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SubjectRe: [SUGGESTION]: drop virtual merge accounting in I/O requests
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From: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Date: Tue, 15 Jul 2008 02:45:03 +0900

> dma_4u_map_sg() has:
>
> if (segstart != s) {
> /* We cannot merge if:
> * - allocated dma_addr isn't contiguous to previous allocation
> */
> if ((dma_addr != dma_next) ||
> (outs->dma_length + s->length > max_seg_size) ||
> (is_span_boundary(out_entry, base_shift,
> /* Can't merge: create a new segment */
> segstart = s;
> outcount++;
> outs = sg_next(outs);
>
> So if the IOMMU allocated dma_addr isn't contiguous to previous
> allocation, it might not merge segments that the block layer expected
> the IOMMU to merge.
>
> We need kinda two phase merging code such as the old SPARC64 IOMMU
> code and PARISC IOMMUs though I like the new simple SPARC64 IOMMU
> code.

I see.

I wonder if all that complexity is really worth it. Also, all of this
IOMMU allocation and mapping code runs under a spinlock with hw IRQs
disabled.

More and more I'm seeing that it's likely better to remove the VMERGE
code. I can't see what it really buys us anymore, and to make it work
requires quite a large amount of complexity in the IOMMU layer.


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