Re: [RFC PATCH] fix problems with NETIF_F_HIGHDMA in networking drivers

[FUJITA Tomonori]

On Sat, 27 Feb 2010 12:15:19 -0600
Robert Hancock <hancockrwd@xxxxxxxxx> wrote:

> On Sat, Feb 27, 2010 at 6:05 AM, David Miller <davem@xxxxxxxxxxxxx> wrote:
> > From: Bartlomiej Zolnierkiewicz <bzolnier@xxxxxxxxx>
> > Date: Sat, 27 Feb 2010 12:59:31 +0100
> >
> >> Having IOMMU (even if it is only a software one, i.e. this would
> >> mean swiotlb for x86-32/highmem) always in place would simplify
> >> things greatly..
> >
> > I agree, things would be a lot simpler.
> 
> Yeah, the situation kind of sucks on the platforms that don't have any
> IOMMU support, since it means that the DMA API can't handle anything
> over 4GB properly and you need all these hacks in the block layer,
> networking layer, etc. It would be nice if some kind of IOMMU support
> could be relied upon always.

When I proposed such approach (always use swiotlb) before, IIRC,
the objections were:

- better to make allocation respect dma_mask. (I don't think that this
  approach is possible since we don't know which device handles data
  later when we allocate memory).

- swiotlb is not good for small systems since it allocates too much
  memory (we can fix this though).

There might be more.
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