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

[Joerg Roedel]

On Thu, Sep 11, 2008 at 01:29:54AM +0900, FUJITA Tomonori wrote:
> On Wed, 10 Sep 2008 17:29:18 +0200
> Joerg Roedel <joerg.roedel@xxxxxxx> 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@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.
> > 
> > What hacks do you need with __get_free_pages? The memory it returns is
> > _always_ aligned at its size.
> 
> Is it guaranteed (documented somewhere) ?

I don't know if there is a formal definition for it. It is documented in
some books about the Linux kernel (I read this in some book the first
time). This alignment results from the buddy algorithm the page alloctor
uses. You can definitly rely on that.

Joerg

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