Re: [PATCH 0/5 v3] Fix NVMe driver support on Power with 32-bit DMA

[Nishanth Aravamudan]

[Sorry, subject should have been 0/7!]

On 23.10.2015 [13:54:20 -0700], Nishanth Aravamudan wrote:
> We received a bug report recently when DDW (64-bit direct DMA on Power)
> is not enabled for NVMe devices. In that case, we fall back to 32-bit
> DMA via the IOMMU, which is always done via 4K TCEs (Translation Control
> Entries).
> 
> The NVMe device driver, though, assumes that the DMA alignment for the
> PRP entries will match the device's page size, and that the DMA aligment
> matches the kernel's page aligment. On Power, the the IOMMU page size,
> as mentioned above, can be 4K, while the device can have a page size of
> 8K, while the kernel has a page size of 64K. This eventually trips the
> BUG_ON in nvme_setup_prps(), as we have a 'dma_len' that is a multiple
> of 4K but not 8K (e.g., 0xF000).
> 
> In this particular case, and generally, we want to use the IOMMU's page
> size for the default device page size, rather than the kernel's page
> size.
> 
> This series consists of five patches:
> 
> 1) add a generic dma_get_page_shift implementation that just returns
> PAGE_SHIFT
> 2) override the generic implementation on Power to use the IOMMU table's
> page shift if available
> 3) allow further specific overriding on power with machdep platform
> overrides
> 4) use the machdep override on pseries, as the DDW code puts the TCE
> shift in a special property and there is no IOMMU table available
> 5) move some sparc code around to make IOMMU_PAGE_SHIFT available in
> include/asm
> 6) override the generic implementation on sparce to use IOMMU_PAGE_SHIFT
> 7) leverage the new API in the NVMe driver
> 
> With these patches, a NVMe device survives our internal hardware
> exerciser; the kernel BUGs within a few seconds without the patch.
> 
>  arch/powerpc/include/asm/dma-mapping.h |  3 +++
>  arch/powerpc/include/asm/machdep.h     |  3 ++-
>  arch/powerpc/kernel/dma.c              | 11 +++++++++++
>  arch/powerpc/platforms/pseries/iommu.c | 36 ++++++++++++++++++++++++++++++++++++
>  arch/sparc/include/asm/dma-mapping.h   |  8 ++++++++
>  arch/sparc/include/asm/iommu_common.h  | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  arch/sparc/kernel/iommu.c              |  2 +-
>  arch/sparc/kernel/iommu_common.h       | 51 ---------------------------------------------------
>  arch/sparc/kernel/pci_psycho.c         |  2 +-
>  arch/sparc/kernel/pci_sabre.c          |  2 +-
>  arch/sparc/kernel/pci_schizo.c         |  2 +-
>  arch/sparc/kernel/pci_sun4v.c          |  2 +-
>  arch/sparc/kernel/psycho_common.c      |  2 +-
>  arch/sparc/kernel/sbus.c               |  3 +--
>  drivers/block/nvme-core.c              |  3 ++-
>  include/linux/dma-mapping.h            |  7 +++++++
>  16 files changed, 127 insertions(+), 61 deletions(-)
> 
> v1 -> v2:
>   Based upon feedback from Christoph Hellwig, rather than using an
>   arch-specific hack, expose the DMA page shift via a generic DMA API and
>   override it on Power as needed.
> v2 -> v3:
>   Based upon feedback from Christoph Hellwig, put the generic
>   implementation in include/linux/dma-mapping.h, since not all archs use
>   include/asm-generic/dma-mapping-common.h.
>   Add sparc implementation, as that arch seems to have a different IOMMU
>   page size.

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