linux-kernel - Re: [PATCH 4/8] dma-buf: add peer2peer flag

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Date:   Fri, 20 Apr 2018 05:46:25 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Christian König <christian.koenig@....com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jerome Glisse <jglisse@...hat.com>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 4/8] dma-buf: add peer2peer flag

On Fri, Apr 20, 2018 at 12:44:01PM +0200, Christian König 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.