Re: libata .sg_tablesize: why always dividing by 2 ?

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

> James B.  suggests that we stick a WARN_ON() into libata to let us
> know if that precondition is violated.  Sounds like an easy thing to do
> for a couple of -rc cycles someday.

If the block layer gives us a 32k block aligned on a 32k boundary
(aligned), we have no guarantee that the iommu will not turn that into
something unaligned crossing a 32k (and thus possibly a 64k) boundary.

On powerpc, the iommu operates on 4k pages and only provides that level
of alignment to dma_map_sg() (dma_alloc_coherent are naturally aligned,
but not map_sg, that would put way too much pressure on the allocator on
machines that have pinhole-sized iommu space).

Cheers,
Ben.