Re: IOMMU and scatterlist limits

[Jens Axboe]

On Thu, Nov 17 2005, Pierre Ossman wrote:
> I'm writing a PCI driver for the first time and I'm trying to wrap my
> head around the DMA mappings in that world. I've done a ISA driver which
> uses DMA, but this is a bit more complex and the documentation doesn't
> explain everything.
> 
> What I'm particularly confused about is how the IOMMU should be handled
> with regard to scatterlist limits. My hardware cannot handle
> scatterlists, only a single DMA address. But from what I understand the

What kind of hardware can't handle scatter gather?

> IOMMU can be very similar to a normal "CPU" MMU. So it should be able to
> aggregate pages that are non-continuous in physical memory into one
> single block in bus memory. Now the question is what do I set
> nr_phys_segments and nr_hw_segments to? Of course the code also needs to
> handle systems without an IOMMU.

nr_hw_segments is how many segments your driver will see once dma
mapping is complete (and the IOMMU has done its tricks), so you want to
set that to 1 if the hardware can't handle an sg list.

That'll work irregardless of whether there's an IOMMU there or not. Note
that the mere existence of an IOMMU will _not_ save your performance on
this hardware, you need one with good virtual merging support to get
larger transfers.

-- 
Jens Axboe

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