Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Mon, Feb 02, 2015 at 03:30:21PM -0500, Rob Clark wrote:
> On Mon, Feb 2, 2015 at 11:54 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
> >> My initial thought is for dma-buf to not try to prevent something than
> >> an exporter can actually do.. I think the scenario you describe could
> >> be handled by two sg-lists, if the exporter was clever enough.
> >
> > That's already needed, each attachment has it's own sg-list. After all
> > there's no array of dma_addr_t in the sg tables, so you can't use one sg
> > for more than one mapping. And due to different iommu different devices
> > can easily end up with different addresses.
> 
> 
> Well, to be fair it may not be explicitly stated, but currently one
> should assume the dma_addr_t's in the dmabuf sglist are bogus.  With
> gpu's that implement per-process/context page tables, I'm not really
> sure that there is a sane way to actually do anything else..

That's incorrect - and goes dead against the design of scatterlists.

Not only that, but it is entirely possible that you may get handed
memory via dmabufs for which there are no struct page's associated
with that memory - think about display systems which have their own
video memory which is accessible to the GPU, but it isn't system
memory.

In those circumstances, you have to use the dma_addr_t's and not the
pages.

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