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    SubjectRe: [PATCH] usb: fix number of mapped SG DMA entries
    On Thu, 15 Dec 2011, Sebastian Andrzej Siewior wrote:

    > * Alan Stern | 2011-12-04 11:59:32 [-0500]:
    > >On Sat, 3 Dec 2011, Clemens Ladisch wrote:
    > >
    > >> Add a new field num_mapped_sgs to struct urb so that we have a place to
    > >> store the number of mapped entries and can also retain the original
    > >> value of entries in num_sgs. Previously, usb_hcd_map_urb_for_dma()
    > >> would overwrite this with the number of mapped entries, which would
    > >> break dma_unmap_sg() because it requires the original number of entries.
    > >>
    > >> This fixes warnings like the following when using USB storage devices:
    > >> ------------[ cut here ]------------
    > >> WARNING: at lib/dma-debug.c:902 check_unmap+0x4e4/0x695()
    > >> ehci_hcd 0000:00:12.2: DMA-API: device driver frees DMA sg list with different entry count [map count=4] [unmap count=1]
    > >
    > >Interesting. Do you have any idea why this warning didn't show up
    > >earlier? Is there perhaps a Kconfig option I ought to be using?
    > My question here is why don't we map the complete sg list but only
    > one entry. What happens to the remaining few sg list entries? Don't we
    > sent too less data?

    I don't know exactly what happened in this particular case -- probably
    there's no way to find out. However, all the data is supposed to be
    mapped, regardless of the number of entries.

    > Is there anything special you do to get into this kind of situation?
    > Like 16GiB of memory on 32bit with highmem and a brutal disk/io test
    > case so?

    As I understand it, the typical case is that two SG entries in a row
    refer to adjacent pages of physical memory. The mapping routine then
    collapses them into a single entry referring to all the pages.

    However, this is not the sort of thing you can deliberately cause,
    unless you set up your SG list by hand.

    Alan Stern

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