Re: PATCH: Further aacraid work

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

> The AMD64 IOMMU could do it too (and the code to do it exists in
> 2.6). But the problem is that the current IO layer doesn't provide a
> sufficient fallback path when this fails. You have to promise in
> advance that you can merge and then later it's too late to change your
> mind without signalling an IO error.

Well, the way I do it on ppc64 works with failure cases too. The IO
layer is given my phusical limitations, that is it provides me with an
SG list that will always fit. If I can do merging, great, that will
improve, but I don't enforce merging.

You could do exactly the same.

The problem I agree is that this forces the IO layer to give you small
enough requests, it would be nice to have a "try big and retry smaller"
path but that require invasive changes.

> I had a chat with James about this at last year's OLS. The Consensus
> was iirc that it needs driver interface changes at least.

Ben.