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

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Message-ID: <20241127214232.GQ29862@gate.crashing.org>
Date: Wed, 27 Nov 2024 15:42:32 -0600
From: Segher Boessenkool <segher@...nel.crashing.org>
To: Michael Ellerman <mpe@...erman.id.au>
Cc: "Rob Herring (Arm)" <robh@...nel.org>,
        Saravana Kannan <saravanak@...gle.com>, linuxppc-dev@...ts.ozlabs.org,
        Conor Dooley <conor@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] of: WARN on deprecated #address-cells/#size-cells handling

On Tue, Nov 26, 2024 at 02:36:32PM +1100, Michael Ellerman wrote:
> Michael Ellerman <mpe@...erman.id.au> writes:
> > "Rob Herring (Arm)" <robh@...nel.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.

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 :-)


Segher