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

[Clemens Ladisch <clemens@ladisch.de>]

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?

Documentation/DMA-API.txt says:
| Returns: the number of physical segments mapped (this may be shorter
| than <nents> passed in if some elements of the scatter/gather list are
| physically or virtually adjacent and an IOMMU maps them with a single
| entry).

> Is there anything special you do to get into this kind of situation?

It appears that enabling CONFIG_DMA_API_DEBUG is quite special.  :o)

> Like 16GiB of memory on 32bit with highmem and a brutal disk/io test
> case so?

Plain x86-64; very few file operations done by hand.


Regards,
Clemens