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    SubjectRe: Mmap device performance
    On Thu, Dec 11, 1997 at 06:27:10PM +0000, Linus Torvalds wrote:

    > One thing to look out for is the "memory type registers" on the PPro and
    > the PII. Somebody had a kernel loadable module to read and set the
    > registers, it may be that your windows setup has a different memory type
    > setup than the Linux setup.
    >
    > Similarly, when you do the mapping, look into the effects of the "cache
    > disable" and "write-through" bits in the page tables. You mention that
    > you copied the code from the /dev/mem map code, and that code does:
    >
    > #if defined(__i386__)
    > /*
    > * hmm.. This disables high-memory caching, as the XFree86 team
    > * wondered about that at one time.
    > * The surround logic should disable caching for the high device
    > * addresses anyway, but right now this seems still needed.
    > */
    > if (x86 > 3 && offset >= __pa(high_memory))
    > pgprot_val(vma->vm_page_prot) |= _PAGE_PCD;
    > #endif
    >
    > the code _shouldn't_ make any difference (as per the comment), but
    > depending on how the card works it might.

    I'll have to add a syscall cachectl() make the caching of a memory area
    controlable for MIPS. Will that one also be useful for Intel (or other
    architectures)?

    Ralf

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