linux-kernel - RE: [PATCH v2 11/14] Documentation: dt: binding: omap: am43x timer

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Message-ID: <C8443D0743D26F4388EA172BF4E2A7A93EAFB174@DBDE04.ent.ti.com>
Date:	Wed, 29 May 2013 09:58:06 +0000
From:	"Mohammed, Afzal" <afzal@...com>
To:	"Cousson, Benoit" <b-cousson@...com>
CC:	Stephen Warren <swarren@...dotorg.org>,
	Jon Hunter <jgchunter@...il.com>,
	Russell King <linux@....linux.org.uk>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <rob.herring@...xeda.com>,
	"Grant Likely" <grant.likely@...aro.org>,
	Benoit Cousson <benoit.cousson@...aro.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH v2 11/14] Documentation: dt: binding: omap: am43x timer

Hi Benoit,

On Wed, May 29, 2013 at 14:09:18, Cousson, Benoit wrote:
> On 05/29/2013 10:06 AM, Mohammed, Afzal wrote:
> > On Wed, May 29, 2013 at 03:35:10, Stephen Warren wrote:
> >> On 05/28/2013 03:25 PM, Jon Hunter wrote:

> >>> If you are adding more compatibility strings, then this implies that the
> >>> AM43x timers are not 100% compatible with any other device listed (such
> >>> as am335x or any omap device). That's fine but you should state that in
> >>> the changelog. If the AM43x timer registers are 100% compatible with
> >>> existing devices you should not add these.
> >>
> >> I'm not sure that's true; .dts files should always include a compatible
> >> value that describes the most specific model of the HW, plus any
> >> baseline compatible value that the HW is compatible with. This allows
> >> any required quirks/fixes/... to be applied for the specific HW model
> >> later even if nobody knows right now they'll be needed. Hence, defining
> >> new compatible values doesn't necessarily mean incompatible HW.
> > 
> > Stephen took words out of my finger ;)
> >  
> > Some explanations,I don;t 
> > 
> > 1. first compatible should be exact device [A], followed by compatible
> > model (if one)
> > 2. Minor effort in getting DT right the first time may help prevent
> > difficult effort later modifying it (if a necessity comes), considering
> > the fact that DT sources has  to move out of Kernel at some point of
> > time. And DT is not supposed to be modified, which may cause difficulty
> > for the users (I had been a minor victim of this during rebase).
> > 
> > As we both were in GPMC land earlier, an example,
> > 
> > If my memory is right, GPMC IP in am335x is rev 6, and IP has 8 chip
> > select, but one is not pinned out. Now assume that same IP is integrated
> > in another SoC (probably OMAP4 has rev 6). Here if we use same compatible
> > for both, driver cannot handle it properly (w/o knowledge about platform).
> > But if exact compatible is mentioned, without modifying DT (which should
> > be considered as a firmware) just by modifying Kernel, deciding based on
> > compatible would help achieve what is required.
> 
> That's true for the DTS itself, but here your are changing the binding
> documentation which is supposed to reflect the driver "interface" in the
> Device Tree model description.
> 
> Since the driver does not support any new compatible string, you should
> not update the binding.

I have a different opinion: Binding documentation is to be considered an
entity that is not a part of the Kernel, but currently is a part of the
Kernel for want of a better place. And binding is to be considered OS
agnostic being ignorant of any piece of software (driver here). Binding has
the authority to allow its usage in DT sources.

> The driver and the binding will have to be changed the day you will have
> to update the driver to handle a bug / feature specific to that revision
> (ti,am4372-timer).
> 
> Since this series does not seems to update the driver, you should not
> update the bindings.

I believe that updating binding is a prerequisite for making a new
DTS change, hence binding was updated first, then DTS. 

Regards
Afzal