linux-kernel - Re: [patch 2/7] dt: dtb version: document chosen/dtb-info node binding

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Date:	Thu, 19 Mar 2015 18:41:40 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Rob Herring <robherring2@...il.com>
Cc:	Frank Rowand <frowand.list@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	Grant Likely <grant.likely@...aro.org>,
	Michal Marek <mmarek@...e.cz>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	linux-kbuild@...r.kernel.org,
	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [patch 2/7] dt: dtb version: document chosen/dtb-info node
 binding

On Thu, Mar 19, 2015 at 08:23:29AM -0500, Rob Herring wrote:
> On Wed, Mar 18, 2015 at 10:33 PM, Frank Rowand <frowand.list@...il.com> wrote:
> > +version
> > +       The version of the DTB.  This is analagous to the linux kernel version.
> > +
> > +       This is a format free field intended for human consumption.  User space
> > +       programs should not have any expections about this property.
> > +
> > +       The DTB number in this property is incremented each time a make that
> > +       creates one or more DTBs is invoked.  If the make creates multiple
> > +       DTBs then this number is only incremented once.
> > +
> > +       The DTB number is stored in file .version_dtb.
> > +
> > +version-linux
> > +       The version of the linux kernel most recently built in the source
> > +       control system that contains the source used to build the DTB.
> > +
> > +       The linux kernel version number is not incremented for a make that
> > +       creates a DTB.
> > +
> > +dtb-path
> > +       The build directory relative path of the DTB.
> > +
> > +dts-path
> > +       The absolute path of the .dts file compiled to create the DTB.
> 
> So these become an ABI and we can never change the directory structure?
> 
> The problem with informational fields is someone, somewhere will rely
> on them and then we are stuck with them. Look at /proc/cpuinfo.

There's a bigger problem: including the Linux kernel specific version
means that we will _never_ be able to move them out of the kernel.

Moreover, what it means is that people can then write code to test
what the dtb version is, and we'll end up with the dtb version being
tested to determine various features.

Since the design goal of DTB is to describe the hardware, including
the Linux kernel version is totally against that: the Linux kernel
version should be totally irrelevant.  What's more relevant would be
a _hardware_ version field, but even that's questionable...

And yes, you are right - anything like this which we add immediately
becames a user ABI which becomes *very* difficult to change later, so
in order to accept anything like this, we have to be absolutely sure
that this is something we really really really really want to do.  Let
me put it another way: once this is accepted, it will probably be
impossible to ever change it.

... just like Bogomips in /proc/cpuinfo.

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