You're picking on the vocabulary in my post, not the meaning: that's just bad form.
I'm honestly sorry if I misunderstood what you meant by the word "design", but I really was doing my best to read in proper context. You seemed to be saying that, even after all the borrowed bcm43xx code was replaced by from-scratch replacement code, some Platonic essence of the original, the "design", inevitably would remain. And, sorry, that's just not how copyright caselaw has developed.
But let's discuss how it would be applied. I think we turn out to be in substantive agreement on that. The court (again, assuming US jurisdiction) would look for both literal and non-literal copying. For the latter, it would apply the "abstraction, filteration, comparison" test developed in CAI v. Altai, looking for expressive elements copied in a non-literal fashion. (Non-expressive, e.g., strictly functional elements are not entitled to copyright.) If there is a substantive amount of copying of expressive elements, and it wasn't explicitly permitted, and it doesn't fall into one of the allowed categories, and plaintiff has valid title, then there would be a ruling of infringement.
And indeed, there isn't a bright line. The courts had a rough time working out even those guidelines. If you'd read the caselaw, you'd have known that quite a while ago. ;-)
Also, sorry, I'm not going to try to understand software law and leave it solely to the courtroom gladiators. It's too important to remain ignorant of. You shouldn't, either.
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