On Thu, Jul 10, 2014 at 11:57:38AM +0100, Thierry Reding wrote:

On Thu, Jul 10, 2014 at 11:23:34AM +0100, Will Deacon wrote:

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On Thu, Jul 10, 2014 at 10:49:10AM +0100, Thierry Reding wrote:

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On Wed, Jul 09, 2014 at 07:10:48PM +0100, Will Deacon wrote:

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On Wed, Jul 09, 2014 at 03:21:27PM +0100, Thierry Reding wrote:

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Anything beyond that (e.g. logical grouping of masters) isn't directly
within the scope of the binding (it doesn't describe hardware but some
policy pertaining to some specific use-case).

This *is* for hardware. I can use PCI as an example, but this could equally
apply to other types of bus. If you have a bunch of PCI master devices
sitting being a non-transparent bridge, they can end up sharing the same
master device ID (requester ID). This means that there is no way in the
IOMMU to initialise a translation for one of these devices without also
affecting the others. We currently have iommu_groups to deal with this, but
it *is* a property of the hardware and we absolutely need a way to describe
it. I'm happy to add it later, but we need to think about it now to avoid
merging something that can't easily be extended.

For PCI, the topology is probable but even then, we need this information to
describe the resulting master device ID emitted by the bridge for the
upstream group. One way to do this with your binding would be to treat all
of the upstream masters as having the same device ID.

Yes, I think that makes most sense. After all from the IOMMU's point of
view requests from all devices behind the bridge will originate from the
same ID.

So technically it's not really correct to encode the master ID within
each of the devices, but rather they should be inheriting the ID from
the non-transparent bridge.

Indeed. Is that possible with your binding, or would we just duplicate the
IDs between the masters?

No, the binding only describes direct relationships between the IOMMU
and masters. There's no way to translate them inbetween or inherit them.

[...]

? That way some code could walk up the IOMMU tree to resolve this. Or
perhaps even easier:

	iommu {
		#iommu-cells = <1>;
	};

	bridge {
		iommus = <&/iommu 42>;

		device at 0 {
			...
		};

		device at 1 {
			...
		};

		...
	};

Yes, I like that. Good thinking!

And we could enhance the binding by defining that the iommus node is
inherited by devices on a bus, which by what you're saying would be the
sensible thing to do anyway.

In the second example above, the presence of an iommus property in the
bridge would indicate that it's non-transparent regarding IOMMU
translation and therefore the master ID should be inherited. Devices
could still override by providing their own iommus property, though I'd
be a little surprised if there ever was hardware like that.

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With virtualisation, we may want to assign a group of devices to a guest but
without emulating the bridge. This would need something the device-tree to
describe that they are grouped together.

But that's also a software decision, isn't it? Virtualization doesn't
have anything to do with the hardware description. Or am I missing
something? Of course I guess you could generate a DTB for the guest and
group device together, in which case you're pretty much free to do what
you want since you're essentially defining your own hardware.

If you're doing device passthrough and you want to allow the guest to
program the IOMMU, I think that virtualisation is directly related to the
hardware description, since the guest will be bound by physical properties
of the system.

Evidently you know much better what the requirements are here and what
will actually be required. I guess we'll need to have more discussions
along with examples of use-cases.

It's still early days for getting this stuff up and running on ARM, so I
agree that we'll have to come back to it a few times once we've got concrete
examples and code.

Will