On Tue, 2008-02-26 at 00:43 -0500, Mark Lord wrote: > > 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. It is totally possible, and I know as wrote part of the powerpc iommu code :-) The iommu code makes no guarantee vs. preserving the alignment of a segment, at least not below PAGE_SIZE. Thus if you pass to dma_map_sg() a 64K aligned 64K segment, you may well get back a 4K aligned 64K segment. Enforcing natural alignment in the iommu code only happens for dma_alloc_coherent (it uses order-N allocations anyway), it doesn't happen for map_sg. If we were to do that, we would make it very likely for iommu allocations to fail on machine with small DMA windows. Ben. - To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
