Messages in this thread | | | Date | Thu, 11 Sep 2008 01:29:54 +0900 | Subject | Re: [PATCH] x86: avoid unnecessary low zone allocation in AMD IOMMU's alloc_coherent | From | FUJITA Tomonori <> |
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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) ?
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