From: Will Deacon <will.deacon@arm.com>
To: Thierry Reding <thierry.reding@gmail.com>
Cc: Rob Herring <robh+dt@kernel.org>, Pawel Moll <Pawel.Moll@arm.com>,
Mark Rutland <Mark.Rutland@arm.com>,
Ian Campbell <ijc+devicetree@hellion.org.uk>,
Kumar Gala <galak@codeaurora.org>,
Stephen Warren <swarren@wwwdotorg.org>,
Arnd Bergmann <arnd@arndb.de>, Joerg Roedel <joro@8bytes.org>,
Cho KyongHo <pullip.cho@samsung.com>,
Grant Grundler <grundler@chromium.org>,
Dave P Martin <Dave.Martin@arm.com>,
Marc Zyngier <Marc.Zyngier@arm.com>,
Hiroshi Doyu <hdoyu@nvidia.com>,
Olav Haugan <ohaugan@codeaurora.org>,
Varun Sethi <varun.sethi@freescale.com>,
"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
"iommu@lists.linux-foundation.org"
<iommu@lists.linux-foundation.org>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
"linux-tegra@vger.kernel.org" <linux-tegra@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4] devicetree: Add generic IOMMU device tree bindings
Date: Thu, 10 Jul 2014 13:38:24 +0100 [thread overview]
Message-ID: <20140710123824.GN2449@arm.com> (raw)
In-Reply-To: <20140710105737.GD21583@ulmo>
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:
> > 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?
>
> 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@0 {
> ...
> };
>
> device@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.
>
> > > > 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
next prev parent reply other threads:[~2014-07-10 12:38 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-07-04 15:29 [PATCH v4] devicetree: Add generic IOMMU device tree bindings Thierry Reding
2014-07-09 13:40 ` Will Deacon
2014-07-09 14:21 ` Thierry Reding
2014-07-09 18:10 ` Will Deacon
2014-07-10 9:49 ` Thierry Reding
2014-07-10 10:23 ` Will Deacon
2014-07-10 10:57 ` Thierry Reding
2014-07-10 12:38 ` Will Deacon [this message]
2014-07-11 20:55 ` Rob Clark
2014-07-12 9:39 ` Will Deacon
2014-07-12 11:26 ` Rob Clark
2014-07-12 12:22 ` Arnd Bergmann
2014-07-12 12:57 ` Rob Clark
2014-07-13 9:43 ` Will Deacon
2014-07-13 11:43 ` Rob Clark
2014-07-16 1:25 ` Olav Haugan
2014-07-16 10:10 ` Will Deacon
2014-07-16 20:24 ` Rob Clark
2014-07-14 6:44 ` Thierry Reding
2014-07-14 10:08 ` Will Deacon
2014-07-14 6:24 ` Thierry Reding
2014-07-14 10:13 ` Rob Clark
2014-07-14 6:15 ` Thierry Reding
2014-07-30 11:04 ` Will Deacon
2014-07-30 13:23 ` Thierry Reding
2014-07-30 13:33 ` Joerg Roedel
2014-07-30 17:37 ` Olof Johansson
2014-07-30 14:30 ` Will Deacon
2014-07-30 18:08 ` Rob Herring
2014-07-30 20:11 ` Arnd Bergmann
2014-07-30 15:26 ` Mark Rutland
2014-07-30 17:35 ` Olof Johansson
2014-07-30 18:18 ` Mark Rutland
2014-07-31 10:09 ` Thierry Reding
2014-07-31 10:50 ` Mark Rutland
2014-07-31 11:14 ` Thierry Reding
2014-07-31 9:51 ` Thierry Reding
2014-07-31 8:39 ` Thierry Reding
2014-07-31 9:22 ` Mark Rutland
2014-07-31 10:18 ` Thierry Reding
2014-07-31 10:23 ` Joerg Roedel
2014-07-31 10:46 ` Thierry Reding
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