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

Joerg Roedel
2008-09-10T15:29:56Z

On Thu, Sep 11, 2008 at 12:09:43AM +0900, FUJITA Tomonori wrote:
> On Wed, 10 Sep 2008 16:52:49 +0200
> Joerg Roedel <joerg.roedel@amd.com> 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.

What hacks do you need with __get_free_pages? The memory it returns is
_always_ aligned at its size.

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

The other possible way is removing this requirement from the
documentation. But if the spec has this requirement the code _has_ to
fullfill it, even if its meaningless for most drivers.

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

This message stated nothing about _always_ map GART alloc_coherent
allocations using the aperture.

Joerg

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