linux-kernel - Re: [PATCH v9 06/13] pwm: add support for sl28cpld PWM controller

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Message-ID: <20200908081227.mxumgqipyod6iltr@pengutronix.de>
Date:   Tue, 8 Sep 2020 10:12:27 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Michael Walle <michael@...le.cc>
Cc:     linux-gpio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hwmon@...r.kernel.org,
        linux-pwm@...r.kernel.org, linux-watchdog@...r.kernel.org,
        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>,
        Lee Jones <lee.jones@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        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>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH v9 06/13] pwm: add support for sl28cpld PWM controller

Hello,

just a bit of nitpicking left. If Lee is going to apply, I can care for
a followup patch if need be.

On Mon, Sep 07, 2020 at 11:37:55PM +0200, Michael Walle wrote:
> [..]
> +config PWM_SL28CPLD
> +	tristate "Kontron sl28cpld PWM support"
> +	depends on MFD_SL28CPLD ||  COMPILE_TEST

s/  / / (@Lee, maybe fixup during application?)

> +	help
> +	  Generic PWM framework driver for board management controller
> +	  found on the Kontron sl28 CPLD.
> [...]
> +#define SL28CPLD_PWM_CLK			32000 /* 32 kHz */
> +#define SL28CPLD_PWM_MAX_DUTY_CYCLE(prescaler)	(1 << (7 - (prescaler)))
> +#define SL28CPLD_PWM_PERIOD(prescaler) \
> +	(NSEC_PER_SEC / SL28CPLD_PWM_CLK * SL28CPLD_PWM_MAX_DUTY_CYCLE(prescaler))
> +
> +/*
> + * We calculate the duty cycle like this:
> + *   duty_cycle_ns = pwm_cycle_reg * max_period_ns / max_duty_cycle
> + *
> + * With
> + *   max_period_ns = 1 << (7 - prescaler) / pwm_clk * NSEC_PER_SEC
> + *   max_duty_cycle = 1 << (7 - prescaler)
> + * this then simplifies to:
> + *   duty_cycle_ns = pwm_cycle_reg / pwm_clk * NSEC_PER_SEC
> + *
> + * NSEC_PER_SEC is a multiple of SL28CPLD_PWM_CLK, therefore we're not losing
> + * precision by doing the divison first.

The division you're talking about is NSEC_PER_SEC / pwm_clk which isn't
obvious in the formula in the line above. Maybe:

	...
	this then simplifies to:

	  duty_cycle_ns = NSEC_PER_SEC / SL28CPLD_PWM_CLK * pwm_cycle_reg

	NSEC_PER_SEC is a multiple of SL28CPLD_PWM_CLK, therefor ...

to make it easier to understand the comment.

> + */
> +#define SL28CPLD_PWM_TO_DUTY_CYCLE(reg) \
> +	(NSEC_PER_SEC / SL28CPLD_PWM_CLK * (reg))
> +#define SL28CPLD_PWM_FROM_DUTY_CYCLE(duty_cycle) \
> +	(DIV_ROUND_DOWN_ULL((duty_cycle), NSEC_PER_SEC / SL28CPLD_PWM_CLK))
> [...]
> +	/*
> +	 * To avoid glitches when we switch the prescaler, we have to make sure
> +	 * we have a valid duty cycle for the new mode.
> +	 *
> +	 * Take the current prescaler (or the current period length) into
> +	 * account to decide whether we have to write the duty cycle or the new
> +	 * prescaler first. If the period length is decreasing we have to
> +	 * write the duty cycle first.
> +	 */
> +	write_duty_cycle_first = pwm->state.period > state->period;
> +
> +	if (write_duty_cycle_first) {
> +		ret = sl28cpld_pwm_write(priv, SL28CPLD_PWM_CYCLE, cycle);
> +		if (ret)
> +			return ret;
> +	}
> +
> +	ret = sl28cpld_pwm_write(priv, SL28CPLD_PWM_CTRL, ctrl);
> +	if (ret)
> +		return ret;
> +
> +	if (!write_duty_cycle_first) {
> +		ret = sl28cpld_pwm_write(priv, SL28CPLD_PWM_CYCLE, cycle);
> +		if (ret)
> +			return ret;
> +	}

Nice! I didn't spend the necessary brain cycles to confirm this
algorithm, but it seems you did :-)

> +
> +	return 0;
> +}
> [...]

Best regards
Uwe

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

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