We live in a 64-bit world, to the point that many distributors want to stop
supporting 32-bit systems at all. However, lurking within our 64-bit
kernels is a subsystem that has not really managed to move past 32-bit
addresses. The quick merge-window failure of an attempt to use
64-bit addresses in the I/O memory-management unit (IOMMU) subsystem shows
how hard it can be to leave all of one’s 32-bit history behind.

Source: LWN.net – [$] The trouble with 64-bit DMA