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    SubjectRe: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct device
    On Wed, Mar 25, 2020 at 03:51:36PM +1100, Alexey Kardashevskiy wrote:
    > >> This is for persistent memory which you can DMA to/from but yet it does
    > >> not appear in the system as a normal memory and therefore requires
    > >> special handling anyway (O_DIRECT or DAX, I do not know the exact
    > >> mechanics). All other devices in the system should just run as usual,
    > >> i.e. use 1:1 mapping if possible.
    > >
    > > On other systems (x86 and arm) pmem as long as it is page backed does
    > > not require any special handling. This must be some weird way powerpc
    > > fucked up again, and I suspect you'll have to suffer from it.
    >
    >
    > It does not matter if it is backed by pages or not, the problem may also
    > appear if we wanted for example p2p PCI via IOMMU (between PHBs) and
    > MMIO might be mapped way too high in the system address space and make
    > 1:1 impossible.

    How can it be mapped too high for a direct mapping with a 64-bit DMA
    mask?

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