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    SubjectRe: [RFC Patch] use MTRR for write combining if PAT is not available
    Suresh Siddha wrote:
    > If we are going to make ioremap() and set_memory_wc() add mtrr's in
    > non-pat case, then we need to delete the added mtrr(s) in the
    > corresponding iounmap() and set_memory_wb() aswell.
    >
    > hmm, this is becoming too complex. The way i915 and other graphics
    > drivers are using set_memory_wc(), it is def a bad idea to start adding
    > mtrr's behind the back for non-pat case.

    Yes, maybe it's better to drop it for ioremap() and set_memory_wc(). But I'd like
    to keep it for mmapping the PCI region. It should help all the people with
    PAT-incapable CPUs and graphics chips without DRM support (for them there
    simply is no driver that should set up the MTRR entries...).

    > Can't we just force PAT option always and we probably don't care about
    > ioremap_wc() on processors were PAT doesn't get enabled because of known
    > errata.

    I don't think this is a good idea, Robert Hancock wrote there may be millions of
    such Laptops (Core Solo/Duo erratum AE7, Pentium M erratum Y31) :
    http://marc.info/?l=linux-kernel&m=125537136105246&w=2

    > or Perhaps just try to add mtrr only for the pci mmap case like the 4th
    > patch in this series..

    I'd prefer this! ;-)

    Kind regards,
    Thomas


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