Re: [PATCH v5 1/1] iommu-api: Add map_sg/unmap_sg functions

[Olav Haugan <ohaugan-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

On 8/19/2014 9:11 AM, Laurent Pinchart wrote:
> On Tuesday 19 August 2014 13:59:54 Joerg Roedel wrote:
>> On Mon, Aug 18, 2014 at 03:47:56PM -0700, Olav Haugan wrote:
>>> If the alignment is not correct then iommu_map() will return error. Not
>>> sure what other option we have here (and why make it different behavior
>>> than iommu_map which just return error when it is not aligned properly).
>>> I don't think we want to force any kind of alignment automatically. I
>>> would rather have the API tell me I am doing something wrong than having
>>> the function aligning the values and possibly undermap or overmap.
>>
>> But sg->offset is an offset into the page (at least it is used that way
>> in the DMA-API and since you do 'page_len = s->offset + s->length' you
>> use it the same way).
>> So when you pass iova + offset the result will no longer be
>> page-aligned. You should force sg->offset == 0 and sg->length to be
>> page-aligned instead. This makes more sense because the IOMMU-API works
>> on (io)-page granularity and not on arbitrary phys-addr ranges like the
>> DMA-API.
>>
>>> Yes, I am aware of that. However, several people prefer this than
>>> passing in scatterlist. It is not very convenient to pass a scatterlist
>>> in some use cases. Someone mentioned a use case where they would have to
>>> create a dummy sg list and populate it with the iova just to do an
>>> unmap. I believe we would have to do this also. There is no use for
>>> sglist when unmapping. However, would like to keep separate API from
>>> iommu_unmap() to keep the API function names symmetric (map_sg/unmap_sg).
>>
>> Keeping it symetric is not more complicated, the caller just needs to
>> keep the sg-list used for mapping around. I prefer the unmap_sg call to
>> work in sg-lists too.
> 
> Do we have a use case where the unmap_sg() implementation would be different 
> than a plain iommu_unmap() call ? If not I'd rather remove unmap_sg() 
> completely.
> 
>>> I thought that was why we added the default fallback and set all the
>>> drivers to point to these fallback functions. Several people wanted this
>>> so that we don't have to have NULL-check in these functions (and have
>>> the functions be simple inline functions).
>>
>> Okay, since you add these call-backs to all drivers I think I can live
>> with not doing a pointer check here.
> 
> I suggested doing a
> 
> if (ops is not NULL)
> 	return ops();
> else
> 	return default_ops();
> 
> to avoid modifying all drivers. I'm not sure why that wasn't received with 
> much enthusiasm.
> 

Both Thierry R. and Konrad W. argued for modifying the drivers instead
so I implemented what the majority wanted. :-)


Olav

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