linux-kernel - Re: [PATCH v7 5/8] pwm: core: Support new PWM_STAGGERING

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Date:   Thu, 8 Apr 2021 20:14:08 +0200
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        linux-pwm@...r.kernel.org, Sven Van Asbroeck <TheSven73@...il.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 5/8] pwm: core: Support new PWM_STAGGERING_ALLOWED flag

On Thu, Apr 08, 2021 at 07:36:37PM +0200, Uwe Kleine-König wrote:
> On Thu, Apr 08, 2021 at 05:51:36PM +0200, Clemens Gruber wrote:
> > On Thu, Apr 08, 2021 at 02:50:40PM +0200, Thierry Reding wrote:
> > > Yes, I think that's basically what this is saying. I think we're perhaps
> > > getting hung up on the terminology here. PWM_STAGGERING_ALLOWED gives
> > > the impression that we're dealing with some provider-specific feature,
> > > whereas what we really want to express is that the PWM doesn't care
> > > exactly when the active cycle starts and based on that a provider that
> > > can support it may optimize the EMI behavior.
> > > 
> > > Maybe we can find a better name for this? Ultimately what this means is
> > > that the consumer is primarily interested in the power output of the PWM
> > > rather than the exact shape of the signal. So perhaps something like
> > > PWM_USAGE_POWER would be more appropriate.
> > 
> > Yes, although it would then no longer be obvious that this feature leads
> > to improved EMI behavior, as long as we mention that in the docs, I
> > think it's a good idea
> > 
> > Maybe document it as follows?
> > PWM_USAGE_POWER - Allow the driver to delay the start of the cycle
> > for EMI improvements, as long as the power output stays the same
> 
> I don't like both names, because for someone who is only halfway into
> PWM stuff it is not understandable. Maybe ALLOW_PHASE_SHIFT?

Sounds good to me.

> When a consumer is only interested in the power output than
> 
> 	.period = 20
> 	.duty_cycle = 5
> 
> would also be an allowed response for the request
> 
> 	.period = 200
> 	.duty_cycle = 50
> 
> and this is not what is in the focus here.

Right.

If Thierry agrees, I can spin up a new revision.

Maybe we can get it into 5.13 after all.

Thanks,
Clemens