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SubjectRe: [PATCH] usb: fix number of mapped SG DMA entries
Alan Stern wrote:
> 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?

CONFIG_DMA_API_DEBUG; its help warns of performance degradation.


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