Benjamin Herrenschmidt wrote:
On Mon, 2008-02-25 at 23:38 -0500, Mark Lord wrote:
Benjamin Herrenschmidt wrote:
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.

Certainly, but never any worse than what the block layer gave originally.

The important note being:  IOMMU only ever *merges*, it never *splits*.

Yes, but it will also change the address and doesn't guarantee the

Which means that, by the time we split up any mis-merges again for 64K 
we can never have more SG segments than what the block layer originally
fed to the IOMMU stuff.

Or so the IOMMU and SCSI experts here at LSF'08 have assured me,
even after my own skeptical questioning.

I suppose so. I don't remember all of the details, but iirc, it has to
do with crossing 64K boundaries. Some controllers can't handle it.

It's not only the _size_ of the segments, it's their alignment.

The iommu will not keep alignement beyond the page size (and even
then... on powerpc with a 64k base page size, you may still end up with
a 4k aligned result, but let's not go there now).

That's just not possible, unless the IOMMU *splits* segments.
And the IOMMU experts here say that it never does that.

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