docs(explanation): add Ubuntu kernel patch life cycle page

docs/explanation/ubuntu-kernel-patch-life-cycle.md

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+---
+myst:
+  html_meta:
+    description: "Understand how a kernel patch moves from a bug report through upstream review and into the Ubuntu kernel."
+---
+
+# Ubuntu kernel patch life cycle
+
+A patch does not move straight from a developer's tree into the Ubuntu kernel.
+It follows a path designed to keep Ubuntu aligned with upstream, maintainable
+over the long term, and safe for the wide range of hardware that Ubuntu
+supports.
+
+## Typical kernel patch life cycle
+
+The typical life cycle of a patch looks like this:
+
+1. A bug or missing feature is identified.
+2. A patch is developed and thoroughly tested.
+3. A Launchpad bug is filed to track the issue.
+4. The patch is submitted to the upstream Linux kernel.
+5. Once accepted upstream, the patch flows down into the Ubuntu kernel.
+6. If a patch needs to land in Ubuntu before the next upstream sync, it can be
+   cherry-picked or backported through the {doc}`Ubuntu stable release update
+   process </explanation/stable-release-updates>`.
+
+This flow reflects a core principle of Ubuntu kernel development: patches that
+benefit the whole kernel community should be accepted upstream first. Ubuntu
+then consumes those changes through its regular rebasing and update process.
+
+## Why upstream first
+
+The Ubuntu kernel is not a separate fork. It is an Ubuntu packaging and
+configuration layer on top of the upstream Linux kernel, plus a small set of
+Ubuntu-specific changes. Because of this, the easiest and safest way for a
+patch to get into Ubuntu is for it to be accepted into the upstream kernel
+first.
+
+Submitting upstream has several advantages:
+
+- The patch receives review from the maintainers who know the affected
+  subsystem best.
+- Wider testing across different hardware and configurations reduces the risk
+  of regressions.
+- Once the patch is in upstream, it automatically reaches Ubuntu through the
+  normal update cycle.
+
+Patches that are not accepted upstream are harder for the Ubuntu kernel team
+to carry. They must be maintained across rebases and may conflict with future
+upstream changes.
+
+## Tracking work in Launchpad
+
+Every change that lands in an Ubuntu stable kernel is tied to a Launchpad bug.
+The bug serves as the record of the problem, the proposed fix, and the
+justification for the update.
+
+Before a patch is sent anywhere, the bug description should explain:
+
+- The observable problem or the missing capability.
+- The impact on users.
+- How the patch resolves the issue.
+- How the fix was tested.
+
+The patch can be attached to the bug so reviewers can inspect it, and the bug
+number is later referenced in the patch itself through the `BugLink:` line.
+
+## From upstream into Ubuntu
+
+Once a patch is accepted into the upstream Linux kernel or the stable kernel
+tree, it will eventually be picked up by Ubuntu through the team's regular
+rebase process. The timing depends on which Ubuntu series the patch is needed
+for and where it sits in the upstream release cycle.
+
+For patches that need to reach Ubuntu more quickly, the Ubuntu kernel team can
+cherry-pick or backport the upstream commit into the relevant Ubuntu kernel
+series. This is done through the stable release update process and requires the
+patch to meet SRU criteria.
+
+## When a patch is not accepted
+
+A patch may be rejected upstream for many reasons: it may need more testing,
+require a different approach, or not fit the upstream direction. When this
+happens, the path into Ubuntu becomes much harder.
+
+The Ubuntu kernel team generally does not carry patches that have been
+rejected upstream unless there is a strong Ubuntu-specific reason to do so.
+In most cases, the correct response is to address the upstream feedback and
+resubmit.
+
+## Related topics
+
+- {doc}`/how-to/source-code/send-patches` explains how to send patches to
+  the Ubuntu kernel team mailing list.
+- {doc}`/reference/patch-acceptance-criteria` describes the criteria for
+  accepting patches into the Ubuntu kernel.
+- {doc}`/reference/stable-patch-format` covers the required patch format.
+- {doc}`/explanation/stable-release-updates` explains how stable release
+  updates work.