There is a good discussion/interview¹ between Alan Kay & Joe Armstrong about how most code is developed backwards b/c none of the code has a formal specification that can be "compiled" into different targets. If there was a specification other than the old driver code then the process of porting over the driver would be a matter of recompiling the specification for a new kernel target. In absence of such specification you have to substitute human expertise to make sure the invariants in the old code are maintained in the new one b/c the LLMs has no understanding of any of it other than pattern matching to other drivers w/ similar code.
1. The original hardware spec is usually proprietary, and
2. The spec is often what the hardware was supposed to do. But hardware prototype revisions are expensive. So at some point, the company accepts a bunch of hardware bugs, patches around them in software, ships the hardware, and reassigns the teams to a newer product. The hardware documentation won't always be updated.
This is obviously an awful process, but I've seen and heard of versions of it for over 20 years. The underlying factors driving this can be fixed, if you really want to, but it will make your product totally uncompetitive.