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From: Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: [PATCH v2 6/7] Holly DTS
Date: Sat, 12 May 2007 14:02:42 +0200
To: David Gibson <dwg@au1.ibm.com>
Cc: Olof Johansson <olof@lixom.net>, linuxppc-dev@ozlabs.org,
	Loeliger Jon-LOELIGER <jdl@Freescale.com>
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>>> What exactly did this ?? syntax imply?  I must have missed that 
>>> thread.
>>
>> A property in a DTS file can use that to say
>> the property should have a value, but it will
>> be filled in by the bootwrapper instead (memory
>> address/size, or network MAC address, for
>> instance).  Presumably the kernel parser would
>> complain when it sees the this-is-still-undefined
>> marker.
>>
>> The original proposal specified the length of
>> the property value IIRC, but that isn't necessary
>> anymore.
>
> Well.. sort of.  The first cut at the idea is that ? would be
> equivalent to zeroes in terms of dtc output, but would act as internal
> documentation that that property is supposed to be filled in by the
> bootloader.

Yes.

> Refinements to try to enforce that would be nice:

Exactly my point.

> easiest is to
> replace it with a configurable poison value.  When using asm output,
> or with a map file, it might be possible to generate out-of-band
> information which we can use to check that the right things are filled
> in.
>
> It's not a simply a matter of making the kernel parser recognize the
> uninitialized info, because there's no way to encode that in the dtb
> itself.

There can be, with a new DTB version -- perhaps simply
define proplen == -1 to mean "undefined" or similar.


Segher