sys/dev/ipmi/ipmivars.h contains the following definition:
#define MAX_TIMEOUT 3 * hz
This determines how long ipmi(4) is willing to wait in its attach
routine for the management device to identify itself. Unfortunately,
it is too short. The IPMI specification recommends a timeout not less
than five seconds, and in practice on this platform it takes between
five and ten seconds.
Increase the timeout to ten seconds. If this is felt to be too long,
make it a tunable.
Boot FreeBSD on a Quanta QSSC-S99Q server. Sometimes ipmi will
attach; most of the time it will time out.
Fixed by sbruno in r253812.