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

FUJITA Tomonori
2008-09-10T16:30:22Z

On Wed, 10 Sep 2008 17:29:18 +0200
Joerg Roedel <joerg.roedel@amd.com> wrote:

> 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.

Is it guaranteed (documented somewhere) ?