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    SubjectRe: How to map high memory for block io
    On Fri, Jan 27, 2006 at 09:04:33PM +0100, Pierre Ossman wrote:
    > Russell King wrote:
    > > I don't see what the problem is. A sg entry is a list of struct page
    > > pointers, an offset, and a size. As such, it can't describe a transfer
    > > which crosses a page because such a structure does not imply that one
    > > struct page follows another struct page.
    >
    > If the pages do not strictly follow each other then there is a lot of
    > broken code in the kernel. drivers/mmc/mmci.c and drivers/block/ub.c
    > being two occurences since both assume they can access the entire entry
    > through a single mapping.

    We don't make that assumption. What we do is:

    - map the current sg using kmap_atomic()
    - copy up to sg->length into or out of that mapping
    - unmap current sg
    - if we have reached the end of this sg, move on to the next

    What this means is that we assume sg->offset + sg->length <= PAGE_SIZE
    in all cases, which is the same assumption architecture DMA code makes.
    If that's invalid, there's likely to be a lot of architecture DMA support
    which is broken.

    --
    Russell King
    Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
    maintainer of: 2.6 Serial core
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