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    SubjectRe: [PATCH] usb: fix number of mapped SG DMA entries
    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?

    Documentation/DMA-API.txt says:
    | Returns: the number of physical segments mapped (this may be shorter
    | than <nents> passed in if some elements of the scatter/gather list are
    | physically or virtually adjacent and an IOMMU maps them with a single
    | entry).

    > Is there anything special you do to get into this kind of situation?

    It appears that enabling CONFIG_DMA_API_DEBUG is quite special. :o)

    > Like 16GiB of memory on 32bit with highmem and a brutal disk/io test
    > case so?

    Plain x86-64; very few file operations done by hand.


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