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SubjectRe: [PATCH] usb: fix number of mapped SG DMA entries
On Thu, 15 Dec 2011, 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?

I don't know exactly what happened in this particular case -- probably
there's no way to find out. However, all the data is supposed to be
mapped, regardless of the number of entries.

> Is there anything special you do to get into this kind of situation?
> Like 16GiB of memory on 32bit with highmem and a brutal disk/io test
> case so?

As I understand it, the typical case is that two SG entries in a row
refer to adjacent pages of physical memory. The mapping routine then
collapses them into a single entry referring to all the pages.

However, this is not the sort of thing you can deliberately cause,
unless you set up your SG list by hand.

Alan Stern

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