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

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Date:	Fri, 26 Jul 2013 07:38:09 -0400
From:	Jason Cooper <jason@...edaemon.net>
To:	"jonsmirl@...il.com" <jonsmirl@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <robherring2@...il.com>,
	Olof Johansson <olof@...om.net>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"ksummit-2013-discuss@...ts.linuxfoundation.org" 
	<ksummit-2013-discuss@...ts.linuxfoundation.org>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Catalin Marinas <catalin.marinas@....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>
Subject: Re: DT bindings as ABI [was: Do we have people interested in
 device tree janitoring / cleanup?]

On Thu, Jul 25, 2013 at 08:27:15PM -0400, jonsmirl@...il.com wrote:
> On Thu, Jul 25, 2013 at 7:18 PM, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
> > On Thu, Jul 25, 2013 at 03:31:35PM -0400, Jason Cooper wrote:
> >> On Thu, Jul 25, 2013 at 02:11:31PM -0500, Rob Herring wrote:
> >> > On Thu, Jul 25, 2013 at 11:09 AM, Olof Johansson <olof@...om.net> wrote:
> >>
> >> > > One problem that needs to be solved is obviously how a binding
> >> > > graduates from tentative to locked. This work isn't going to be very
> >> > > interesting to most people, I suspect. Think standards committee type
> >> > > work.
> >> >
> >> > I think a time based stabilization period would be better than a
> >> > separate directory to apply bindings too. Or time plus periodic review
> >> > perhaps.
> >>
> >> The only problem with a time-based versus separate directory is how do
> >> users who've downloaded the tree determine which bindings are stable?
> >> If they pull a tarball, or receive an SDK, there is most likely no git
> >> history attached.
> >>
> >> I think the idea of a 'tentative' directory (or 'locked') is churnish,
> >> but necessary.  If I DL'd a tarball and had to type 'tentative' to get
> >> to the binding doc I wanted, that would be a pretty clear clue to be
> >> delicate about how I trust/use/plan with that binding.
> >
> > It's actually extremely simple.  If the bindings are in development,
> > they must not appear in a -final released kernel.  Anything that appears
> > in a -final kernel becomes part of the ABI at that point.
> >
> > That obviously does not mean you remove them in the last -rc and put
> > them back during the merge window!
> >
> > That's how we handle every other ABI thing in the kernel tree, why should
> > DT files be any different?  (I've added Linus and Grant to this discussion.)
> >
> > As I've already stated, it is intended to eventually remove the DT files
> > from the kernel tree and have them as a separately maintained project,
> > which means they will be independent of the kernel version.
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at  http://www.tux.org/lkml/
> 
> Having a schema system for the device trees is closely related to this
> discussion.  In this case the schema would probably be equal to the
> stable set of nodes. This has been discussed before on the device tree
> mailing list. The dtc compiler would take this schema and validate the
> trees it compiles against it issuing warnings for 'non-standard'
> usage. Over time the schema would be updated to allow these usages
> when everyone agrees to it. Note that there would be a single schema
> describing all possible legal Linux device trees.

s/Linux// ?

> The scheme is also quite useful for new tree developers since it will
> show them the universe of device tree attributes that have already
> been standardized. By using comments, you could probably turn the
> device tree documentation into the schema source files.

One more note on schema, since DT is a description of hardware, it would
be useful to have two comments, a url to the datasheet, and a canonical
name of the datasheet suitable for $searchengine.  Where available, of
course.

thx,

Jason.
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