From: "David S. Miller" <davem@redhat.com>

Currently, for an existing DMA mapping, there is a way to transfer buffer
ownership back to the cpu, yet there is no way to give it back to the device
again explicitly.  The latter really is needed on platforms where the PCI
subsystem does not snoop the cpu caches, MIPS is one example.

Many drivers were expecting the existing DMA sync interface to handle both
directions, which was wrong.

Now, with this change, we have explicit interfaces for DMA syncing to/from
the device and the cpu.