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

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Date:	Sun, 4 Dec 2011 11:59:32 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Clemens Ladisch <clemens@...isch.de>
cc:	Greg Kroah-Hartman <gregkh@...e.de>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: fix number of mapped SG DMA entries

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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