libata .sg_tablesize: why always dividing by 2 ?

[Mark Lord <liml@rtr.ca>]

Jeff,

We had a discussion here today about IOMMUs,
and they *never* split sg list entries -- they only ever *merge*.

And this happens only after the block layer has
already done merging while respecting q->seg_boundary_mask.

So worst case, the IOMMU may merge everything, and then in
libata we unmerge them again.  But the end result can never
exceed the max_sg_entries limit enforced by the block layer.

So.. why are we still specifying .sg_tablesize as half of
what the LLD can really handle?

This can cost a lot of memory, as using NCQ effectively multiplies
everything by 32..

Based on this information, I should be able to do this in sata_mv,
for example:

-      .sg_tablesize           = MV_MAX_SG_CT / 2,
+      .sg_tablesize           = MV_MAX_SG_CT,


???