> I hope I'm stating the obvious, but to do this you will need a HBA driver
> that handles target mode, unless you plan to use a third device as an
> in-between.  This is not a trivial task and will probably involve a major
> rewrite of the current driver.  Target mode and initiator mode are
> disjoint, and depending on the hardware it is possible to have a single
> controller operate in both initiator and target mode at the same time.

The aic7xxx driver in FreeBSD supports target mode and can be configured
to be both an initiator and target at the same time (You lose the ability
to respond as a target on multiple IDs in the multi-mode configuration,
but this may not be an issue for you).  FreeBSD also includes a target mode
peripheral driver that emulates a processor target device so there are
examples present for how to talk to a controller that implements the
target mode API.

--
Justin