linux-kernel - Re: [PATCH v4 05/11] pwm: add support for sl28cpld PWM controller

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Date:   Fri, 5 Jun 2020 12:33:01 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     Michael Walle <michael@...le.cc>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-hwmon@...r.kernel.org, linux-pwm@...r.kernel.org,
        linux-watchdog@...r.kernel.org,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Shawn Guo <shawnguo@...nel.org>, Li Yang <leoyang.li@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <maz@...nel.org>, Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v4 05/11] pwm: add support for sl28cpld PWM controller

On Fri, Jun 5, 2020 at 11:51 AM Lee Jones <lee.jones@...aro.org> wrote:
> On Thu, 04 Jun 2020, Michael Walle wrote:

...

> > +     cycle = state->duty_cycle * config->max_duty_cycle;
> > +     do_div(cycle, state->period);
>
> Forgive my ignorance (I'm new here!), but what are these 2 lines
> doing?  Here we are multiplying the current duty_cycle with the
> maximum value, then dividing by the period.
>
> So in the case of PWM_MODE_1KHZ with a 50% duty cycle, you'd have:
>
>    (500000 * 0x20[16]) / 1000000 = [0x10]16
>
> Thus, the above gives as a proportional representation of the maximum
> valid value for placement into the cycle control register(s), right?
>
> Either way (whether I'm correct or not), I think it would be nice to
> mention this in a comment.  Maybe even clarify with a simple example.

IIRC PWM has a helper for that (to calc period based on PWM state and
new duty cycle %).

-- 
With Best Regards,
Andy Shevchenko