Date:   Tue, 1 Dec 2020 14:22:14 +0100
From:   Jonathan Neuschäfer <j.neuschaefer@....net>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     Jonathan Neuschäfer <j.neuschaefer@....net>,
        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

On Tue, Dec 01, 2020 at 08:20:26AM +0100, Uwe Kleine-König wrote:
> 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.

Good point, will do.

> 
> > +{
> > +	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.

In this case, I thought it helps the readability, because the lines are
quite similar.

> > +	period /= TIME_BASE_NS;
> > +	duty   /= TIME_BASE_NS;

Here, I did it because I had already aligned the previous two lines.

> > +
> > +	/*
> > +	 * 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

Good idea, but I'm not sure if the EC handles this kind of interleaving
correctly.

> but it gets less readable. Maybe move that to a function to have the
> reg_sequence and the actual write nearer together?

Indeed, a separate function would keep register names and values
together (without resorting to declarations-after-statements).

> Or somehow name the indexes to make it more obvious?

Too much unnecessary complexity, IMHO.

> > +	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?

Good point. I'll add something to the 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)?

I think it does.


Thanks,
Jonathan

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