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Date: Sun, 1 Jun 2014 10:55:46 +0100
From: Will Deacon <will.deacon@arm.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Hiroshi Doyu <hdoyu@nvidia.com>, Rob Herring <robherring2@gmail.com>,
        Thierry Reding <thierry.reding@gmail.com>,
        Rob Herring <robh+dt@kernel.org>, Pawel Moll <Pawel.Moll@arm.com>,
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        Ian Campbell <ijc+devicetree@hellion.org.uk>,
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        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
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Subject: Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings
Message-ID: <20140601095546.GA8625@arm.com>
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On Fri, May 30, 2014 at 08:54:37PM +0100, Arnd Bergmann wrote:
> On Friday 30 May 2014 22:29:13 Hiroshi Doyu wrote:
> > Tegra,SMMU has a similar problem and we have used a fixed size bitmap(64
> > bit) to afford 64 stream IDs so that a single device can hold multiple
> > IDs. If we apply the same bitmap to the above exmaple:
> > 
> >        iommu {
> >                /* the specifier represents the ID of the master */
> >                #address-cells = <1>;
> >                #size-cells = <0>;
> >        };
> > 
> >         master@a {
> >                 ...
> >                 iommus = <&smmu (BIT(1) | BIT(2) | BIT(3))>; # IDs 1 2 3
> >         };
> > 
> >         master@b {
> >                 ...
> >                 iommus = <&smmu BIT(4)>;     # ID 4
> >         };
> > 
> > The disadvantage of this is that this limits the max number of streamIDs
> > to support. If # of streamID is increased later more than 64, this
> > format cannot cover any more. You have to predict the max # of streamIDs
> > in advance if steamID is statically assigned.
> > 
> 
> Well, the iommu specific binding could allow a variable #address-cells.
> That way, you just need to know the number of stream IDs for that instance
> of the iommu.

In general, though, the SMMU will be able to support a large number of
stream IDs (say a 16-bit space). The restriction we're interested in here is
how many different stream IDs can be emitted by a single master device
coming into the SMMU. *That* is a property of the master, not the SMMU.

In the current arm,smmu binding I have a #stream-id-cells property in each
master. I can then feed that straight into of_parse_phandle_with_args to
enumerate the IDs for that master. The problem with that is we're
artificially restricted by MAX_PHANDLE_ARGS.

Will