Re: [PATCH 6/6] IB/srp: Fix srp_map_sg_fr()

[Sagi Grimberg <sagig-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org>]

On 01/12/2015 20:39, Bart Van Assche wrote:
> On 12/01/2015 10:35 AM, Sagi Grimberg wrote:
>>> After dma_map_sg() has been called the return value of that function
>>> must be used as the number of elements in the scatterlist instead of
>>> scsi_sg_count().
>>
>> Umm, but ib_map_mr_sg iterates on the sg list. Say you have sg_nents=3
>> and after mapping you got dma_nents=2 (2 entries are contig) and you
>> pass that, ib_sg_to_pages will only iterate on 2 elements won't it? am
>> I missing something?
>
> Hello Sagi,
>
>  From https://www.kernel.org/doc/Documentation/DMA-API-HOWTO.txt:
>
> <quote>
> With scatterlists, you map a region gathered from several regions by:
>
>      int i, count = dma_map_sg(dev, sglist, nents, direction);
>      struct scatterlist *sg;
>
>      for_each_sg(sglist, sg, count, i) {
>          hw_address[i] = sg_dma_address(sg);
>          hw_len[i] = sg_dma_len(sg);
>      }
>
> where nents is the number of entries in the sglist.
> </quote>

 From Documentation/DMA_API.txt

<quote>
         int
         dma_map_sg(struct device *dev, struct scatterlist *sg,
                 int nents, enum dma_data_direction direction)

Returns: the number of DMA address 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).
</quote>
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