HP's README when they published this driver on the download page of
the former HPOJ project:

-------------------------------------------------------------------------
HP cannot provide Free/open-source Linux drivers for the LaserJet 3100
and 3150 all-in-one peripherals, because they were developed for HP by
another company, and HP doesn't have rights to disclose the necessary
protocol information.

The information and sample code in this directory were obtained from
legitimate reverse-engineering by a non-HP individual and posted to a
public mailing list. Therefore, we (HP) believe it is fair game for us to
reproduce it here for anybody who wants to experiment with and/or
extend it. However, we are unable to provide any support for it.

Anyone who wants to finish this work is welcome and is encouraged to
post here on the HP Forum of www.openprinting.org.
-------------------------------------------------------------------------