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    SubjectRe: [RFC] MMIO accessors & barriers documentation
    Ar Llu, 2006-09-11 am 19:17 +1000, ysgrifennodd Benjamin Herrenschmidt:
    > > > 3- memcpy_to_io, memcpy_from_io: #1 semantics apply (all MMIO loads or
    > > > stores are performed in order to each other). #2+#4 (stores) or #3
    > >
    > > What is "in order" here. "In ascending order of address" would be
    > > tighter.
    > In program order. Every time I say "in order", I mean "in program
    > order". I agree that this is not enough precision as it's not obvious
    > that memcpy will copy in ascending order of addresses (it doesn't have
    > to), I'll add that precision... or not. THat could be another question.
    > What do we want here ? I would rather have those strongly ordered for
    > Class 1.

    I'd rather memcpy_to/from_io only made guarantees about the start/end of
    the transfer and not order of read/writes or size of read/writes. The
    reason being that a more restrictive sequence can be efficiently
    expressed using read/writefoo but the reverse is not true.

    > > "Except where the underlying device is marked as cachable or
    > > prefetchable"
    > You aren't supposed to use MMIO accessors on cacheable memory, are you ?

    Why not. Providing it is in MMIO space, consider ROMs for example or
    write path consider frame buffers.

    > with cacheable mappings of anything behind HT... I'd keep use of
    > cacheable mapping as an arch specific special case for now, and that
    > definitely doesn't allow for MMIO accessors ...

    I'm describing existing semantics 8)

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      Last update: 2006-09-11 11:49    [W:0.021 / U:10.808 seconds]
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