Re: [RFC PATCH v1 18/18] nvme-pci: use new dma API

[Christoph Hellwig]

> This is exactly how dma_map_sg() works.

Which dma_map_sg?  swiotlb handling is implemented in the underlying
ops, dma-direct and dma-iommu specifically.

dma-direct just iterates over the entries and calls dma_direct_map_page,
which does a per-entry decision to bounce based on
is_swiotlb_force_bounce, dma_capable and dma_kmalloc_needs_bounce.