[PATCH v2] of: WARN on deprecated #address-cells/#size-cells handling

[Rob Herring]

On Wed, Nov 27, 2024 at 3:47 PM Segher Boessenkool
<segher at kernel.crashing.org> wrote:
>
> On Tue, Nov 26, 2024 at 02:36:32PM +1100, Michael Ellerman wrote:
> > Michael Ellerman <mpe at ellerman.id.au> writes:
> > > "Rob Herring (Arm)" <robh at kernel.org> writes:
> > >> While OpenFirmware originally allowed walking parent nodes and default
> > >> root values for #address-cells and #size-cells, FDT has long required
> > >> explicit values. It's been a warning in dtc for the root node since the
> > >> beginning (2005) and for any parent node since 2007. Of course, not all
> > >> FDT uses dtc, but that should be the majority by far. The various
> > >> extracted OF devicetrees I have dating back to the 1990s (various
> > >> PowerMac, OLPC, PASemi Nemo) all have explicit root node properties.
> > >
> > > I have various old device trees that have been given to me over the
> > > years, and as far as I can tell they all have these properties (some of
> > > them are partial trees so it's hard to be 100% sure).
> > >
> > > So LGTM.
> >
> > Turns out I was wrong.
> >
> > The warning about #size-cells hits on some powermacs, possible fixup
> > patch here:
> >
> >   https://lore.kernel.org/linuxppc-dev/20241126025710.591683-1-mpe@ellerman.id.au/
>
> The Open Firmware specification is extremely clear that a "missing"
> "#size-cells" property means this bus has the default value of 1.

And the default for #address-cells is 2, but yet every architecture
except Sparc has that wrong.

If I have a node without #size-cells, is the default of 1 used or do
we check parent nodes? My read of the spec would be the former, but
the kernel does the latter.

> https://www.openfirmware.info/data/docs/of1275.pdf (page 186).
>
> DTC or FDT might want to do things differently, but expecting decades
> older stuff to conform to its ill-conceived unnecessarily super wordy
> stuff is, well, not a plan that is likely to work very well :-)

That is not the intention. The intention is to identify what doesn't
conform and exclude those systems from this check (or apply a fixup if
that works).

Rob