Re: [PATCH v2 4/7] pwm: Add support for Azoteq IQS620A PWM generator

From: Jeff LaBundy <jeff@labundy.com>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: "mark.rutland@arm.com" <mark.rutland@arm.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"lars@metafoo.de" <lars@metafoo.de>,
	"kernel@pengutronix.de" <kernel@pengutronix.de>,
	"linux-pwm@vger.kernel.org" <linux-pwm@vger.kernel.org>,
	"linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
	"dmitry.torokhov@gmail.com" <dmitry.torokhov@gmail.com>,
	"robh+dt@kernel.org" <robh+dt@kernel.org>,
	"thierry.reding@gmail.com" <thierry.reding@gmail.com>,
	"pmeerw@pmeerw.net" <pmeerw@pmeerw.net>,
	"linux-input@vger.kernel.org" <linux-input@vger.kernel.org>,
	"lee.jones@linaro.org" <lee.jones@linaro.org>,
	"jic23@kernel.org" <jic23@kernel.org>,
	"knaack.h@gmx.de" <knaack.h@gmx.de>
Subject: Re: [PATCH v2 4/7] pwm: Add support for Azoteq IQS620A PWM generator
Date: Wed, 1 Jan 2020 22:39:36 +0000	[thread overview]
Message-ID: <20200101223933.GB14339@labundy.com> (raw)
In-Reply-To: <20191222214851.kapsro6b6qylke43@pengutronix.de>

Hi Uwe,

On Sun, Dec 22, 2019 at 10:48:51PM +0100, Uwe Kleine-König wrote:
> Hello Jeff,
> 
> On Sat, Dec 21, 2019 at 03:28:01AM +0000, Jeff LaBundy wrote:
> > I heard back from the vendor today; they've acknowledged the limitation and
> > are considering adding support for 0% in a future ROM spin. In the meantime,
> > they've agreed to describe the high-impedance behavior in the data sheet as
> > well as include the pull-down resistor in an example schematic.
> 
> Oh wow, seems like a good vendor then. :-)
> 
> > > > Option (3) seems like overkill for such a simple PWM, and ultimately doesn't
> > > > add any value because I don't want to allow option (1) behavior in any case.
> > > > Whether the PWM is disabled because it is truly disabled or to simulate a 0%
> > > > duty cycle as in option (2), the pull-down is ultimately required regardless
> > > > of whether or not the data sheet happens to go into such detail.
> > > 
> > > Actually I like option 3 best.
> > >  
> > 
> > Based on your other feedback, I'm moving forward under the impression that
> > you'll still accept option (2); please let me know if I have misunderstood
> > (thank you for being flexible).
> 
> Yeah, that's fine. If in the end it shows that this is a bad idea we can
> still change to (3).
> 

Sounds great. As soon as 5.5-rc5 lands this weekend, I'll rebase v3 and
send it out.

I failed to catch this in my previous reply, but the comment I've added
to iqs620_pwm_get_state actually reads as follows:

/*
 * Since the device cannot generate a 0% duty cycle, requests to do so
 * force subsequent calls to iqs620_pwm_get_state to report the output
 * as disabled with duty cycle equal to that which was in use prior to
 * the request. This is not ideal, but is the best compromise based on
 * the capabilities of the device.
 */

This matches the present implementation, not your proposed comment that
claims duty cycle is clamped to 1 / 256 ms following a request for a 0%
duty cycle.

This seems OK since the concept of a duty cycle or period aren't really
relevant if the output is disabled in my opinion. However if you prefer
I update iqs620_pwm_apply to clamp duty cycle to 1 / 256 ms (instead of
leaving it untouched) in this case, please let me know.

> Best regards
> Uwe
> 
> -- 
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | https://www.pengutronix.de/ |

Wishing you a Happy New Year,
Jeff LaBundy