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

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Message-ID: <51A73B97.2020701@ti.com>
Date:	Thu, 30 May 2013 13:44:23 +0200
From:	Benoit Cousson <b-cousson@...com>
To:	Stephen Warren <swarren@...dotorg.org>
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

Hi Stephen,

On 05/29/2013 05:27 PM, Stephen Warren wrote:
> 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.

Well, I tend to agree on the principle, but so far it was never really
done like that. That's not necessarily a good excuse, but if we start
adding new bindings for the huge number of OMAP|AM variants TI has been
introduced for 10 years, I'd rather use a wildcard than a exhaustive
list of all the devices.
Something like ti,[omap|am]*-timer for example .

Regards,
Benoit

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