Re: [PATCH] [ppc64] watch IOMMU virtual merging

On Tue, Aug 03, 2004 at 07:45:51AM +1000, Benjamin Herrenschmidt wrote:

[adding linux-scsi with full quote]

Actually, we could tune the ratio between large allocs and small allocs,
the real problem is the failure case. We can't really afford to break
down a physical segment at the iommu level because that would increase
the size of the SG list which can't be dealt at the upper level (we don't
know how much space has been allocated and the HW may have limitations on
the number of entries).

What we really need is a way for drivers to return the BIO upstream and
ask for a split in case of iommu allocation error, but I've been told that
would be horribly complex...

Not very, according to James B. It just needs driver changes.
I would like to have it too. 

-Andi