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    SubjectRe: Cache incoherencies
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    Alan Cox wrote:
    > > > And the rest of that 4Mb range is loaded with critical data now running
    > > > uncached. Your box crawls like a 286.
    > >
    > > But you access it through the other window, which is still cached.
    >
    > Which as Russell says means you hack vmalloc().

    Back to the start.

    get_free_page now handles two different types of memory. DMA-able and
    non-DMA. Currently that suffices. The time will come pretty soon when
    however this is no longer sufficient. For example we'll be confronted
    with a 32-bit PCI card which can DMA into the lower 4G of memory pretty
    quickly.

    We're in the process of getting "> 2G mem" support. This adds another
    class: bigmem.

    I think we should take a step back, abstract the GFP mechansism to the
    point where it can satisfy a "gimmy any page" request, which
    technically turns into "preferably a page from non-DMA memory, but if
    you're out of those, give me a DMA-able page".

    A driver wanting to do DMA will request "a DMA-ABLE page" (with either
    the 16M ISA limit, the 1M XT limit, or the 4G PCI limit!), with "if
    you're out of those, give up".

    I think that GFP may need to be redesigned one time or another.
    However, once that is done, taking along things as "uncached" pages,
    should become a breeze. Yes, the current implementation makes it a
    quick hack to vmalloc.

    I think that generalizing gfp is a good idea in the end.


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    ** R.E.Wolff@BitWizard.nl ** http://www.BitWizard.nl/ ** +31-15-2137555 **
    *-- BitWizard writes Linux device drivers for any device you may have! --*
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