[Xen-devel] Re: mthca use of dma_sync_single is bogus

To:	Lukas Hejtmanek <xhejtman@xxxxxxxxxxx>
Subject:	[Xen-devel] Re: mthca use of dma_sync_single is bogus
From:	Roland Dreier <rdreier@xxxxxxxxx>
Date:	Tue, 10 Jul 2007 12:24:02 -0700
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Cc:	Keir Fraser <keir@xxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, "Michael S. Tsirkin" <mst@xxxxxxxxxxxxxxxxxx>, general@xxxxxxxxxxxxxxxxxxxxx
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In-reply-to:	<20070710191639.GL3885@xxxxxxxxxxx> (Lukas Hejtmanek's message of "Tue, 10 Jul 2007 21:16:39 +0200")
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 > > coherent and consistent are synonyms.  It's confusing because there is
 > > pci_alloc_consistent(), which is in general just a wrapper for
 > > dma_alloc_coherent().
 > 
 > According to DMA-mapping.txt they are not. Alpha, M68000 wihtout MMU, PPC,
 > Sparc, Sparc64, V850 have own implementation of pci_alloc_consistent().
 > 
 > Yes, on i386, the pci_alloc_consistent() is just wrapper for
 > dma_alloc_coherent().

Sorry, I was a little confusing.  The implementations may be different
but in general there is no real difference between consistent and
coherent memory.  Using either pci_alloc_consistent() or
dma_alloc_coherent() will exhaust the same small pool of address space
on powerpc 4xx for example.

 > So, what about some #ifdefs ? E.g., allow config option - Xen optimizations?

Seems pretty ugly, especially given that Xen is not upstream.  I think
the Xen tree should just carry such patches, at least until Xen is
merged.  Even then I'm quite dubious about having two code paths for this.

 - R.

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[Xen-devel] Re: mthca use of dma_sync_single is bogus