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    SubjectRe: Adding kmap_atomic_prot_pfn (was: [git pull] drm patches for 2.6.27-rc1)
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    On Thu, 2008-10-23 at 14:24 -0700, Linus Torvalds wrote:

    > The whole point of that function has absolutely nothing to do with
    > highmem, and it *must* be useful on non-highmem configurations to be
    > appropriate.

    Right, I'd just like my io_mapping_map_atomic_wc to be able to rapidly
    map random pages from my PCI BAR in WC mode. The code in your tree uses
    kmap_atomic_pfn, which does the mapping, but use the wrong prot bits. On
    a system with MTRR failure, this all ends badly, with factors of 20
    performance drops for copying data from the CPU to the aperture.

    > So I'd much rather create a new <linux/kmap.h> or something. Or just
    > expose this from to <asm/fixmap.h> or something. Let's not confuse this
    > with highmem, even if the implementation _historically_ was due to that.

    Sure, we readily admit that we're abusing the highmem API. So we wrapped
    that abusive code in a pretty package that can be re-implemented however
    you desire.

    How (and when) would you like to see this fixed?

    --
    keith.packard@intel.com
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