[$] Dancing the DMA two-step

Direct memory access (DMA) I/O is simple in concept: a peripheral devicemoves data directly to or from memory while the CPU is busy doing otherthings. As is so often the case, DMA is rather more complicated inpractice, and the kernel has developed a complicated internal API tosupport it. It turns out that the DMA API, as it exists now, can affectthe performance of some high-bandwidth devices. In an effort to addressthat problem, Leon Romanovsky is making the API even more complex with this patch seriesadding a new two-step mapping API.