Linux has been developed at a fast pace, with a new version being released every 8 to 12 weeks. The way that the version numbers are constructed has changed a bit in recent years. Before July 2011, there was a three number version scheme with version numbers that looked like 2.6.39. The middle number indicated whether it was a developer or stable release, odd numbers (2.1.x, 2.3.x, 2.5.x) were for developers and even numbers were for end users. From version 2.6 onwards, the idea of a long-lived development branch (the odd numbers) was dropped as it slowed down the rate at which new features were made available to users. The change in numbering from 2.6.39 to 3.0 in July 2011 was purely because Linus felt that the numbers were becoming too large: there was no huge leap in the features or architecture of Linux between those two versions. He also took the opportunity to drop the middle number. Since then, in April 2015, he bumped the major from 3 to 4, again purely for neatness, not because of any large architectural shift.

Linus manages the development kernel tree. You can follow him by cloning his git tree like so:

$ git clone  git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

This will check out into subdirectory linux. You can keep up to date by running the command git pull in that directory from time to time.

Currently, a full cycle of kernel development begins with a merge window of two weeks, during which Linus will accept patches for new features. At the end of the merge window, a stabilization phase begins, during which Linus will produce release candidates with version numbers ending in -rc1, -rc2, and so on, usually up to -rc7 or -rc8. During this time, people test the candidates and submit bug reports and fixes. When all significant bugs have been fixed, the kernel is released.

The code incorporated during the merge window has to be fairly mature already. Usually, it is pulled from the repositories of the many subsystem and architecture maintainers of the kernel. By keeping to a short development cycle, features can be merged when they are ready. If a feature is deemed not sufficiently stable or well developed by the kernel maintainers, it can simply be delayed until the next release.

Keeping a track of what has changed from release to release is not easy. You can read the commit log in Linus' git repository but, with roughly 10,000 or more entries per release, it is not easy to get an overview. Thankfully, there is the Linux Kernel Newbies website, http://kernelnewbies.org where you will find a succinct overview of each version, at http://kernelnewbies.org/LinuxVersions.

The rapid rate of change of Linux is a good thing in that it brings new features into the mainline code base, but it does not fit very well with the longer life cycle of embedded projects. Kernel developers address this in two ways. Firstly, it is acknowledged that a release may contain bugs that need to be fixed before the next kernel release comes around. That is the role of the stable Linux kernel, maintained by Greg Kroah-Hartman. After release, the kernel moves from being mainline (maintained by Linus) to stable (maintained by Greg). Bug fix releases of the stable kernel are marked by a third number, 3.18.1, 3.18.2, and so on. Before version 3, there were four release numbers, 2.6.29.1, 2.6.39.2, and so on.

You can get the stable tree by using the following command:

$ git clone 
git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git

You can use git chckout to get a particular version, for example version 4.1.10:

$ cd linux-stable
$ git checkout v4.1.10

Usually, the stable kernel is maintained only until the next mainline release, 8 to 12 weeks later, so you will see that there is just one or sometimes two stable kernels at kernel.org. To cater for those users who would like updates for a longer period of time and be assured that any bugs will be found and fixed, some kernels are labeled long term and maintained for two or more years. There is at least one long term kernel each year. Looking at kernel.org at the time of writing, there are a total of eight long term kernels: 4.1, 3.18, 3.14, 3.12, 3.10, 3.4, 3.2, and 2.6.32. The latter has been maintained for five years and is at version 2.6.32.68. If you are building a product that you will have to maintain for this length of time the latest long term kernel might well be a good choice.

In an ideal world, you would be able to download a kernel from kernel.org and configure it for any device that claims to support Linux. However, that is not always possible: in fact mainline Linux has solid support for only a small subset of the many devices that can run Linux. You may find support for your board or SoC from independent open source projects, Linaro or the Yocto Project, for example, or from companies providing third party support for embedded Linux, but in many cases you will be obliged to look to the vendor of your SoC or board for a working kernel. As we also know, some are better than others.

The Linux source code is licensed under GPL v2, which means that you must make the source code of your kernel available in one of the ways specified in the license.

The actual text of the license for the kernel is in the file COPYING. It begins with an addendum written by Linus that states that code calling the kernel from user space via the system call interface is not considered a derivative work of the kernel and so is not covered by the license. Hence, there is no problem with proprietary applications running on top of Linux.

However, there is one area of Linux licensing that causes endless confusion and debate: kernel modules. A kernel module is simply a piece of code that is dynamically linked with the kernel at runtime, thereby extending the functionality of the kernel. The GPL makes no distinction between static and dynamic linking, so it would appear that the source for kernel modules is covered by the GPL. But, in the early days of Linux, there were debates about exceptions to this rule, for example, in connection with the Andrew filesystem. This code predates Linux and therefore (it was argued) is not a derivative work, and so the license does not apply. Similar discussions took place over the years with respect to other pieces of code, with the result that it is now accepted practice that the GPL does not necessarily apply to kernel modules. This is codified by the kernel MODULE_LICENSE macro, which may take the value Proprietary to indicate that it is not released under the GPL. If you plan to use the same arguments yourself, you may want to read though an oft-quoted email thread titled Linux GPL and binary module exception clause? (http://yarchive.net/comp/linux/gpl_modules.html).

The GPL should be considered a good thing because it guarantees that when you and I are working on embedded projects, we can always get the source code for the kernel. Without it, embedded Linux would be much harder to use and more fragmented.