Re: Scatter-gather segment merges by IOMMU?

[Grant Grundler]

On Fri, Aug 8, 2008 at 12:44 PM, Stefan Richter
<stefanr@xxxxxxxxxxxxxxxxx> wrote:
> Hi all,
>
> the block layer usually tries to merge s/g segments if consecutive segments
> combined fit into the queue's max_segment_size.  When such a scatter gather
> list is DMA-mapped, can it happen that an IOMMU collapses the elements even
> further, so that sg_dma_len() of a DMA-mapped s/g segment exceeds
> max_segment_size?

I don't see how. The IOMMU code only collapses the "physical" mappings and
does not add new elements to the SG list. ergo sg_dma_len() shouldn't change.

grant

>
> As I understood some discussions in the past, this could indeed happen,
> which is a nuisance.  But I may have misunderstood something, or something
> may have changed in the meantime...
> --
> Stefan Richter
> -=====-==--- =--- -=---
> http://arcgraph.de/sr/
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