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

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Date:	Tue, 18 Mar 2014 18:06:03 -0700
From:	Tim Kryger <tim.kryger@...aro.org>
To:	Thierry Reding <thierry.reding@...il.com>
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 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:

>>> +
>>> +     /* There is polarity support in HW but it is easier to manage in SW */
>>> +     if (pwm->polarity == PWM_POLARITY_INVERSED)
>>> +             duty_ns = period_ns - duty_ns;
>>
>> No, this is wrong. You're not actually changing the *polarity* here.
>
> Please elaborate.  I don't understand what is wrong here.
>
> Does polarity influence the output while a PWM is disabled?
>

>>> +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?
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