May 042011

With the soon to be legendary “roadmap” Thomas Gleixner presented at the 2011 Embedded Linux Conference for the PREEMPT_RT patch series, people are likely wondering what that means for projects like The Yocto Project which make the PREEMPT_RT patch series available as part of a larger integration project.

It is our goal to maintain the linux-yocto recipes and git repositories as closely aligned with the mainline kernel as possible. This primary goal sets the schedule for which kernel version will be used as the basis for the next major release. 0.9 released with 2.6.34, 1.0 with 2.6.37, and 2.6.39 or 40 are likely to follow. These won’t necessarily align with the supported PREEMPT_RT kernel versions.

We have a few options. First, we could port PREEMPT_RT to whatever the next kernel version turns out to be. Second, we could release a separate kernel tree just for RT. Third, we could just not include PREEMPT_RT when the versions do not align. There is precedent for the first option, and indeed this exactly how we provide a 2.6.34 PREEMPT_RT tree with 0.9 and 1.0. We took the third option for 1.0 and the 2.6.37 tree, preferring to stick with the 2.6.34 version for RT kernels rather than do yet another forward port to 2.6.37, duplicating much of Thomas’ work toward a 2.6.38 PREEMPT_RT. This is time consuming, error prone, and doesn’t contribute to the overall quality of the PREEMPT_RT patch series as it is somewhat apart from that which Thomas releases. The second option, a separate kernel tree, is contrary to another key goal of The Yocto Project, which is to reduce duplication of effort. A separate kernel tree would require duplication of the meta branch, and all the bug fixes, security fixes, and feature sets would have to be applied to each tree.

Going forward, our approach to PREEMPT_RT will be as follows. We will strive to align with the official PREEMPT_RT releases. When that is not possible, we will favor skipping RT support in a kernel version or two just as Thomas does with PREEMPT_RT. In very rare occasions, such as with the 2.6.34 kernel, we may opt to forward port PREEMPT_RT to align with the current linux-yocto recipe’s base kernel version.

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