Re: [PATCH v12 11/31] documentation: iommu: add binding document of Exynos System MMU

[Thierry Reding <thierry.reding@gmail.com>]

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On Sun, Apr 27, 2014 at 08:23:06PM +0200, Arnd Bergmann wrote:
> On Sunday 27 April 2014 13:07:43 Shaik Ameer Basha wrote:
> > +- mmu-masters: A phandle to device nodes representing the master for which
> > +               the System MMU can provide a translation. Any additional values
> > +              after the phandle will be ignored because a System MMU never
> > +              have two or more masters. "#stream-id-cells" specified in the
> > +              master's node will be also ignored.
> > +              If more than one phandle is specified, only the first phandle
> > +              will be treated.
> 
> This seems completely backwards: Why would you list the masters for an IOMMU
> in the IOMMU node?
> 
> The master should have a standard property pointing to the IOMMU instead.
> 
> We don't have a generic binding for IOMMUs yet it seems, but the time is
> overdue to make one.
> 
> Consider this NAKed until there is a generic binding for IOMMUs that all
> relevant developers have agreed to.

I'd like to take this opportunity and revive one of the hibernating
patch sets that we have for Tegra. The last effort to get things merged
was back in January I think. I haven't bothered to look up the reference
since it's probably good to start from scratch anyway.

The latest version of the binding that was under discussion back then I
think looked something like this:

	device@... {
		iommus = <&iommu [spec]>[, <&other_iommu [other_spec]>...];
	};

And possibly with a iommu-names property to go along with that. The idea
being that a device can be a master on possibly multiple IOMMUs. Using
the above it would also be possible to have one device be multiple
masters on the same IOMMU.

On Tegra the specifier would be used to encode a memory controller's
client ID. One discussion point back at the time was to encode the ID as
a bitmask to allow more than a single master per entry. Another solution
which I think is a little cleaner and more generic, would be to use one
entry per master and use a single cell to encode the client ID. Devices
with multiple clients to the same IOMMU could then use multiple entries
referencing the same IOMMU.

I've added Hiroshi Doyu on Cc since he knows the Tegra IOMMU best.
Hiroshi, can you summarize exactly what the proposed bindings were. If
my memory serves me well they were mostly along the lines of what Arnd
proposes here, and perhaps they are something that can also be used for
Exynos.

Will Deacon (I think) had some comments on the earlier discussion as
well, so I've added him on Cc for visibility. Sorry if I'm confusing you
with someone else, Will. In that case perhaps you know who to include in
the discussion from the ARM side.

Also adding Stephen Warren for visibility.

Thierry

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