> How much work is it to port drivers between Free and Open BSD?

IIRC there are two problems at play:

First, I'm not a C coder so this is a bit above my pay-grade, but from what little I do remember about the subject, the problem relates to the OpenBSD requirement to adopt their security mechanisms such as pledge, unveil and strlcpy. IIRC the OpenBSD compiler is also (unsurprisingly !) more anal about stack protector W^X etc. So the porting process is perhaps more time-consuming and low-level than it might otherwise be on other porting projects.

Second, the licensing thing might come into it. OpenBSD has a high preference to most-permissive, and so things like GPL-licensed origins might not be acceptable. IIRC FreeBSD is a little more relaxed within reason ? And when you're working with network cards I would think that is perhaps hard to avoid to some extent if you're relying on certain bits being ultimately derived from Intel chipsets or whatever.

I'm open to correction by those more knowledgable than me on porting intricacies. ;)

The difficulty of porting NIC drivers is probably not in differences in the userland API; kernel drivers don't likely pledge anything. But OpenBSD and FreeBSD diverged a long time ago, and I'd be surprised if their kernel APIs are very close anymore. How to detect and interface with devices is probably a bit different, and rx/tx packets will be different too.

I think most of the vendor supplied NIC drivers in FreeBSD are BSD licensed, so that shouldn't be an issue. I checked Intel, Melanox (now NVidia), Cavium/QLogic/Broadcom, Solarflare. The realtek driver in the tree is BSD licensed but not vendor provided; the vendor driver in ports is also BSD licensed. I'm not sure if there's a datacenter ethernet provider with in kernel drivers I missed; but I don't think license is a problem here either --- anyway you could ship a driver module out of tree if it was.