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    SubjectRe: can device drivers return non-ram via vm_ops->nopage?
    On Sun, 2004-03-21 at 15:51 -0800, Linus Torvalds wrote:
    > Note that there is really two different kinds of IO memory:
    > - real IO-mapped memory on the other side of a bus
    > - real RAM which is on the CPU side of the bus, but that has additionally
    > been "mapped" some way as to be visible from devices.
    > The second kind is what you seem to be talking about,


    > So the minimal fix for any misuses would be to just have a
    > "dma_map_to_page()" reverse mapping for "dma_alloc_coherent()". For x86,
    > that's just the same thing as "virt_to_page()". For others, you have to
    > look more carefully at undoing whatever mapping the iommu has been set up
    > for.

    You are assuming that dma_alloc_coherent() will always return memory of
    that second kind -- host-side RAM, not PCI-side. That hasn't previously
    been a requirement, and there are machines out there on which it makes a
    lot more sense for dma_alloc_coherent() to use some SRAM which happens
    to be hanging off the I/O bus than it does to use host RAM.

    Doing dma_map_to_pfn() instead of dma_map_to_page() would work. That
    means you can't use nopage() for mappings of dma_coherent memory. That's
    fine though.

    > Would that make people happy?

    No. It'd be OK if you make it dma_map_to_pfn() instead of
    dma_map_to_page() though. As discussed, that means you can't use
    nopage() for mappings of dma_coherent memory. That's fine though.

    I think it would be better to provide arch-specific functions for
    mapping dma_coherent allocations and SG lists. On most architectures we
    can just do it with virt_to_page() and nopage() and it'll be OK. On
    others we can do the right thing as appropriate.


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