linux-kernel - Re: [PATCH v5 4/7] pwm: ntxec: Add driver for PWM function in Netronix EC

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Date:   Tue, 1 Dec 2020 08:20:26 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Jonathan Neuschäfer <j.neuschaefer@....net>
Cc:     linux-kernel@...r.kernel.org,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Heiko Stuebner <heiko@...ech.de>, linux-pwm@...r.kernel.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Fabio Estevam <festevam@...il.com>, linux-rtc@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Daniel Palmer <daniel@...f.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Andreas Kemnade <andreas@...nade.info>,
        NXP Linux Team <linux-imx@....com>,
        devicetree@...r.kernel.org, Stephan Gerhold <stephan@...hold.net>,
        allen <allen.chen@....com.tw>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Lubomir Rintel <lkundrak@...sk>,
        Rob Herring <robh+dt@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        linux-arm-kernel@...ts.infradead.org,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Mark Brown <broonie@...nel.org>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Heiko Stuebner <heiko.stuebner@...obroma-systems.com>,
        Josua Mayer <josua.mayer@....eu>,
        Shawn Guo <shawnguo@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v5 4/7] pwm: ntxec: Add driver for PWM function in
 Netronix EC

Hello Jonathan,

very nice driver, just a few minor comments below.

On Tue, Dec 01, 2020 at 02:15:10AM +0100, Jonathan Neuschäfer wrote:
> +static struct ntxec_pwm *pwmchip_to_priv(struct pwm_chip *chip)

a function prefix would be great here, I'd pick ntxec_pwm_from_chip as
name.

> +{
> +	return container_of(chip, struct ntxec_pwm, chip);
> +}
> +
> +[...]
> +static int ntxec_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm_dev,
> +			   const struct pwm_state *state)
> +{
> +	struct ntxec_pwm *priv = pwmchip_to_priv(pwm_dev->chip);
> +	unsigned int period, duty;
> +	struct reg_sequence regs[] = {
> +		{ NTXEC_REG_PERIOD_HIGH },
> +		{ NTXEC_REG_PERIOD_LOW },
> +		{ NTXEC_REG_DUTY_HIGH },
> +		{ NTXEC_REG_DUTY_LOW }
> +	};
> +	int res;
> +
> +	if (state->polarity != PWM_POLARITY_NORMAL)
> +		return -EINVAL;
> +
> +	period = min_t(u64, state->period, MAX_PERIOD_NS);
> +	duty   = min_t(u64, state->duty_cycle, period);

I'm not a big fan of aligning =. (As if you have to add a longer
variable you have to realign all otherwise unrelated lines.) But that's
subjective and it's up to you if you want to change this.

> +	period /= TIME_BASE_NS;
> +	duty   /= TIME_BASE_NS;
> +
> +	/*
> +	 * Changes to the period and duty cycle take effect as soon as the
> +	 * corresponding low byte is written, so the hardware may be configured
> +	 * to an inconsistent state after the period is written and before the
> +	 * duty cycle is fully written. If, in such a case, the old duty cycle
> +	 * is longer than the new period, the EC may output 100% for a moment.
> +	 */
> +
> +	regs[0].def = ntxec_reg8(period >> 8);
> +	regs[1].def = ntxec_reg8(period);
> +	regs[2].def = ntxec_reg8(duty >> 8);
> +	regs[3].def = ntxec_reg8(duty);

You could even minimize the window by changing the order here to

	NTXEC_REG_PERIOD_HIGH
	NTXEC_REG_DUTY_HIGH
	NTXEC_REG_PERIOD_LOW
	NTXEC_REG_DUTY_LOW

but it gets less readable. Maybe move that to a function to have the
reg_sequence and the actual write nearer together? Or somehow name the
indexes to make it more obvious?

> +	res = regmap_multi_reg_write(priv->ec->regmap, regs, ARRAY_SIZE(regs));
> +	if (res)
> +		return res;
> +
> +	/*
> +	 * Writing a duty cycle of zero puts the device into a state where
> +	 * writing a higher duty cycle doesn't result in the brightness that it
> +	 * usually results in. This can be fixed by cycling the ENABLE register.
> +	 *
> +	 * As a workaround, write ENABLE=0 when the duty cycle is zero.

If the device already has duty_cycle = 0 but ENABLE = 1, you might get
a failure. But I guess this doesn't need addressing in the code. But
maybe point it out in a comment?

> +	 */
> +	if (state->enabled && duty != 0) {
> +		res = regmap_write(priv->ec->regmap, NTXEC_REG_ENABLE, ntxec_reg8(1));
> +		if (res)
> +			return res;
> +
> +		/* Disable the auto-off timer */
> +		res = regmap_write(priv->ec->regmap, NTXEC_REG_AUTO_OFF_HI, ntxec_reg8(0xff));
> +		if (res)
> +			return res;
> +
> +		return regmap_write(priv->ec->regmap, NTXEC_REG_AUTO_OFF_LO, ntxec_reg8(0xff));

Given that you cannot read back period and duty anyhow: Does it make
sense to write these only if (state->enabled && duty != 0)?

> +	} else {
> +		return regmap_write(priv->ec->regmap, NTXEC_REG_ENABLE, ntxec_reg8(0));
> +	}
> +}

Thanks
Uwe

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

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