A.1. Yocto Project Kernel Development and Maintenance

Kernels available through the Yocto Project (Yocto Linux kernels), like other kernels, are based off the Linux kernel releases from http://www.kernel.org. At the beginning of a major Linux kernel development cycle, the Yocto Project team chooses a Linux kernel based on factors such as release timing, the anticipated release timing of final upstream kernel.org versions, and Yocto Project feature requirements. Typically, the Linux kernel chosen is in the final stages of development by the Linux community. In other words, the Linux kernel is in the release candidate or "rc" phase and has yet to reach final release. But, by being in the final stages of external development, the team knows that the kernel.org final release will clearly be within the early stages of the Yocto Project development window.

This balance allows the Yocto Project team to deliver the most up-to-date Yocto Linux kernel possible, while still ensuring that the team has a stable official release for the baseline Linux kernel version.

As implied earlier, the ultimate source for Yocto Linux kernels are released kernels from kernel.org. In addition to a foundational kernel from kernel.org, the available Yocto Linux kernels contain a mix of important new mainline developments, non-mainline developments (when no alternative exists), Board Support Package (BSP) developments, and custom features. These additions result in a commercially released Yocto Project Linux kernel that caters to specific embedded designer needs for targeted hardware.

You can find a web interface to the Yocto Linux kernels in the Source Repositories at http://git.yoctoproject.org. If you look at the interface, you will see to the left a grouping of Git repositories titled "Yocto Linux Kernel". Within this group, you will find several Linux Yocto kernels developed and included with Yocto Project releases:

Notes

Long Term Support Initiative (LTSI) for Yocto Linux kernels is as follows:
  • For Yocto Project releases 1.7, 1.8, and 2.0, the LTSI kernel is linux-yocto-3.14.

  • For Yocto Project releases 2.1, 2.2, and 2.3, the LTSI kernel is linux-yocto-4.1.

  • For Yocto Project release 2.4, the LTSI kernel is linux-yocto-4.9

  • linux-yocto-4.4 is an LTS kernel.

Once a Yocto Linux kernel is officially released, the Yocto Project team goes into their next development cycle, or upward revision (uprev) cycle, while still continuing maintenance on the released kernel. It is important to note that the most sustainable and stable way to include feature development upstream is through a kernel uprev process. Back-porting hundreds of individual fixes and minor features from various kernel versions is not sustainable and can easily compromise quality.

During the uprev cycle, the Yocto Project team uses an ongoing analysis of Linux kernel development, BSP support, and release timing to select the best possible kernel.org Linux kernel version on which to base subsequent Yocto Linux kernel development. The team continually monitors Linux community kernel development to look for significant features of interest. The team does consider back-porting large features if they have a significant advantage. User or community demand can also trigger a back-port or creation of new functionality in the Yocto Project baseline kernel during the uprev cycle.

Generally speaking, every new Linux kernel both adds features and introduces new bugs. These consequences are the basic properties of upstream Linux kernel development and are managed by the Yocto Project team's Yocto Linux kernel development strategy. It is the Yocto Project team's policy to not back-port minor features to the released Yocto Linux kernel. They only consider back-porting significant technological jumps ‐ and, that is done after a complete gap analysis. The reason for this policy is that back-porting any small to medium sized change from an evolving Linux kernel can easily create mismatches, incompatibilities and very subtle errors.

The policies described in this section result in both a stable and a cutting edge Yocto Linux kernel that mixes forward ports of existing Linux kernel features and significant and critical new functionality. Forward porting Linux kernel functionality into the Yocto Linux kernels available through the Yocto Project can be thought of as a "micro uprev." The many “micro uprevs” produce a Yocto Linux kernel version with a mix of important new mainline, non-mainline, BSP developments and feature integrations. This Yocto Linux kernel gives insight into new features and allows focused amounts of testing to be done on the kernel, which prevents surprises when selecting the next major uprev. The quality of these cutting edge Yocto Linux kernels is evolving and the kernels are used in leading edge feature and BSP development.