linux-kernel - Re: [PATCH 3/3] of: make default address and size cells sizes private

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Date:   Wed, 5 Sep 2018 07:12:20 -0500
From:   Rob Herring <robh@...nel.org>
To:     mark.cave-ayland@...nde.co.uk
Cc:     Frank Rowand <frowand.list@...il.com>, devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        David Miller <davem@...emloft.net>, sparclinux@...r.kernel.org
Subject: Re: [PATCH 3/3] of: make default address and size cells sizes private

On Tue, Sep 4, 2018 at 11:59 PM Mark Cave-Ayland
<mark.cave-ayland@...nde.co.uk> wrote:
>
> On 05/09/18 02:55, Frank Rowand wrote:
>
> > On 08/30/18 12:05, Rob Herring wrote:
> >> Only some old OpenFirmware implementations rely on default sizes. Any
> >> FDT and modern implementation should have explicit properties. Make the
> >> OF_ROOT_NODE_*_CELLS_DEFAULT defines private so we don't get any outside
> >> users.
> >>
> >> This also gets us one step closer to removing the asm/prom.h dependency on
> >> Sparc.
>
> Just for the record: you say "any FDT and modern implementation should
> have explicit properties", however the default values of these
> properties when missing are clearly defined in the IEEE-1275
> specification (Annex A):

The spec may define defaults, but best practice is to be explicit. For
FDT, dtc doesn't allow defaults (and never has). No arch added in the
last 10 years has relied on the defaults.

> "#address-cells"
> Standard property name to define the package’s address format.
> ...
> In a package with a "decode-unit" method, a missing "#address-cells"
> property signifies that the number of
> address cells is two.

So only Sparc is compliant as the default for everyone else is 1.

> "#size-cells"
> Standard property name to define the package’s address size format.
> ...
> A missing "#size-cells" property signifies the default value of one.
>
> I can't speak for FDT but it isn't completely unreasonable for a guest
> parsing a DT without these properties to assume these default values.

I'm not removing the defaults. Just not allowing for code outside of
drivers/of/ to use the defaults.

Rob