[$] Noncoherent DMA mappings

While it is sometimes possible to perform I/O by moving data through the
CPU, the only way to get the required level of performance is usually for devices
to move data directly to and from memory. Direct memory access (DMA) I/O
has been well supported in the Linux kernel since the early days, but there
are always ways in which that support can be improved, especially when
hardware adds some challenges of its own. The somewhat confusingly named
"non-contiguous" DMA API that
was added for 5.13 shows the kinds of things that have to be done to get
the best performance on current systems.