linux-kernel - Re: [PATCH v3 2/5] pwm: kona: Introduce Kona PWM controller support

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Date:	Thu, 20 Mar 2014 15:48:38 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Tim Kryger <tim.kryger@...aro.org>
Cc:	Matt Porter <mporter@...aro.org>, Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Rob Landley <rob@...dley.net>,
	Christian Daudt <bcm@...thebug.org>,
	Grant Likely <grant.likely@...aro.org>,
	Linux PWM List <linux-pwm@...r.kernel.org>,
	Device Tree List <devicetree@...r.kernel.org>,
	Linux Doc List <linux-doc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Broadcom Kernel Feedback List 
	<bcm-kernel-feedback-list@...adcom.com>,
	Linux ARM Kernel List <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 2/5] pwm: kona: Introduce Kona PWM controller support

On Tue, Mar 18, 2014 at 06:06:03PM -0700, Tim Kryger wrote:
> On Tue, Mar 18, 2014 at 4:47 PM, Tim Kryger <tim.kryger@...aro.org> wrote:
> > On Tue, Mar 18, 2014 at 2:52 PM, Thierry Reding <thierry.reding@...il.com> wrote:
> >> On Wed, Mar 12, 2014 at 01:15:43PM -0700, Tim Kryger wrote:
[...]
> >>> +static int kona_pwmc_set_polarity(struct pwm_chip *chip, struct pwm_device *pwm,
> >>> +                               enum pwm_polarity polarity)
> >>> +{
> >>> +     /*
> >>> +      * The framework only allows the polarity to be changed when a PWM is
> >>> +      * disabled so no immediate action is required here.  When a channel is
> >>> +      * enabled, the polarity gets handled as part of the re-config step.
> >>> +      */
> >>> +
> >>> +     return 0;
> >>> +}
> >>
> >> See above. If you don't want to implement the hardware support for
> >> inversed polarity, then simply don't implement this.
> >
> > I had originally planned to omit polarity support but because it
> > affects the binding (which is treated as ABI), it wouldn't be possible
> > to add it in later without defining a new compatible string.
> 
> I would like to get this right but it occurred to me that there may be
> a way to defer the implementation of this feature without disrupting
> the binding.
> 
> Would it be acceptable to continue using #pwm-cells = <3> and
> of_pwm_xlate_with_flags but return -EINVAL from kona_pwmc_set_polarity
> if PWM_POLARITY_INVERSED is specified?

This was recently discussed for the pwm-imx driver. And you can easily
support #pwm-cells = <2> and #pwm-cells = <3> with the same binding. So
you could start with #pwm-cells = <2>, leaving out .set_polarity() and
implement it later on, extending the binding in a backwards-compatible
way to support the polarity flag.

Thierry

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