There are two primary types of IO-MMU supported by parisc-linux.
The first is U2/U-Turn, found on machines with PA7200 and PA8000/8200 processors. On these machines, as I understand it, U2/U-Turn act as a Runway-to-GSC bus bridge. (TODO: Writeup U2/U-Turn info on another page)
The second type of IO-MMU is Astro. Astro (or the successor IKE and Pluto) is found on all machines with PA8500 and newer processors. (TODO: Better Astro description on another page)
There are a few more types of IO-MMU found on PA-RISC machines, such as the JAVA IO-MMU found on T600 servers. However these are not supported, and quite likely will never be, because the chipset is not IO-Coherent.