On Tue, 16 Sep 2008 15:43:35 +0200
Andi Kleen <andi@firstfloor.org> wrote:

quoted text
> > What the need_iommu() does is seeing if GART needs to do virtual
> > mappings or not.
> > 
> > (After need_iommu() checking) What this patchset does is to guarantee
> > that GART provides a virtual address that a device can access to.
> 
> Ah you care about masks < 32bit?

Yes,


quoted text
> Those always are handled elsewhere in the block layer (using the bounce_pfn
> mechanism)

I don't think that the bounce guarantees that dma_alloc_coherent()
returns an address that a device can access to.


quoted text
> or in various other ways in other subsystems. e.g. on networking
> the rule is that you just don't announce direct SG when you have 
> less than 32bit mask.

I'm not familiar with what the networking does, for example, seems
that b44 sets dev->dma_mask to DMA_30BIT_MASK and b44_start_xmit()
does:

mapping = ssb_dma_map_single(bp->sdev, skb->data, len, DMA_TO_DEVICE);
if (ssb_dma_mapping_error(bp->sdev, mapping) || mapping + len > DMA_30BIT_MASK) {
	struct sk_buff *bounce_skb;

	/* Chip can't handle DMA to/from >1GB, use bounce buffer */
	if (!ssb_dma_mapping_error(bp->sdev, mapping))
		ssb_dma_unmap_single(bp->sdev, mapping, len,
				     DMA_TO_DEVICE);

IOMMUs can try to return an address that the NIC can access to.
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