Re: [PATCH] of: support an enumerated-bus compatible value

On 07/02/2012 11:23 AM, Mitch Bradley wrote:

On 7/2/2012 5:59 AM, Stephen Warren wrote:

quoted

On 07/01/2012 01:36 PM, Rob Herring wrote:

quoted

On 06/28/2012 06:05 PM, Stephen Warren wrote:

quoted

From: Stephen Warren <redacted>

An "enumerated" bus is one that is not memory-mapped, hence hence
typically has #address-cells=1, and #size-cells=0. Such buses would be
used to group related non-memory-mapped nodes together, often just
under
the top-level of the device tree. The ability to group nodes into a
non-
memory-mapped subnode of the root is important, since if nodes exist to
describe multiple entities of the same type, the nodes will have the
same name, and hence require a unit address to differentiate them. It
doesn't make sense to assign bogus unit addresses from the CPU's own
address space for this purpose. An example:

    regulators {
        compatible = "enumerated-bus";
        #address-cells = <1>;
        #size-cells = <0>;

        regulator@0 {
            compatible = "regulator-fixed";
            reg = <0>;
        };

        regulator@1 {
            compatible = "regulator-fixed";
            reg = <1>;
        };
    };

Finally, because such buses are not memory-mapped, we avoid creating
any IO/memory resources for the device.

This seems like a work-around to use reg instead of using cell-index
(which is discouraged). reg in this case is really not a hardware
description. Do you have an intended use or just trying to fix the error
messages?

I'm not familiar with cell-index; can you please describe it some more.
Looking at some existing files in arch/powerpc/boot/dts, it looks like
something that exists alongside reg rather than replacing it, so I don't
see how it'd solve the problem.

The portion of .dts file quoted above is the use-case. In more general
terms, I need to add a bunch of non-memory-mapped devices to DT. There
are multiple devices of a given type. The DT node names should be named
after the class of device not the instance, and hence all get named the
same. Hence, I need a unit address to differentiate the node names.
Hence I need to use the reg property in order that the unit address
matches the reg property. Is there some other way of solving these
requirements other than using a unit address to make the node names
unique?

One of Rob's objections was that, in this case, the reg property is not
a hardware description.  That's an interesting point.  If in fact
numerous such devices exist, there must be some mechanism for software
to choose which device to talk to, typically a number.  Is that the case
for these devices?  If so, that number is a perfectly valid "reg"
property value.  If not, how does software choose to talk to a specific
device?

Yes, the reg value is purely a unique ID that exists to satisfy DT
semantics.

These regulators will eventually be referenced by phandles from the
drivers that use them, just like interrupts/GPIOs/... I just haven't
hooked up any clients that do so yet.