Report from 2013 ARM kernel summit

[Will Deacon]

On Tue, Nov 19, 2013 at 08:45:02PM +0000, Rob Herring wrote:
> On 11/19/2013 11:35 AM, Will Deacon wrote:
> > Adding Andreas and Rob for input on potential binding additions to the SMMU.
> 
> The above proposal would be an incompatible change. However, I think we
> could still deal with a change in this binding at this stage.
> 
> One way approach to handle this without changing the binding would be to
> scan the DT for all iommu's up front and create a list of all nodes and
> their iommu parent. The fact that the hierarchy is described in a way
> that doesn't fit Linux well is really a Linux implementation detail.
> 
> If changing the binding, a simple approach would be to allow
> 'smmu-parent' to be a bus and/or device property and not just for
> chained iommu's. This could be a global or bus property that is
> inherited. Like interrupt-parent, you would have to deal with the parent
> being itself. Also, perhaps iommu-parent would be a better name. In any
> case, I'd like to see this all be a generic iommu binding.

I like that idea. I've recently been toying with removing the chained IOMMU
support, since I don't think anybody is using it who is interested in
mainline. However, making it more general sounds like a better idea.

One potential issue is that I think the nvidia guys want to describe masters
that master via multiple SMMUs (which I believe was the motivation for
moving the stream-ids out into the master nodes, rather than keeping them in
the SMMU). Again, that's not something we can easily add to the arm-smmu,
because the incoming stream-ids are a property of the SMMU node.

So the question is: do we actually need to describe masters that master
through multiple SMMUs as a single node in the devicetree?

Will