But of course it doesn't help much in practice because all the interesting
block devices support DAC anyways and the IOMMU is disabled for that.
Platform dependant. SAC DMA transfers are faster on sparc64 so
we only allow the device to specify a 32-bit DMA mask successfully.
And actually, I would recommend other platforms that have a IOMMU do
this too (unless there is some other reason not to) since virtual
merging causes less scatter-gather entries to be used in the device
and thus you can stuff more requests into it.
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