Re: Report from 2013 ARM kernel summit

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

On Wed, Nov 20, 2013 at 08:02:10PM +0000, Rob Herring wrote:
> On Wed, Nov 20, 2013 at 12:39 AM, Hiroshi Doyu <hdoyu-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org> wrote:
> >   smmu_a: iommu@xxxxxxxx {
> >         #iommu-cells = <2>;
> >         ....
> >   };
> >
> >   smmu_b: iommu@xxxxxxxx {
> >         #iommu-cells = <3>;
> >         ....
> >   };
> >
> >   device_a {
> >          iommus = <&smmu_a param1 param2>,
> >                   <&smmu_b param1 param2 param3>;
> >   };
> >
> > This can describe the relation between a device and an iommu
> > independently. The number of params needed for each IOMMU can be
> > sepcified by #iommu-cells in its iommu entry.
> >
> >   device_a <-> smmu_a, needs 2 params for a device
> >   device_a <-> smmu_b, needs 3 params for a device
> >
> > For example, "smmu_a" can be an bus level global IOMMU where all child
> > devices can be an master of "smmu_a", and "smmu_b" is a local IOMMU
> > only for "device_a".
> >
> > "memory controller"---"smmu_a"---bus--+--"smmu_b"--"device_a"
> >                                       |
> >                                       |
> >                                       +--"device_b"
> 
> I think the above binding would be the correct way to describe things
> if you have 1 device connected to 2 IOMMUs (directly rather than
> chained). IIUC, that is something you have on tegra?
> 
> For the topology above where you are chaining iommu's, I think
> something like this is more accurately describing the hierarchy:
> 
>   smmu_b: iommu@xxxxxxxx {
>         #iommu-cells = <3>;
>          iommus = <&smmu_a param1 param2>;
>        ....
>   };
>   device_a {
>          iommus = <&smmu_b param1 param2 param3>;
>   };
> 
> I remember discussing this with Will and seem to recall some issue
> with describing things this way. But looking at it now, I don't see
> what that was.

I think it was the usual issue with StreamID remastering. For example, if
you had a device going into two chained IOMMUs, you need a way to relate the
StreamIDs of the device to the remastered IDs coming out of the first SMMU.

Will