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From: Arnd Bergmann <arnd@arndb.de>
To: Hiroshi Doyu <hdoyu@nvidia.com>
Cc: Rob Herring <robherring2@gmail.com>,
        Thierry Reding <thierry.reding@gmail.com>,
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Subject: Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings
Date: Fri, 30 May 2014 21:54:37 +0200
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On Friday 30 May 2014 22:29:13 Hiroshi Doyu wrote:
> 
> IIUC the original problem, "a master with 8 streamIDs" means something
> like below, where some devices have multiple IDs but some have a
> single. A sinle #address-cells cannot afford those 2 masters at once.
> 
>        iommu {
>                /* the specifier represents the ID of the master */
>                #address-cells = <1>;
>                #size-cells = <0>;
>        };
> 
>         master@a {
>                 ...
>                 iommus = <&smmu 1 2 3>; # 3 IDs
>         };
> 
>         master@b {
>                 ...
>                 iommus = <&smmu 4>;     # 1 ID
>         };

This would not be the usual format really. It should instead be

		iommus = <&smmu 1>, <&smmu 2>, <&smmu 3>;

which can be tedious to type.

> 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.

	Arnd