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

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Date:	Wed, 29 May 2013 09:27:28 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Benoit Cousson <b-cousson@...com>
CC:	"Mohammed, Afzal" <afzal@...com>, 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

On 05/29/2013 02:39 AM, Benoit Cousson wrote:
> Hi Afzal,
> 
> On 05/29/2013 10:06 AM, Mohammed, Afzal wrote:
>> Hi Jon,
>>
>> On Wed, May 29, 2013 at 03:35:10, Stephen Warren wrote:
>>> On 05/28/2013 03:25 PM, Jon Hunter wrote:
>>
>>>>>  			ti,am335x-timer	(applicable to AM335x devices)
>>>>>  			ti,am335x-timer-1ms (applicable to AM335x devices)
>>>>> +			"ti,am4372-timer-1ms", "ti,am335x-timer-1ms" for AM43x 1ms timer
>>>>> +			"ti,am4372-timer", "ti,am335x-timer" for AM43x timers other than 1ms one
>>
>>>> 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 don't agree here; the DT binding should define all the required and/or
allowed values that must/should/can be present in the DT - the entire
legal schema. The set of all compatible values is included in that,
irrespective of whether a particular value actually (currently) defines
a different HW interface or not.
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