linux-kernel - Re: [PATCH v2 3/6] pwm: jz4740: Apply configuration atomically

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Message-ID: <20190724064745.7ghecdpg3gmxsiim@pengutronix.de>
Date:   Wed, 24 Jul 2019 08:47:45 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>, od@...c.me,
        linux-pwm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH v2 3/6] pwm: jz4740: Apply configuration atomically

Hello Paul,

On Tue, Jul 23, 2019 at 04:46:40PM -0400, Paul Cercueil wrote:
> Le lun. 22 juil. 2019 à 15:34, Uwe =?iso-8859-1?q?Kleine-K=F6nig?=
> <u.kleine-koenig@...gutronix.de> a écrit :
> > On Fri, Jun 07, 2019 at 05:44:07PM +0200, Paul Cercueil wrote:
> > >  -	is_enabled = jz4740_timer_is_enabled(pwm->hwpwm);
> > >  -	if (is_enabled)
> > >  -		jz4740_pwm_disable(chip, pwm);
> > >  +	jz4740_pwm_disable(chip, pwm);
> > 
> > I assume this stops the PWM. Does this complete the currently running
> > period? How does the PWM behave then? (Does it still drive the output?
> > If so, on which level?)
> 
> Some PWM channels work in one mode "TCU1" and others work in "TCU2". The
> mode in which channels work depends on the version of the SoC.
> 
> When stopped, the pins of TCU1 channels will be driven to the inactive
> level (which depends on the polarity). It is unknown whether or not the
> currently running period is completed. We set a bit to configure for
> "abrupt shutdown", so I expect that it's not, but somebody would need
> to hook up a logic analyzer to see what's the exact behaviour with
> and without that bit.

This might be done even without a logic analyzer. Just do something
like:

	pwm_apply_state(pwm, { .enabled = 1, .period = 5s })
	pwm_apply_state(pwm, { .enabled = 1, .period = 5s, .duty = 5s })

and if that takes less then 5s the period is not completed.

And note that "abrupt shutdown" is a bug.

> TCU2 channels on the other hand will stop in the middle of a period,
> leaving the pin hanging at whatever level it was before the stop.
> That's the rationale behind the trick in commit 6580fd173070 ("pwm:
> jz4740: Force TCU2 channels to return to their init level").

Strange, but ok.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |