Replying to my own email,
dma_map_sg returns the actual number of entries to iterate. At least
historically some IOMMU implementations would do strange tricks like:
If entries 2 and 3 could be merged dma_len for 2 would span 2 and 3,
and then entry 3 would actually have the dma addr and len for entry 4.
So what would be in the last entry {dma_addr, dma_len}? zeros?
I'm not sure anyone still does that, but the first spot to check would
be the Parisc IOMMU drivers.
So how does that sit with the fact that dma_unmap requires the
same sg_nents as in dma_map and not the actual value of dma entries?
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