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

[Alan Stern]

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?

Alan Stern

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