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    SubjectRe: [PATCH v7 16/16] ARM: LPAE: Add the Kconfig entries
    On Wed, Aug 10, 2011 at 04:03:39PM +0100, Catalin Marinas wrote:
    > +config ARCH_DMA_ADDR_T_64BIT
    > + def_bool ARM_LPAE
    > +

    I think this should be selected only when we have a DMA engine supporting
    64-bit addresses. Technically LPAE itself doesn't give us that assurance.

    If you have this kind of a setup:

    CPU <==++==> RAM
    DMA device

    where == and || means >32-bit addressing, and | means 32-bit addressing.

    In such a setup, having dma_addr_t be 64-bit is pointless because it
    should never see 64-bit addresses (the DMA API should deal with the
    IOMMU and provide a list of DMA addresses to be placed into RAM for
    the DMA device which takes account of the mappings setup in the IOMMU.)

    So, I think 64-bit dma_addr_t should be a property of the DMA devices
    present in the system rather than whether the CPUs MMU can deal with
    >32-bit addresses or not.

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      Last update: 2011-11-02 18:23    [W:0.020 / U:2.316 seconds]
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