32-bit powerpc needs this support - I was actually about to push a similar set of patches. We have several processors that support 36 bits of physical address space and do not have any iommu capability. The rest of the kernel support for those processors is now in place, so swiotlb is the last piece of the puzzle for that to be fully functional. I need to take a closer look at this series to see exactly what it's doing and how it differs from what I've been testing.
So there is another immediate use case, and I'd really hate to see this code duplicated. It should be entirely possible to remove the assumption that we can save off the VA of the original buffer, which is the thing that precludes HIGHMEM support, and still have nice readable, maintainable code.