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 <h...@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.

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