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Date:   Thu, 6 Apr 2023 15:53:58 +0200
From:   Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= <u.kleine-koenig@pengutronix.de>
To:     Thierry Reding <thierry.reding@gmail.com>
Cc:     Lorenz Brun <lorenz@brun.one>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        linux-pwm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org
Subject: Re: [PATCH v2] pwm: mediatek: support inverted polarity
Message-ID: <20230406135358.x3et6gvvxqsknfn6@pengutronix.de>
References: <20230309010410.2106525-1-lorenz@brun.one>
 <ZC7LaC19YjNwTIi1@orome>
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Hello Thierry,

On Thu, Apr 06, 2023 at 03:38:48PM +0200, Thierry Reding wrote:
> On Thu, Mar 09, 2023 at 02:04:10AM +0100, Lorenz Brun wrote:
> > +	 * appear to have the capability to invert the output.
> > +	 * This means that inverted mode can not be fully supported as the
> > +	 * waveform will always start with the low period and end with the hi=
gh
> > +	 * period. Thus reject non-normal polarity if the shape of the wavefo=
rm
> > +	 * matters, i.e. usage_power is not set.
> > +	 */
> > +	if (state->polarity !=3D PWM_POLARITY_NORMAL && !state->usage_power)
> >  		return -EINVAL;
> > =20
> >  	if (!state->enabled) {
> > @@ -213,7 +221,11 @@ static int pwm_mediatek_apply(struct pwm_chip *chi=
p, struct pwm_device *pwm,
> >  		return 0;
> >  	}
> > =20
> > -	err =3D pwm_mediatek_config(pwm->chip, pwm, state->duty_cycle, state-=
>period);
> > +	duty_cycle =3D state->duty_cycle;
> > +	if (state->polarity =3D=3D PWM_POLARITY_INVERSED)
> > +		duty_cycle =3D state->period - state->duty_cycle;
>=20
> That's not really what state->usage_power was meant to address.

I don't understand your concern here. I don't like .usage_power, but
AFAICT this is a legitimite use. With .usage_power =3D true, the lowlevel
driver is free to shift the phase_offset and even modify the period size
and the goal is just that the average power-output matches.

Lorenz's patch does exactly this: It even keeps the period and only
shifts the phase (by period - duty_cycle). If you consider this not
legitmate, I think we have to improve the docs about .usage_power.

> What's wrong with just reversing the duty cycle in the pwm-fan? If you
> use DT it's quite trivial to do that by just reversing the entries in
> your cooling-levels property. Does that not work for you?

That's an option, too. With a different PWM (i.e. one that can do proper
inverted polarity) Lorenz's solution would be ok, though, right? And the
pwm-fan only cares about the relative duty_cycle and not the phase
shift, so setting .usage_power =3D true is fine, too?!

Best regards
Uwe

--=20
Pengutronix e.K.                           | Uwe Kleine-K=F6nig            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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