Here is a link to the dfs(4) man page for 5.0-release on the macppc:
http://www.openbsd.org/cgi-bin/man.c...pc&format=html
In it, you will see a description of the applicable hardware. If this driver is loaded, you will see it in your dmesg. Please also note that there is a SEE ALSO section that refers you to both a driver for on-board GPIO controllers, and to sysctl(8). I assume, therefore, that you will be able to use the sysctl hw.setperf or a similar hw.* sysctl setting.