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    SubjectRe: PCI DMA to small buffers on cache-incoherent arch
       From: Roland Dreier <>
    Date: 08 Jun 2002 13:38:53 -0700

    The USB stack was doing PCI DMA into buffers that were
    allocated on the stack

    This is illegal.

    which causes stack corruption: on the PPC
    440GP, pci_map_single() with PCI_DMA_FROMDEVICE just invalidates the
    cache for the region being mapped. However, if a buffer is smaller
    than a cache line, then two bad things can happen.

    There is no allocation scheme legal for PCI DMA which gives you
    smaller than a cacheline of data, this includes SLAB. This is why
    stack buffers and the like are illegal for PCI DMA.

    The solution to this was simply to use kmalloc() to allocate buffers
    instead of using automatic variables.


    However, this leads to my first question: is this safe on all

    It must be. If the architecture allows SLAB to give smaller than
    cacheline sized data, it must handle PCI DMA map/unmap flushing
    in an appropriate fashion (ie. handle sub-cacheline buffers).

    DMA-mapping.txt says kmalloc()'ed memory is OK for DMAs and does not
    mention cache alignment.

    It doesn't mention cache alignment because that is an implementation
    specific issue. The user of the interfaces need not be concerned
    about any of this.

    There need be no changes to the documentation. If you do as the
    documentation states (use kmalloc or get_free_page to get your
    buffers) then it will just work.

    Franks a lot,
    David S. Miller
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