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

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Date:	Thu, 20 Mar 2014 10:12:51 -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 Thu, Mar 20, 2014 at 8:57 AM, Thierry Reding
<thierry.reding@...il.com> wrote:
> On Tue, Mar 18, 2014 at 04:47:36PM -0700, Tim Kryger wrote:

> Perhaps the comment for enum pwm_polarity in include/linux/pwm.h makes
> this more obvious. What you do here is invert the duty cycle, rather
> than the polarity. While it is true that the result is the same for
> things like LEDs or backlight (because the signal power remains the
> same), but there's a slight difference to what the PWM signal looks
> like.

Thanks, I missed that comment before.

>> Does polarity influence the output while a PWM is disabled?
>
> Yes, there is apparently hardware where the polarity causes the PWM pin
> to be 1 when the PWM is disabled. But that's really a separate issue.

Do you have a preference on how this should be handled?


> Things are starting to get confusing here. Looking at the register
> definitions, there's PWM_CONTROL_ENABLE_SHIFT(chan), which I suspect
> controls whether or not a channel is enabled (if that's not the case
> then please add a comment that explains what it does).

I've tried to do this but the unfortunate name for these bits and
their nuanced behavior makes it difficult.

>
> But a little further up you said that the hardware does only support a
> configure operation and not an enable/disable.

If you define disabled as zero duty output, then this is true.

When the smooth bit is off and the "enable" bit is off output is 100% duty.

>
> The comment above further confuses me. What I read from it is that you
> can in fact disable a channel by clearing the "enable" bit in the
> control register. But the reason why you don't do it that way is because
> that change won't take effect until "settings start to take effect". So
> in order to disable the PWM immediately you resort to writing a 0 duty
> cycle.

Sorry if my comments were confusing.  New settings are only applied on
a rising edge of the "enable" bit.  You should think of it more as a
trigger bit than an enable.

>
> Perhaps I misunderstood, in which case it might be good to revise that
> comment to be more explicit or accurate.

Perhaps it would be clearest to deviate from the hw docs and rename
PWM_CONTROL_ENABLE_SHIFT to PWM_CONTROL_TRIGGER_SHIFT to more closely
match its function.  What do you think of the following?

/*
* New duty and period settings are only reflected in the PWM output
* after a rising edge of the trigger bit.  After a rising edge, if the
* smooth bit is set, the settings are also delayed until the current
* period has completed. Furthermore, if the smooth bit is set, the PWM
* continues to output a waveform based on the old settings during the
* time that the trigger bit is low.  Otherwise the output is a constant
* high signal while the trigger bit is low.
*/
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