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

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Message-ID: <52339479.6030402@wwwdotorg.org>
Date:	Fri, 13 Sep 2013 16:40:57 -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 09/13/2013 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, ...).

> diff --git a/Documentation/devicetree/bindings/pinctrl/atmel,at91-pinctrl.txt b/Documentation/devicetree/bindings/pinctrl/atmel,at91-pinctrl.txt

> +Optional properties for iomux controller:
> +- atmel,default-debounce-div: array of debounce divisors (one divisor per bank)
> +  which describes the debounce timing in use for all pins of a given bank
> +  configured with the DEBOUNCE option (see the following description).
> +  Debounce timing is obtained with this formula:
> +  Tdebounce = 2 * (debouncediv + 1) / Fslowclk
> +  with Fslowclk = 32KHz
> +
>  Required properties for pin configuration node:
>  - atmel,pins: 4 integers array, represents a group of pins mux and config
>    setting. The format is atmel,pins = <PIN_BANK PIN_BANK_NUM PERIPH CONFIG>.
> @@ -91,7 +99,6 @@ DEGLITCH	(1 << 2): indicate this pin need deglitch.
>  PULL_DOWN	(1 << 3): indicate this pin need a pull down.
>  DIS_SCHMIT	(1 << 4): indicate this pin need to disable schmit trigger.
>  DEBOUNCE	(1 << 16): indicate this pin need debounce.
> -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.
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