Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

[Thierry Reding]

On Sun, Jun 01, 2014 at 10:55:46AM +0100, Will Deacon wrote:
> 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.

Maybe I don't fully understand, but since we leave it up to the IOMMU
binding to define the exact meaning of #iommu-cells, can't you simply
use that to your advantage and define something like:

	iommus = <&smmu 0 7>;

to mean IDs 0 to 7 for this particular IOMMU type?

Thierry

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