linux-kernel - Re: [PATCH 3/4] pwm: mxs: add support for inverse polarity

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Message-ID: <20190923085414.6d3gbby6gglpjsfe@pengutronix.de>
Date:   Mon, 23 Sep 2019 10:54:14 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     devicetree@...r.kernel.org, linux-pwm@...r.kernel.org,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/4] pwm: mxs: add support for inverse polarity

On Mon, Sep 23, 2019 at 10:45:56AM +0200, Rasmus Villemoes wrote:
> On 23/09/2019 10.27, Uwe Kleine-König wrote:
> > On Mon, Sep 23, 2019 at 10:13:47AM +0200, Rasmus Villemoes wrote:
> >>
> >>  
> >> +	pol_bits = state->polarity == PWM_POLARITY_NORMAL ?
> >> +		PERIOD_POLARITY_NORMAL : PERIOD_POLARITY_INVERSE;
> >> +
> >>  	writel(duty_cycles << 16,
> >>  	       mxs->base + PWM_ACTIVE0 + pwm->hwpwm * 0x20);
> >> -	writel(PERIOD_PERIOD(period_cycles) | PERIOD_POLARITY_NORMAL | PERIOD_CDIV(div),
> >> +	writel(PERIOD_PERIOD(period_cycles) | pol_bits | PERIOD_CDIV(div),
> > 
> > When will this affect the output? Only on the next start of a period, or
> > immediatly? Can it happen that this results in a mixed output (i.e. a
> > period that has already the new duty cycle from the line above but not
> > the new polarity (or period)?
> 
> The data sheet says "Also, when the user reprograms the channel in this
> manner, the new register values will not take effect until the beginning
> of a new output period. This eliminates the potential for output
> glitches that could occur if the registers were updated while the
> channel was enabled and in the middle of a cycle.". So I think this
> should be ok. "this manner" refers to the registers being written in the
> proper order (first ACTIVEn, then PERIODn).

OK. IMHO this is worth a code comment.

Best regards
Uwe

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