Re: [PATCH v4] devicetree: Add generic IOMMU device tree bindings

Will Deacon
2014-07-10T10:23:34Z

On Thu, Jul 10, 2014 at 10:49:10AM +0100, Thierry Reding wrote:
> On Wed, Jul 09, 2014 at 07:10:48PM +0100, Will Deacon wrote:
> > On Wed, Jul 09, 2014 at 03:21:27PM +0100, Thierry Reding wrote:
> > > 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?

> > 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.

Will