linux-kernel - Re: [Ksummit-2013-discuss] DT bindings as ABI [was: Do we have people interested in device tree janitoring / cleanup?]

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Date:	Fri, 26 Jul 2013 14:14:23 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	David Woodhouse <dwmw2@...radead.org>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Mark Rutland <mark.rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"ksummit-2013-discuss@...ts.linuxfoundation.org" 
	<ksummit-2013-discuss@...ts.linuxfoundation.org>,
	Pawel Moll <Pawel.Moll@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Domenico Andreoli <cavokz@...il.com>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.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>,
	Ian Campbell <ian.campbell@...rix.com>
Subject: Re: [Ksummit-2013-discuss] DT bindings as ABI [was: Do we have
	people interested in device tree janitoring / cleanup?]

On Fri, Jul 26, 2013 at 03:09:29PM +0200, Richard Cochran wrote:
> On Fri, Jul 26, 2013 at 10:42:24AM +0100, David Woodhouse wrote:
> > On Fri, 2013-07-26 at 10:01 +0200, Richard Cochran wrote:
> > > On Thu, Jul 25, 2013 at 03:37:53PM -0600, Jason Gunthorpe wrote:
> > > > 
> > > > We use DT has a kernel configuration input. Our environment is
> > > > designed to guarantee 100% that the kernel and DT match exactly. DT
> > > > very deliberately isn't an ABI boundary in our systems.
> > > 
> > > Think about what you just said.
> > > 
> > > The DT describes the *hardware* not the kernel. Why should you ever
> > > need to update your DT at all?
> > 
> > Well, the nodes which describe hardware devices, according to the
> > bindings which form an ABI contract between DT and drivers, should not
> > normally change. Although they *can* change, if for example you change
> > the MAC address and that's stored there. Or you change the PHY you want
> > it to use. Or something like that. The *ABI* doesn't change, but the
> > data you express *using* that ABI can change. That's kind of the point.
> 
> Unless I totally misunderstood, the thread is talking about letting
> established bindings change with each new kernel version.  I am
> opposed to that.
> 
> Of course, a user may want to change the values of his MAC addresses,
> if he needs to. But he should never have to change *how* he specifies
> those addresses.
> 
> So, as long as everyone can agree to the principle that a working DT
> should remain working after a kernel upgrade, then I'll shut up. In
> actual fact, this is not the case today, nor was it ever so in the
> past, AFAICT, but it never hurts to set goals.

We agree.

That's precisely why I'm saying that once a binding appears in a -final
released kernel, it is _stable_ by that very fact.  If there were any
doubts about it, it should never have been accepted into the kernel tree
in the first place.

It's no different from how we treat syscalls.  Once we accept a syscall
and it's published in a -final kernel, it can't then be altered - it can
be supplemented by a new syscall with a different interface, but the old
one remains and keeps its semantics for a great many years.

There's no reason for DT to be any different in this regard.
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