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

[Russell King - ARM Linux]

On Tue, Feb 03, 2015 at 11:22:01AM -0500, Rob Clark wrote:
> On Tue, Feb 3, 2015 at 11:12 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > I agree for the case you are describing here. From what I understood
> > from Rob was that he is looking at something more like:
> >
> > Fig 3
> > CPU--L1cache--L2cache--Memory--IOMMU---<iobus>--device
> >
> > where the IOMMU controls one or more contexts per device, and is
> > shared across GPU and non-GPU devices. Here, we need to use the
> > dmap-mapping interface to set up the IO page table for any device
> > that is unable to address all of system RAM, and we can use it
> > for purposes like isolation of the devices. There are also cases
> > where using the IOMMU is not optional.
> 
> 
> Actually, just to clarify, the IOMMU instance is specific to the GPU..
> not shared with other devices.  Otherwise managing multiple contexts
> would go quite badly..
> 
> But other devices have their own instance of the same IOMMU.. so same
> driver could be used.

Okay, so that is my Fig.2 case, and we don't have to worry about Fig.3.

One thing I forgot in Fig.1/2 which my original did have were to mark
the system MMU as optional.  (Think an ARM64 with SMMU into a 32-bit
peripheral bus.)  Do we support stacked MMUs in the DMA API?  We may
need to if we keep IOMMUs in the DMA API.

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