[PATCH] powerpc: document new interrupt-array property

[Segher Boessenkool]

>> what problem is this trying to solve?
>
> As he says, it's for devices with interrupts routed to multiple
> controllers.  The normal interrupts property assumes a single
> interrupt parent for a device.  This is needed on most 4xx boards,
> where the MAL has 5 interrupts, some of which are on UIC0 and some of
> which are on UIC1.  At the moment the Ebony port deals with this using
> an "interesting" hack, treating the MAL as an interrupt nexus for
> itself, using an interrupt-map to remap its interrupts to the two
> controllers.

Not really a hack, this is documented in the interrupt
binding:

> 3.3.  "interrupt-map" property
> At any level in the interrupt tree, a mapping may need to take
> place between the child interrupt domain and the parent’s.  This
> is represented by a new property called "interrupt-map".

Note it says "*any* level".

> The interrupt-array proposal does the same thing much
> more neatly and compactly.

Sure, for every specific case one can envision a more neat
and compact device tree ;-P

If in a certain tree you have this "problem" with not only
the MAL but with lots of devices, you could introduce a
"fake" interrupt nexus that doesn't represent a physical
device as such, but that represents the combined cascaded
interrupt controllers, and maps the interrupts to the nodes
for the "physical" interrupt controllers.  Just don't make
the mistake of putting an "interrupt-controller" property
in there and all is just fine.  As an added bonus you end up
with one single namespace for the interrupts (one interrupt
domain in interrupt-mapping speak), which is probably what
the chip documentation does as well.


Segher