These were developed on github:
https://github.com/bwidawsk/freebsd/tree/s0ix
S0ix is the name of Intel's implementation of Low Power Idle states as described
in ACPI spec via LPIT, and SPMC. It can be thought of as a hardware managed
suspend state whereby all software needs to do is sufficiently idle the system,
and hardware will go into the lower power state until woken by a specified set
of events.
Newer laptops with Intel CPUs prefer a sleep state known as s0ix (where x can be
1, 2, or 3). s0ix entry and benefits are explained in the patches found on
github.