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

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Date:	Sun, 04 Dec 2011 18:04:42 +0100
From:	Clemens Ladisch <clemens@...isch.de>
To:	Alan Stern <stern@...land.harvard.edu>
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

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.


Regards,
Clemens
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