Hey Stephen-
Thanks for the comments.
On Fri, Aug 23, 2013 at 03:58:36PM -0600, Stephen Warren wrote:
> On 08/22/2013 01:59 PM, Josh Cartwright wrote:
> > Signed-off-by: Josh Cartwright <joshc@xxxxxxxxxxxxxx>
> > ---
> > I'm introducing this as an RFC, because there are set of assumptions
> > made in this binding spec, that currently hold true for the supported
> > controller/addressing scheme for the Snapdragon 800 series, but don't
> > necessarily hold true for SPMI in general.
> >
> > 1. No use of Group Slave Identifiers (GSIDs)
> > (SPMI allows for a slave to belong to zero or more groups specified
> > by GSID, however this feature isn't currently implemented)
> >
> > 2. No specification of Master Identifier (MID)
> > (A "system integrator" allocates to each master a 2-bit MID, this
> > currently isn't being specified, as it isn't needed by software for
> > the PMIC Arb; not sure if this is of use to other SPMI controllers)
> >
> > 3. Single SPMI master per controller
> >
> > Effectively, only a subset of possible SPMI configurations are specified
> > in this document.
> >
> > So, if it's considered necessary to provide a generic SPMI binding
> > specification, is it acceptable to only define a subset at this time,
> > expanding only when necessary, or shall I expand the definition to at
> > least address 1 & 2, even though they are of no use in the current
> > implementation?
>
> It's best to define the whole thing from the start if possible. It's
> easier to ensure the whole binding is consistent, and nothing has been
> left out.
That makes sense. I think I'll go down this route for v3 of this
patchset:
For #1 above, extend the 'reg' property of a slave node to include the
group slave ID's in which the slave is a member. The first 'reg' entry
will remain the slave's Unique Slave Identifier (USID) as before.
For #2, add additional required 'spmi-mid' property in the
controller/master node that describes the 2-bit Master Identifier (MID).
For #3, rename the SPMI API's s/controller/master/. The current
controller/master terminology difference is confusing and unnecessary.
> However, it's probably OK to define a subset binding initially and then
> expand it later, as long as some thought it put into how it can be
> expanded in a way that is 100% compatible: old DTs will still operate
> with new kernels and perhaps even new DTs will still operate with old
> kernels.
>
> That said, if the thought is put in to ensure that's possible, it's
> probably just as easy to define the whole binding from the start.
That all makes sense.
If we want to ensure for the generic bindings that we are fulling
characterizing/describing the SPMI bus, then we'll additionally need to
tackle an additional identified assumption:
4. One master per SPMI bus. (The SPMI spec allows for up to 4
masters)
On the Snapdragon 800 series, there exists only one software-controlled
master, but it is conceivably possible to have a setup with two
software-controlled masters on the same SPMI bus.
This necessarily means that the description of the slaves and the
masters will need to be decoupled; I'm imagining a generic binding
supporting multiple masters would look something like this:
master0: master@0 {
compatible = "...";
#spmi-master-cells = <0>;
spmi-mid = <0>;
...
};
master2: master@2 {
compatible = "...";
#spmi-master-cells = <0>;
spmi-mid = <2>;
...
};
spmi_bus {
compatible = "...";
spmi-masters = <&master0 &master2>;
foo@0 {
compatible = "...";
reg = <0 ...>;
};
foo@8 {
compatible = "...";
reg = <8 ...>;
};
};
(This will also necessitate a change in the underlying SPMI driver
model, in the current implementation, a SPMI master 'owns' a particular
device. This is not a valid assumption to make.)
Would this property-containing-phandle-vector be considered the
canonical way of representing nodes with multiple parents in the device
tree?
Thanks,
Josh
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