On 04/15/2018 08:29 PM, Christoph Hellwig wrote:

This way we have one central definition of it, and user can select it as
needed.  Note that we also add a second ARCH_HAS_SWIOTLB symbol to
indicate the architecture supports swiotlb at all, so that we can still
make the usage optional for a few architectures that want this feature
to be user selectable.

Signed-off-by: Christoph Hellwig <hch@lst.de>

snip

+
+config SWIOTLB
+	bool "SWIOTLB support"
+	default ARCH_HAS_SWIOTLB
+	select DMA_DIRECT_OPS
+	select NEED_DMA_MAP_STATE
+	select NEED_SG_DMA_LENGTH
+	---help---
+	  Support for IO bounce buffering for systems without an IOMMU.
+	  This allows us to DMA to the full physical address space on
+	  platforms where the size of a physical address is larger
+	  than the bus address.  If unsure, say Y.
+
 config CHECK_SIGNATURE
 	bool

Pulling DMA_DIRECT_OPS config option by default when SWIOTLB is enabled
makes sense. This option was also needed to be enabled separately even
to use swiotlb_dma_ops.