After last month's release of Android 5.1 updates for HTC's Google Play Edition devices, it was only a matter of time before the kernel sources for those phones were posted. This is especially interesting for fans of the M7 and M8 because these are the first looks we have at the kernel for 5.1 on them.
The M7 release is version 6.04.1700.6 while the M8 is 4.04.1700.6. This is most useful for people who are considering flashing custom ROMs and the like. The source helps software developers at places like XDA Developers.
Even better, since they are the last of the Google Play Edition program, they're actually the only HTC devices that have Android 5.1 yet. The kernel source could potentially have broader appeal to developers trying to bring newer software to the M9 or other similar phones.