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    SubjectRe: pci-gart_64.c: could we get better explanation?
    On Tue 2008-05-27 05:51:39, Benjamin Herrenschmidt wrote:
    >
    > On Mon, 2008-05-26 at 20:47 +0200, Pavel Machek wrote:
    > > Hi!
    > >
    > > +++ b/arch/x86/kernel/pci-gart_64.c
    > > @@ -774,6 +778,7 @@ #endif
    > >
    > > /*
    > > * Try to workaround a bug (thanks to BenH)
    > > * Set unmapped entries to a scratch page instead of 0.
    > > * Any prefetches that hit unmapped entries won't get an bus abort
    > >
    > > Could someone explain? Which bug is being worked around? CPUs
    > > prefetching over NULL?
    >
    > P2P bridge prefetching on DMA reads.

    ---

    Add better explanation to pci-gart.

    Signed-off-by: Pavel Machek <pavel@suse.cz>

    diff --git a/arch/x86/kernel/pci-gart_64.c b/arch/x86/kernel/pci-gart_64.c
    index 838a1d3..e72e583 100644
    --- a/arch/x86/kernel/pci-gart_64.c
    +++ b/arch/x86/kernel/pci-gart_64.c
    @@ -800,10 +800,10 @@ #endif
    wbinvd();

    /*
    - * Try to workaround a bug (thanks to BenH)
    + * Try to workaround a bug (thanks to BenH):
    * Set unmapped entries to a scratch page instead of 0.
    * Any prefetches that hit unmapped entries won't get an bus abort
    - * then.
    + * then. (P2P bridge may be prefetching on DMA reads).
    */
    scratch = get_zeroed_page(GFP_KERNEL);
    if (!scratch)
    --
    (english) http://www.livejournal.com/~pavelmachek
    (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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