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Date:   Fri, 20 Apr 2018 05:46:25 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Jerome Glisse <jglisse@redhat.com>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
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        Logan Gunthorpe <logang@deltatee.com>,
        Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH 4/8] dma-buf: add peer2peer flag
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On Fri, Apr 20, 2018 at 12:44:01PM +0200, Christian K?nig wrote:
> > > What we need is an sg_alloc_table_from_resources(dev, resources,
> > > num_resources) which does the handling common to all drivers.
> > A structure that contains
> > 
> > {page,offset,len} + {dma_addr+dma_len}
> > 
> > is not a good container for storing
> > 
> > {virt addr, dma_addr, len}
> > 
> > no matter what interface you build arond it.
> 
> Why not? I mean at least for my use case we actually don't need the virtual
> address.

If you don't need the virtual address you need scatterlist even list.

> What we need is {dma_addr+dma_len} in a consistent interface which can come
> from both {page,offset,len} as well as {resource, len}.

Ok.

> What I actually don't need is separate handling for system memory and
> resources, but that would we get exactly when we don't use sg_table.

At the very lowest level they will need to be handled differently for
many architectures, the questions is at what point we'll do the
branching out.