Re: [PATCH 0/5 v2] Fix NVMe driver support on Power with 32-bit DMA

From	Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 0/5 v2] Fix NVMe driver support on Power with 32-bit DMA
Date	2015-10-02 23:10 +0200
Message-ID	<qffKy-6Av-17@gated-at.bofh.it> (permalink)
References	<qfc9X-1mW-5@gated-at.bofh.it> <qfeYa-5qP-5@gated-at.bofh.it> <qffKy-6Av-19@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On 03.10.2015 [06:51:06 +1000], Benjamin Herrenschmidt wrote:
> On Fri, 2015-10-02 at 13:09 -0700, Nishanth Aravamudan wrote:
> 
> > 1) add a generic dma_get_page_shift implementation that just returns
> > PAGE_SHIFT
> 
> So you chose to return the granularity of the iommu to the driver
> rather than providing a way for the driver to request a specific
> alignment for DMA mappings. Any specific reason ?

Right, I did start with your advice and tried that approach, but it
turned out I was wrong about the actual issue at the time. The problem
for NVMe isn't actually the starting address alignment (which it can
handle not being aligned to the device's page size). It doesn't handle
(addr + len % dev_page_size != 0). That is, it's really a length
alignment issue.

It seems incredibly device specific to have a an API into the DMA code
to request an end alignment -- no other device seems to have this
issue/design. If you think that's better, I can fiddle with that
instead.

Sorry, I should have called this out better as an alternative
consideration.

-Nish

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Thread

[PATCH 0/2] Fix NVMe driver support on Power with 32-bit DMA Nishanth Aravamudan <nacc@linux.vnet.ibm.com> - 2015-10-02 19:20 +0200
  [PATCH 1/2] powerpc/iommu: expose IOMMU page shift Nishanth Aravamudan <nacc@linux.vnet.ibm.com> - 2015-10-02 19:20 +0200
    [PATCH 2/2] drivers/nvme: default to the IOMMU page size on Power Nishanth Aravamudan <nacc@linux.vnet.ibm.com> - 2015-10-02 19:30 +0200
      Re: [PATCH 2/2] drivers/nvme: default to the IOMMU page size on Power Christoph Hellwig <hch@infradead.org> - 2015-10-02 19:30 +0200
        Re: [PATCH 2/2] drivers/nvme: default to the IOMMU page size on Power Nishanth Aravamudan <nacc@linux.vnet.ibm.com> - 2015-10-02 19:50 +0200
          Re: [PATCH 2/2] drivers/nvme: default to the IOMMU page size on Power Christoph Hellwig <hch@infradead.org> - 2015-10-02 19:50 +0200
  [PATCH 2/5 v2] powerpc/dma-mapping: override dma_get_page_shift Nishanth Aravamudan <nacc@linux.vnet.ibm.com> - 2015-10-02 22:20 +0200
    [PATCH 3/5 v2] powerpc/dma: implement per-platform dma_get_page_shift Nishanth Aravamudan <nacc@linux.vnet.ibm.com> - 2015-10-02 22:20 +0200
      [PATCH 4/5 v2] pseries/iommu: implement DDW-aware dma_get_page_shift Nishanth Aravamudan <nacc@linux.vnet.ibm.com> - 2015-10-02 22:30 +0200
        [PATCH 5/5 v2] drivers/nvme: default to the IOMMU page size Nishanth Aravamudan <nacc@linux.vnet.ibm.com> - 2015-10-02 22:40 +0200
        Re: [PATCH 4/5 v2] pseries/iommu: implement DDW-aware  dma_get_page_shift kbuild test robot <lkp@intel.com> - 2015-10-02 22:40 +0200
  [PATCH 1/5 v2] dma-mapping: add generic dma_get_page_shift API Nishanth Aravamudan <nacc@linux.vnet.ibm.com> - 2015-10-02 22:20 +0200
  [PATCH 0/5 v2] Fix NVMe driver support on Power with 32-bit DMA Nishanth Aravamudan <nacc@linux.vnet.ibm.com> - 2015-10-02 22:20 +0200
    Re: [PATCH 0/5 v2] Fix NVMe driver support on Power with 32-bit DMA Nishanth Aravamudan <nacc@linux.vnet.ibm.com> - 2015-10-02 23:10 +0200
      Re: [PATCH 0/5 v2] Fix NVMe driver support on Power with 32-bit DMA Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-10-02 23:40 +0200
        Re: [PATCH 0/5 v2] Fix NVMe driver support on Power with 32-bit DMA Nishanth Aravamudan <nacc@linux.vnet.ibm.com> - 2015-10-03 00:00 +0200
    Re: [PATCH 0/5 v2] Fix NVMe driver support on Power with 32-bit DMA Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-10-02 23:30 +0200
      Re: [PATCH 0/5 v2] Fix NVMe driver support on Power with 32-bit DMA Christoph Hellwig <hch@infradead.org> - 2015-10-03 10:30 +0200

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