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    SubjectRe: spin_unlock optimization(i386)

    On Fri, 26 Nov 1999, Linus Torvalds wrote:

    > > i believe this still doesnt turn off store forwarding. If two PCI commands
    > > are written to the same register then they might get 'merged', resulting
    > > in the first written command to be lost. This usually doesnt happen, but
    > > can happen, and does happen in some cases.
    >
    > No, the MTRR's turn off store forwarding too.

    ok. Then having ioremap_nocache() is completely pointless, because the
    MTRRs already say that those areas are UC. (this is a different issue, not
    correctness but just interface cleanliness)

    > How do we know? Think about DOS. It doesn't _have_ page tables, it depends
    > entirely on the MTRR's to do the right thing wrt IO memory.

    ok.

    > way to feed a networking card is to map a large buffer in PCI memory space
    > and then use the MTRR's to mark it write combining but uncached (whatever
    > intel calls that combination). That way the core will write combine and
    > burst PCI writes, which is what you need for top performance.

    The interface will likely be an ioremap_relaxed() done by the driver? Then
    we could have ioremap() which does the UC thing we always did. And remove
    ioremap_nocache().

    -- mingo


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