applied to the
+associated stable Ubuntu kernel tree.
+The patch will then be considered for release in an upcoming SRU cycle if all
+the patch acceptance criteria are met.
+
+See the {doc}`Ubuntu patch acceptance criteria </reference/patch_acceptance_criteria>`
+for more information.
+
+(exp-sru-kernel-respins)=
+## SRU kernel respins
+
+A respin is a rebuild of a kernel package replacing a previous build. During
+each SRU cycle, kernel respins may need to happen for several reasons.
+
+- A regression was introduced in a previous cycle or in the current cycle.
+- Additional fixes need to be added.
+- An important update needs to be added mid-cycle which cannot wait until the
+next cycle.
+
+## Kernel streams
+
+Kernels that are ready for the full suite of testing and verification are
+promoted to the "testing" phase, where the built kernel binaries (and artefacts)
+are copied to a proposed location.
+
+As the [Ubuntu archive has a single proposed pocket], the support for multiple
+kernel streams was implemented in the kernel SRU workflow.
+These streams consist of a set of locations (Ubuntu archive pockets or PPAs)
+that can be used for parallel (and generally independent) preparation and
+testing of kernels.
+
+For example, when a respin is required for a regression released in the previous
+cycle it can be prepared while the kernel spin for the current SRU cycle is
+still in progress. These streams are also what enables the 4/2 Kernel SRU cycle
+model.
+
+## Related topics
+
+- [Discourse - Ubuntu Kernel 4/2 SRU Cycle Announcement]
+- [Ubuntu Wiki - Stable Kernel Release Cadence]
+- [Kernel team stable dashboard]
+
+% LINKS
+
+[Ubuntu Kernel Team]: https://kernel.ubuntu.com/
+[Ubuntu Wiki - Stable Patch Format]: https://wiki.ubuntu.com/Kernel/Dev/StablePatchFormat
+[Ubuntu Wiki - Kernel Updates]: https://wiki.ubuntu.com/KernelTeam/KernelUpdates
+[Ubuntu archive has a single proposed pocket]: https://canonical-ubuntu-packaging-guide.readthedocs-hosted.com/en/latest/explanation/archive/#archivepockets
+[Discourse - Ubuntu Kernel 4/2 SRU Cycle Announcement]: https://discourse.ubuntu.com/t/ubuntu-kernel-4-2-sru-cycle-announcement/37478
+[Ubuntu Wiki - Stable Kernel Release Cadence]: https://wiki.ubuntu.com/Kernel/StableReleaseCadence
+[Kernel team stable dashboard]: https://kernel.ubuntu.com/reports/kernel-stable-board/

docs/index.md

@@ -46,9 +46,10 @@ new users
 **Technical information** - specifications, APIs, architecture
 ```
 
-```{grid-item-card} [Explanation](/explanation/ubuntu-linux-kernel-sources.md)
+```{grid-item-card} [Explanation](/explanation/index)
 
-**Discussion and clarification** about Ubuntu Linux kernel source repositories.
+**Discussion and clarification** about different aspects of the Ubuntu kernel
+and kernel development process at Canonical.
 ```
 
 ````
@@ -72,7 +73,7 @@ fixes and constructive feedback.
 :hidden:
 :maxdepth: 2
 
-how-to/index
-reference/index
-Explanation: About Ubuntu Linux kernel sources <explanation/ubuntu-linux-kernel-sources>
+/how-to/index
+/reference/index
+/explanation/index
 ```