Sony is one manufacturer that takes the “open source” part of Android more seriously than others. They contribute a lot of code to AOSP and they have the Xperia Open Devices program that makes it easy for developers to tinker. Sony releases device trees, kernel trees, binaries, and build instruction to compile AOSP for all the devices in the program. You’ll find most of Sony’s recent devices are in the program.
Last summer, the entire Sony X family was close to having a fully working Linux 4.4 kernel. It was available for the Xperia X Performance, Xperia XZ, Xperia XZ Premium, and Xperia XZs. A few other devices needed more work. Now, more devices have been added to the list. The Xperia X, Xperia X Compact, Xperia XZ1, Xperia XZ1 Compact, and Xperia XA2 are now available for building on the latest Linux 4.4 kernel.
Sony’s Community Manager in Developer Relation, who also happens to be XDA Senior Recognized Developer jerpelea, posted updates in all the aforementioned device forums. You’ll find links to resources for building AOSP for your unlocked Xperia device, unified Linux 4.4 kernel sources, Sony’s Github repositories, and bug tracker. Kudos to Sony and the Xperia Dev team for making this happen.