linux-kernel - Re: [RFC PATCH alt 4/4] pinctrl: at91: rework debounce configuration

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Message-ID: <523734B2.6050208@wwwdotorg.org>
Date:	Mon, 16 Sep 2013 10:41:22 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	boris brezillon <b.brezillon@...rkiz.com>
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>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Grant Likely <grant.likely@...aro.org>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Richard Genoud <richard.genoud@...il.com>,
	Jiri Kosina <jkosina@...e.cz>, devicetree@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH alt 4/4] pinctrl: at91: rework debounce configuration

On 09/14/2013 01:08 AM, boris brezillon wrote:
> Hello Stephen,
> 
> Le 14/09/2013 00:40, Stephen Warren a écrit :
>> On 09/13/2013 01:53 AM, Boris BREZILLON wrote:
>>> AT91 SoCs do not support per pin debounce time configuration.
>>> Instead you have to configure a debounce time which will be used for all
>>> pins of a given bank (PIOA, PIOB, ...).
...
>>>   Required properties for pin configuration node:
...
>>> -DEBOUNCE_VAL    (0x3fff << 17): debounce val.
>>
>> This change would break the DT ABI since it removes a feature that's
>> already present.
...
>> I suppose it's still up to the Atmel maintainers to decide whether this
>> is appropriate, or whether the impact to out-of-tree DT files would be
>> problematic.
>>
>> Assuming the DT ABI can be broken, I think I'd prefer to do so, rather
>> than take "non-alt" patch 4/4, since a per-pin DEBOUNCE_VAL clearly
>> doesn't correctly model the HW, assuming the patch description is
>> correct. I don't think arguments re: the generic pinconf debounce
>> property hold; if the Linux-specific/internal generic property doesn't
>> apply, the DT binding should not be bent to adjust to it, but should
>> rather still represent the HW itself.
> 
> What about the last point in my list: "reconfigure debounce after
> startup" ?
> 
> Here is an example that may be problematic:
> 
> Let's say you have one device using multiple configuration of pins
> ("default", "xxx", "yyy").
> The "default" config needs a particular debounce time on a given pin and
> the "xxx" and "yyy"
> configs need different debounce time on the same pin.
> 
> How would you solve this with this patch approach ?

Each state has a different pin configuration node, and hence can specify
a different debounce value. This patch has no impact on that (it just
changes whether the state-specific node specifies the debounce value in
a single standalone property, or encodes it into each entry in the pins
property, all within the same node).
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