The current FreeBSD smsc(4) driver is hard-coded to always enable turbo_mode (coalesce multiple packets into one read). While this increases throughput, it can increase latency and jitter, which is detrimental to timekeeping (NTP) and possibly line UDP (like VoIP).
Because of this, the Linux version makes turbo_mode configurable. This patch does the same. The default leaves it enabled, like it currently is.