Benjamin Herrenschmidt wrote:
On Mon, 2008-02-25 at 19:15 -0500, Jeff Garzik wrote:
Mark Lord wrote:

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.
<shrug> Early experience said otherwise. The split in foo_fill_sg() and resulting sg_tablesize reduction were both needed to successfully transfer data, when Ben H originally did the work.

If Ben H and everyone on the arch side agrees with the above analysis, I would be quite happy to remove all those "/ 2".

The split wasn't done by the iommu. The split was done by the IDE code
itself to handle the stupid 64k crossing thingy. If it's done
differently now, it might be possible to remove it, I haven't looked.

The block layer uses seg_boundary_mask to ensure that we never have
to split them again in the LLD.

A very long time ago, when I wrote the IDE DMA code, this was not the case.

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