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

Arnd Bergmann
2014-05-30T19:54:37Z

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