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

[Will Deacon <will.deacon@arm.com>]

Hi Stuart,

On Mon, Jun 16, 2014 at 05:56:32PM +0100, Stuart Yoder wrote:
> > Do you have use-cases where you really need to change these mappings
> > dynamically?
> 
> Yes.  In the case of a PCI bus-- you may not know in advance how many
> PCI devices there are until you probe the bus.   We have another FSL
> proprietary bus we call the "fsl-mc" bus that is similar.

For that case, though, you could still describe an algorithmic
transformation from RequesterID to StreamID which corresponds to a fixed
mapping.

> Another thing to consider-- starting with SMMUv2, as you know, there
> is a new distributed architecture with multiple TBUs and a centralized
> TCU that walks the SMMU page tables.  So instead of sprinkling multiple
> SMMUs all over an SoC you now have the option a 1 central TCU and sprinkling
> multiple TBUs around.   However, this means that the stream ID namespace
> is now global and can be pretty limited.  In the SMMU implementation we 
> have there are only 64 stream ID total for our Soc.  But we have many more
> masters than that.
> 
> So we look at stream IDs as really corresponding to an 'isolation context'
> and not to a bus master.  An isolation context is the domain you are
> trying to isolate with the SMMU.  Devices that all belong to the same
> 'isolation context' can share the same stream ID, since they share
> the same domain and page tables.

Ok, this is more compelling.

> So, perhaps by default some/most SMMU masters may have a default stream ID
> of 0x0 that is used by the host...and that could be represented
> statically in the device tree.
> 
> But, we absolutely will need to dynamically set new stream IDs
> into masters when a new IOMMU 'domain' is created and devices
> are added to it.   All the devices in a domain will share
> the same stream ID.
> 
> So whatever we do, let's please have an architecture flexible enough
> to allow for this.

What is the software interface to the logic that assigns the StreamIDs? Is
it part of the SMMU, or a separate device (or set of devices)?

Will