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Re: [PATCH] iommu/dma-iommu: properly respect configured address space size

From	Robin Murphy <robin.murphy@arm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] iommu/dma-iommu: properly respect configured address space size
Date	2016-11-10 17:20 +0100
Message-ID	<sC0eZ-77-3@gated-at.bofh.it> (permalink)
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On 10/11/16 15:59, Joerg Roedel wrote:
> On Tue, Nov 08, 2016 at 11:37:23AM +0000, Robin Murphy wrote:
>> TBH I've been pondering ripping the size stuff out of dma-iommu, as it
>> all stems from me originally failing to understand what dma_32bit_pfn is
>> actually for.
> 
> The point of dma_32bit_pfn is to allocate dma-address below 4G by
> default. This is a performance optimization so that even devices capable
> of 64bit DMA are using SAC by default instead of DAC.
> 
> Since it is the goal to share a dma-iommu implemenation between
> architectures, I would rather prefer not to rip this stuff out.

Oh, I didn't mean rip it out entirely, just get rid of the bogus
assumption that it's the "size" of the domain, especially when given a
>32-bit DMA mask, since that defeats the very optimisation I do now
understand (although it might still be OK for platform devices where
SAC/DAC doesn't apply, to avoid the rb_last() overhead every time).

From the patch I've started, "rip it out" turns out to actually be
mostly "rewrite the comments" anyway - I'll post something soon.

Robin.

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[PATCH] iommu/dma-iommu: properly respect configured address space size Marek Szyprowski <m.szyprowski@samsung.com> - 2016-11-07 14:20 +0100
  Re: [PATCH] iommu/dma-iommu: properly respect configured address  space size Robin Murphy <robin.murphy@arm.com> - 2016-11-08 12:50 +0100
    Re: [PATCH] iommu/dma-iommu: properly respect configured address space  size Marek Szyprowski <m.szyprowski@samsung.com> - 2016-11-08 15:00 +0100
      Re: [PATCH] iommu/dma-iommu: properly respect configured address  space size Robin Murphy <robin.murphy@arm.com> - 2016-11-08 15:50 +0100
        Re: [PATCH] iommu/dma-iommu: properly respect configured address space  size Marek Szyprowski <m.szyprowski@samsung.com> - 2016-11-08 16:00 +0100
          Re: [PATCH] iommu/dma-iommu: properly respect configured address  space size Robin Murphy <robin.murphy@arm.com> - 2016-11-10 15:10 +0100
    Re: [PATCH] iommu/dma-iommu: properly respect configured address  space size Joerg Roedel <joro@8bytes.org> - 2016-11-10 17:00 +0100
      Re: [PATCH] iommu/dma-iommu: properly respect configured address  space size Robin Murphy <robin.murphy@arm.com> - 2016-11-10 17:20 +0100

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