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To:     Rob Herring <robh@kernel.org>
Cc:     Frank Rowand <frowand.list@gmail.com>, devicetree@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        David Miller <davem@davemloft.net>, sparclinux@vger.kernel.org
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From:   Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
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Date:   Wed, 5 Sep 2018 17:01:52 +0100
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Subject: Re: [PATCH 3/3] of: make default address and size cells sizes private
Sender: linux-kernel-owner@vger.kernel.org
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On 05/09/18 13:12, Rob Herring wrote:

> On Tue, Sep 4, 2018 at 11:59 PM Mark Cave-Ayland
> <mark.cave-ayland@ilande.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.

Got it - if dtc requires explicit properties, then I agree there should
be no issues with this patch. Sorry for the noise.


ATB,

Mark.