Hi Stuart,

On Mon, Jun 16, 2014 at 05:56:32PM +0100, Stuart Yoder wrote:

quoted

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