What's the minimal compatible Linux kernel version required to run Ubuntu?

QUESTION: What is the policy on compatibility with earlier kernel versions? E.g. no bug reports accepted, must work with all kernel versions back to and including previous LTS, etc.

I'm a member of the Ubuntu BugControl team and I can say that only bugs in non-obsolete Ubuntu packages are considered. If you install your own kernel or if you use a package from a different distribution and report a bug, your bug will be invalidated. See these two stock responses:

• About an obsolete version of the software for reporter's release
• Packages not provided by Ubuntu

Also, the Ubuntu Kernel team has a FAQ that you might find interesting:

The Kernel Team provides support (security updates etc) for the Ubuntu kernels on all currently active releases, we do not support any non-Ubuntu kernels. A full list of the currently active releases can be found on the Releases page. For Long Term Support (LTS) releases the desktop kernels drop from support before the server kernels, this is reflected in the Releases page.

However this just says which kernels are supported, not which ones are considered compatible.

QUESTION: Example case, practically: How likely will I be in trouble when running Lucid's kernel on Precise?

This is a pretty difficult question to answer. Especially because it really depends on what applications/modules you will be using. We can restrict this question to the "standard" Ubuntu Desktop or Server, but even then it would be too difficult to answer: there is not enough documentation and the information available are sparse.

For example, to check whether udev from Quantal is compatible with the Lucid kernel you would have to see the M, N, O, P, Q release notes, kernel changelogs and udev changelogs. And then proceed to an another package, e.g. libc, upstart and so on. All these packages depend on specific kernel versions and all these packages are not controlled directly by Ubuntu (in the sense that it's not the Ubuntu Team that decides the compatibility policies of that packages).

QUESTION: To what extent is software relatively close to the kernel (udev, gvfs, mdadm etc.) being tested on other than the version provided with the release?

The Ubuntu Testing team and the Ubuntu Quality team do not test kernels not provided by Ubuntu. The proof is that there are no test cases nor test activities for obsolete kernels.

QUESTION: How does Desktop/Server edition differ in this?

They do not differ in any way. This is partially proven by the fact that both Desktop and Server edition use the same kernel.

QUESTION: Is Ubuntu simply not bothering about these cases or am I missing a resource on this?

Ubuntu is not bothering about these cases. Not supporting a kernel version, but being compatible with it would be just extra work with few benefits.

Whether one may like it or not, one of the Ubuntu practices is to look forward and try to support the most recent technologies, rather than the most outdated. You can find an example of this when the Ubuntu CD has been dropped in favor of the DVD, or when Unity 2d has been removed from Quantal.

Also, and this is the most important point in my opinion, Ubuntu is not interested in distributing software which works, but software which works and is supported. There are important differences between these two terms.

The only officially supported kernel is the one shipped with that Ubuntu release. If you have issues due to using a different kernel, you will be on your own. If an issue is suspected to be related to using a non standard kernel, you will be asked to at least test the standard one to see if it actually is related.

There are too many potential issues that could result from using an older kernel to have any sort of accurate list of what works and what doesn't; you will just have to try for yourself.

Making an educated guess, I don't think older kernels are even considered for any Ubuntu release. ...and why would they be? The 'required kernel' is simply the one a release ships with.

Why would one want to use an older kernel on a new release, rather then the older release itself?

AFAIK, the kernel team looks forward rather then backward. They backport newer kernels from newer releases, for example, Quantal kernels get backported to Precise, but not the other way around.