Commercially useful IP is orders of magnitude more difficult to create than fixed-configuration IP.
Over time, commercial IP products have morphed from single function blocks to 100% configurable IPs where no two instances are the same. In this article I point out the challenges of creating configurable IP, and the best-known practices to address them.
IP Configurability Spectrum
Throughout the history of chip design, there has been a spectrum of configurability that has been built into internally developed and commercial semiconductor design IP. In the early days of chip design, sections of chip designs (i.e. IP blocks) were mostly fixed-function and therefore were not configurable. We still see this today, with standards-based IP, where there is a very limited set of functionality that is configurable, or even no configurability at all.