linux-kernel - Re: DT bindings as ABI [was: Do we have people interested in device tree janitoring / cleanup?]

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Date:	Thu, 25 Jul 2013 11:53:49 -0700
From:	Stephen Warren <swarren@...dotorg.org>
To:	Richard Cochran <richardcochran@...il.com>
CC:	Mark Rutland <mark.rutland@....com>,
	Olof Johansson <olof@...om.net>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"ksummit-2013-discuss@...ts.linuxfoundation.org" 
	<ksummit-2013-discuss@...ts.linuxfoundation.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Domenico Andreoli <cavokz@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dave P Martin <Dave.Martin@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Pawel Moll <Pawel.Moll@....com>,
	Ian Campbell <ian.campbell@...rix.com>
Subject: Re: DT bindings as ABI [was: Do we have people interested in device
 tree janitoring / cleanup?]

On 07/25/2013 11:48 AM, Richard Cochran wrote:
> On Thu, Jul 25, 2013 at 07:29:20PM +0100, Mark Rutland wrote:
>> On Thu, Jul 25, 2013 at 07:05:48PM +0100, Stephen Warren wrote:
>>>
>>> I don't think having people "rely" on the bindings is the issue so much
>>> as the awareness that if they do, there will be compatibility issues for
>>> unstable bindings.
>>
>> As long as we can make sufficiently clear that trying to use an unstable
>> binding is going to be *very* painful, and not necessarily supported.
> 
> Oh, man.
> 
> The introduction of DT into ARM Linux was supposed to make everyone's
> life sooo much easier. Of course, based on experience with powerpc, I
> never believed it*, but still I would expect to hear that the DT
> bindings are, well, a *binding* contract between the board developer,
> boot loader, and the kernel.
> 
> Once it is working with a particular kernel, a DT board description
> file should continue to work indefinitely with newer kernels. Anything
> less is a regression, pure and simple.
> 
> If you go around changing the bindings willy nilly, then what is point
> of having DT at all?

That's exactly why we're starting to think about which bindings should
be considered stable and immutable, and when that should happen. As Olof
pointed out, we haven't fully enforced that yet. Preferably bindings
will be marked stable very fast, but mistakes are always going to happen
in early development. ABIs are very hard.
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