[RFC] Describing arbitrary bus mastering relationships in DT

[thierry.reding@gmail.com (Thierry Reding)]

On Fri, May 02, 2014 at 02:32:08PM +0200, Arnd Bergmann wrote:
> On Friday 02 May 2014 13:05:58 Thierry Reding wrote:
> > 
> > Let me see if I understood the above proposal by trying to translate it
> > into a simple example for a specific use-case. On Tegra for example we
> > have various units that can either access system memory directly or use
> > the IOMMU to translate accesses for them. One such unit would be the
> > display controller that scans out a framebuffer from memory.
> 
> Can you explain how the decision is made whether the IOMMU gets used
> or not? In all cases I've seen so far, I think we can hardwire this
> in DT, and only expose one or the other. Are both ways used
> concurrently?

It should be possible to hardcode this in DT for Tegra. As I understand
it, both interfaces can't be used at the same time. Once translation has
been enabled for one client, all accesses generated by that client will
be translated.

Hiroshi, please correct me if I'm wrong.

> >         dc at 0,54200000 {
> >                 ...
> > 
> >                 slave {
> >                         /*
> >                          * 2 is the memory controller client ID of the
> >                          * display controller.
> >                          */
> >                         iommu = <&iommu 2>;
> > 
> >                         ...
> >                 };
> >         };
> > 
> > Admittedly this is probably a lot more trivial than what you're looking
> > for. There's no need for virtualization here, the IOMMU is simply used
> > to isolate memory accesses by devices. Still it's a use-case that needs
> > to be supported and one that at least Tegra and Exynos have an immediate
> > need for.
> > 
> > So the above isn't much different from the proposed bindings, except
> > that the iommu property is now nested within a slave node. I guess this
> > gives us a lot more flexibility to extend the description of a slave as
> > needed to represent more complex scenarios.
> 
> This looks rather complicated to parse automatically in the generic
> DT code when we try to decide which dma_map_ops to use. We'd have
> to look for 'slave' nodes in each device we instatiate and then see
> if they use an iommu or not.

But we need to do that now anyway in order to find an iommu property,
don't we? Adding one extra level here shouldn't be all that bad if it
gives us more flexibility or uniformity with more complicated setups.

To some degree this also depends on how we want to handle IOMMUs. If
they should remain transparently handled via dma_map_ops, then it makes
sense to set this up at device instantiation time. But how can we handle
this in situations where one device needs to master on two IOMMUs at the
same time? Or if the device needs physically contiguous memory for
purposes other than device I/O. Using dma_map_ops we can't control which
allocations get mapped via the IOMMU and which don't.

> > Also, are slaves/slave-names and slave subnodes mutually exclusive? It
> > sounds like slaves/slave-names would be a specialization of the slave
> > subnode concept for the trivial cases. Would the following be an
> > equivalent description of the above example?
> > 
> >         dc at 0,54200000 {
> >                 ...
> > 
> >                 slaves = <&iommu 2>;
> >         };
> > 
> > I don't see how it could be exactly equivalent since it misses context
> > regarding the type of slave that's being interacted with. Perhaps that
> > could be solved by making that knowledge driver-specific (i.e. the
> > driver for the Tegra display controller will know that it can only be
> > the master on an IOMMU and therefore derive the slave type). Or the
> > slave's type could be derived from the slave-names property.
> 
> I'd rather have a device-specific property that tells the driver
> about things the iommu driver doesn't need to know but the master
> does. In most cases, we should be fine without a name attached to the
> slave.

For the easy cases where we either have no IOMMU or a single IOMMU per
device, that should work fine. This only becomes problematic when there
are more than one, since you need to distinguish between possibly more
than one type.

As I understand it, Dave's proposal is for generic bus masters, which
may be an IOMMU but could also be something completely different. So in
those cases we need extra meta information so that we can look up the
proper type of object.

FWIW, I do prefer a device-specific property as well. Though I also
think we should agree on common names (like "iommu") as part of the
binding definition. And then, even for devices that can use multiple
IOMMUs they can be differentiated using iommu-names for example. If
another type of master interface is required, then a different set of
property names can be used.

Thierry
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