Re: [PATCH] x86: avoid unnecessary low zone allocation in AMDIOMMU's alloc_coherent

[FUJITA Tomonori]

On Wed, 10 Sep 2008 16:52:49 +0200
Joerg Roedel <joerg.roedel@xxxxxxx> wrote:

> On Wed, Sep 10, 2008 at 11:39:00PM +0900, FUJITA Tomonori wrote:
>  
> > btw, in tip/x86/iommu, GART's alloc_coherent always does virtual
> > mappings to allocate a size-aligned memory (as DMA-mapping.txt
> > defines).
> > 
> > Because someone strongly insisted, I modified GART's alloc_coherent to
> > do so but as I said again and again, it's completely meaningless (only
> > POWER IOMMU does it and drivers don't depend on such requirement).
> > 
> > I guess that it would be better to do virtual mappings only when
> > necessary as the current mainline does since GART I/O space is
> > precious in some systems. But I don't care much. What's your opinion
> > (as a AMD developer)?
> 
> Very true. My original rewrite did the mapping only when necessary too.
> What were the reasons to do the mapping always?

As I said above, it's for allocating a size-aligned memory. Look at
the description of pci_alloc_consistent in DMA-mapping.txt:

The cpu return address and the DMA bus master address are both
guaranteed to be aligned to the smallest PAGE_SIZE order which
is greater than or equal to the requested size.  This invariant
exists (for example) to guarantee that if you allocate a chunk
which is smaller than or equal to 64 kilobytes, the extent of the
buffer you receive will not cross a 64K boundary.

You can't do this with __get_free_pages easily (you need some hacks to
do this). You can do this via iommu_area_alloc() for free.

Well, actually you agreed with adding such requirement (though I said
again and again that it's totally meaningless...):

http://lkml.org/lkml/2008/7/24/162


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