linux-kernel - Re: [RFC PATCH 1/3] pinctrl: add new generic pinconf config for deglitch filter

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Date:	Tue, 27 Aug 2013 09:42:54 +0200
From:	Nicolas Ferre <nicolas.ferre@...el.com>
To:	boris brezillon <b.brezillon@...rkiz.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Linus Walleij <linus.walleij@...aro.org>
CC:	Rob Herring <rob.herring@...xeda.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ian.campbell@...rix.com>,
	Rob Landley <rob@...dley.net>,
	Russell King <linux@....linux.org.uk>,
	"Linus Walleij" <linus.walleij@...aro.org>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Masanari Iida <standby24x7@...il.com>,
	Richard Genoud <richard.genoud@...il.com>,
	"Heiko Stuebner" <heiko@...ech.de>,
	James Hogan <james.hogan@...tec.com>,
	<devicetree@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 1/3] pinctrl: add new generic pinconf config for deglitch
 filter

On 27/08/2013 08:16, boris brezillon :
> On 27/08/2013 05:55, Stephen Warren wrote:
>> On 08/26/2013 11:01 AM, boris brezillon wrote:
>>> Hello Stephen,
>>>
>>> On 26/08/2013 18:50, Stephen Warren wrote:
>>>> On 08/24/2013 03:35 PM, Boris BREZILLON wrote:
>>>>> Add a new parameter to support deglitch filter configuration.
>>>>> A deglitch filter works like a debounce filter but with a smaller
>>>>> delay (nanoseconds).
>>>> Why not use the existing debounce property, just with a small delay
>>>> specified. It seems like that's exactly what the property is for?
>>> That's one of the question I asked in my cover letter :-)
>>>
>>> Indeed the at91 deglitch filter delay is not configurable and is statically
>>> assigned to half a master clk cycle (if master clk = 133MHz -> 8 ns).
>>> The debounce property argument is currently expressed in usecs.
>>>
>>> This will result in always selecting the debounce filter (which is also
>>> available on at91 SoCs) over the deglitch filter.
>>>
>>> Could we add a flag in the deglitch argument to specify the delay unit
>>> (nsecs or usecs) ?
>> If the value is hard-coded in HW, why not use non-zero (or 1) to enable
>> and zero to disable?
>
> Indeed at91 pins support both deglitch and debounce filter and I have to
> choose
> between the two given the argument value (in usec).
>
> Here's what I can do:
>
> if (arg >= 1/2 * slowclock)    /* debounce case */
>       /* choose debounce filter and configure the delay
>           according to the given argument value */
> else                                          /* deglitch case */
>       /* choose deglitch filter */
>
>
> Slow clock is running at 32KHz which gives a 30 usec clock cycle.

I am not in favor for this kind of complicated heuristic. Deglitch and 
Debounce filters are different features in at91 (even if they pursuit 
the same goal). So I do prefer to let the user choose which feature is 
preferred for his application and add a different flag.


>> (this kind of thing is why I'm not convinced that generic pinconf works
>> so well... What if we need psecs in the future?)
>
> Should I keep the at91 native pinconf binding and add the missing flags
> to this binding
> (OUTPUT configuration flags) ?
>
> This was another question I asked in my cover letter: wether or not the
> generic pinconf
> binding should be used.

The question is: how much this "generic" pinconf is... well... generic! 
And it is not a answer I can give.
On the other hand, if the "generic" is not going to overcome the native 
pinctrl, I do not feel like switching to this at the cost of changing 
the whole dtsi/dts entries that we already have.

So, it is more to Linus and Stephen to give us clues about this...

Bye,
-- 
Nicolas Ferre
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