Subject: Master-aware devices and sideband ID data

From: [email-masked] (Mark Rutland)
Date: Wed, 27 May 2015 18:39:53 +0100

On Tue, May 26, 2015 at 11:20:59PM +0100, Chalamarla, Tirumalesh wrote:
> This is some thing we also like to see in ITS and SMMU drivers. 
> > On Mar 24, 2015, at 8:50 AM, Mark Rutland <[email-masked]> wrote:
> > 
> > Hi all,
> > 
> > For some devices, identification of particular masters is critical to
> > their operation (e.g. IOMMUs, MSI controllers). The identity of masters
> > is determined from sideband signals on the bus, and it may or may not be
> > possible to probe and/or configure this information. Worse still, these
> > information may be rewritten by intermediate buses, so the
> > information for a given master may differ at different points in the bus
> > hierarchy.
> > 
> > We currently describe this master information for devices attached to
> > IOMMUs, with a master ID being encoded in the iommu-cells. However this
> > only covers the ID between the master and its IOMMU(s), and we don't
> > currently have a mechanism to describe the master IDs as they are seen
> > by devices beyond the IOMMU(s), or in the absence of any IOMMU(s).
> > 
> > The following are example of master IDs:
> > * PCIe Requester IDs (RIDs) (bus:device.function AKA BDF)
> > * SMMU Stream IDs (SIDs)
> > * GICv3 ITS Device IDs (DIDs)
> > 
> > In the case of a system with PCIe, SMMU and GICv3, the master IDs are
> > rewritten in a chain of RID=>SID=>DID, as in the following diagram:
> > 
> > +-------------+
> > | PCIe master |
> > +-------------+
> >    ||
> >    || Requester ID (RID)
> >    || Probeable from RC.
> >    \/
> > +-------------------+
> > | PCIe Root Complex |
> > +-------------------+
> >    ||
> >    || SMMU Stream ID (SID)
> >    || Derived from RID, static non-probeable mapping.
> >    \/
> > +--------------+
> > | SMMU (IOMMU) |
> > +--------------+
> >    ||
> >    || ITS Device ID (DID)
> >    || Derived from SID, static non-probeable mapping.
> >    \/
> > +----------------------------+
> > | GICv3 ITS (MSI controller) |
> > +----------------------------+
> > 
> > In simpler cases, you might have a simpler set of master ID
> > translations, e.g. just a DID:
> > 
> > +-----------------+
> > | Platform device |
> > +-----------------+
> >    ||
> >    || ITS Device ID (DID)
> >    || Hard-wired on the bus.
> >    \/
> > +----------------------------+
> > | GICv3 ITS (MSI controller) |
> > +----------------------------+
> > 
> > Ignoring current bindings for the moment, I can see how we can describe
> > this with a generic master id-binding, with master-id-map along the
> > lines of interrupt-map, with a tuple format like:
> > <child-id-base child-id-length parent parent-id-base>
> > 
> > For the PCIe example, this would look something like the following (with
> > properties omitted for brevity):
> > 
> > PCI: pci at af000000 {
> > 	...
> > 	
> > 	/* Requester ID of PCIe endpoints, implicit at runtime */
> > 	master-id-cells = <1>;
> > 
> > 	/* RIDS idmapped to SIDS @ SMMU */
> > 	master-id-map = <0 0x10000 &SMMU 0>;
> > }
> > 
> > SMMU: iommu at bf000000 {
> > 	...
> > 
> > 	/* SID, derived from RID */
> > 	master-id-cells = <1>;
> > 
> > 	/* 
> > 	 * For some reason the high bit of the ID was negated.
> > 	 */
> > 	master-id-map = <0x0000 0x8000 &ITS 0x0 0x8000>,
> > 	                <0x8000 0x8000 &ITS 0x0 0x0000>;
> > 
> > };
> > 
> > ITS: its at cf000000 {
> > 	...
> > 
> > 	/* DID, derived from SID */
> > 	master-id-cells = <2>;
> > 
> > 	/* 
> > 	 * Master traffic not propagated beyond this point, so no
> > 	 * master-id-ranges
> > 	 */
> > };
> 
> I think it is nice to have max IDs supported by masters. so that drivers can check and enforce.  

The set of IDs that we expect to transform (i.e. those masters use)
would be implicit in the first master-id-map from a master. In the PCI
example above, no master is expected to generate an ID outside of the
range 0-0x10000 inclusive (and that's all the SMMU would see).

For devices which are not hotpluggable, the nodes for those devices
would describe the specific set of IDs they use.

Generally, between a master and a slave there might not be one
contiguous set of valid IDs as these may be rewritten along the way (as
happens at the SMMU between the PCI root complex and the ITS in the
example above).

Which drivers do you think need this information? What exactly are they
trying to check and enforce?

> > For the simpler case, this would look something like:
> > 
> > DEV: device at af000000 {
> > 	master-id-cells = <1>;
> > 	master-ids = <0xf>, <0xb>;
> > 	master-id-map = <0 0xf &ITS 0 0>;
> > };
> > 
> > ITS: its at cf000000 {
> > 	...
> > 
> > 	/* DID */
> > 	master-id-cells = <2>;
> > };
> > 
> Is this is not depending heavily on discover order, how do drivers
> know which device to get which ID. is it implicitly assumed in
> discovery order? 

I'm not sure I follow the question. Could you elaborate?

> what happens to hot pluggable devices.  

It would be necessary to be able to discover the ID assigned to the
device by the hotpluggable bus. For example, this could depend on which
slot the device is plugged into.

If you can't discover the ID associated with a hotpluggable device from
the bus it is plugged into I can't see how hotplug could work.

>>From that point on I would expect the ID transformations to be static as
in the example.

Thanks,
Mark.