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    SubjectRe: [1/3] Add 4GB DMA32 zone
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    On Llu, 2005-09-12 at 12:42 +0200, Andi Kleen wrote:
    > (I cannot in fact remember a report of someone running especially
    > into this problem) And the cards seem to be essentially dead in the market
    > now. So it's really more a theoretical problem than a practical one.
    > [Proof of it: the current sources don't seem to handle it, so
    > it cannot be that bad ;-]

    Current sources don't handle DMA32, so that can't be bad either. Can we
    stick to sensible discussion, its quicker.

    There have been various reports over time, quite a few early on when we
    had a long series of on list discussions trying to debug what appeared
    to be an iommu bug but was in fact a kernel bug.

    You hit it on any size AMD64 with iommu, but since most aacraid users
    are intel boxes it doesn't hurt too many, and the rest all know about
    turning the iommu off on the box (but that hurts the rest), or just run
    something else because Linux "doesn't work".

    > That is why I essentially ignored the b44. AFAIK the driver
    > has a GFP_DMA bounce workaround anyways, so it would work
    > anyways.

    Usually - the DMA zone at 16MB is too small so allocations sometimes
    fail. It btw would want 1GB limits.

    > Yes I know some soundcards have similar limits, but for all
    > these we still have GFP_DMA and they always have been quite happy
    > with that.

    No current shipping card, also those that need it typically need small
    amounts (they'll live with 8K)

    > > Old aacraid actually cannot use IOMMU. It isn't alone in that
    > > limitation. Most hardware that has a 30/31bit limit can't go via the
    > > IOMMU because IOMMU space appears on the bus above 2GB so is itself
    > > invisible to the hardware.
    >
    > Yes, true. Use GFP_DMA then.

    Doesn't work. The DMA area is way too small with all the users already
    and the aacraid can and does want a lot of outstanding I/O. Using a 1GB
    or 2GB boundary line hurts nobody doing "allocate me some memory below
    4Gb" because nobody asks for .5GB chunks. A 4GB zone means we need to
    either increase the 16MB zone or add yet another one.


    Alan

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