linux-kernel - Re: [RFC PATCH 06/10] pwm: ntxec: Add driver for PWM function in Netronix EC

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Date:   Mon, 22 Jun 2020 10:18:02 +0200
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,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Sam Ravnborg <sam@...nborg.org>,
        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: [RFC PATCH 06/10] pwm: ntxec: Add driver for PWM function in
 Netronix EC

Hello,

On Sun, Jun 21, 2020 at 12:42:17AM +0200, Jonathan Neuschäfer wrote:
> The Netronix EC provides a PWM output, which is used for the backlight

s/,//

> on ebook readers. This patches adds a driver for the PWM output.

on *some* ebook readers


> +#define NTXEC_UNK_A		0xa1
> +#define NTXEC_UNK_B		0xa2
> +#define NTXEC_ENABLE		0xa3
> +#define NTXEC_PERIOD_LOW	0xa4
> +#define NTXEC_PERIOD_HIGH	0xa5
> +#define NTXEC_DUTY_LOW		0xa6
> +#define NTXEC_DUTY_HIGH		0xa7
> +
> +/*
> + * The time base used in the EC is 8MHz, or 125ns. Period and duty cycle are
> + * measured in this unit.
> + */
> +static int ntxec_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm_dev,
> +				 int duty_ns, int period_ns)
> +{
> +	struct ntxec_pwm *pwm = pwmchip_to_pwm(chip);
> +	uint64_t duty = duty_ns;
> +	uint64_t period = period_ns;

As you cannot use values bigger than 8191999 anyhow, I wonder why you
use a 64 bit type here.

> +	int res = 0;
> +
> +	do_div(period, 125);

Please use a define instead of plain 125.

> +	if (period > 0xffff) {
> +		dev_warn(pwm->dev,
> +			 "Period is not representable in 16 bits: %llu\n", period);
> +		return -ERANGE;
> +	}
> +
> +	do_div(duty, 125);
> +	if (duty > 0xffff) {
> +		dev_warn(pwm->dev, "Duty cycle is not representable in 16 bits: %llu\n",
> +			duty);
> +		return -ERANGE;
> +	}

This check isn't necessary as the pwm core ensures that duty <= period.

> +	res |= ntxec_write8(pwm->ec, NTXEC_PERIOD_HIGH, period >> 8);
> +	res |= ntxec_write8(pwm->ec, NTXEC_PERIOD_LOW, period);
> +	res |= ntxec_write8(pwm->ec, NTXEC_DUTY_HIGH, duty >> 8);
> +	res |= ntxec_write8(pwm->ec, NTXEC_DUTY_LOW, duty);

Does this complete the currently running period? Can it happen that a
new period starts between the first and the last write and so a mixed
period can be seen at the output?

> +
> +	return (res < 0) ? -EIO : 0;
> +}
> +
> +static int ntxec_pwm_enable(struct pwm_chip *chip,
> +				 struct pwm_device *pwm_dev)
> +{
> +	struct ntxec_pwm *pwm = pwmchip_to_pwm(chip);
> +
> +	return ntxec_write8(pwm->ec, NTXEC_ENABLE, 1);
> +}
> +
> +static void ntxec_pwm_disable(struct pwm_chip *chip,
> +				   struct pwm_device *pwm_dev)
> +{
> +	struct ntxec_pwm *pwm = pwmchip_to_pwm(chip);
> +
> +	ntxec_write8(pwm->ec, NTXEC_ENABLE, 0);
> +}
> +
> +static struct pwm_ops ntxec_pwm_ops = {
> +	.config		= ntxec_pwm_config,
> +	.enable		= ntxec_pwm_enable,
> +	.disable	= ntxec_pwm_disable,
> +	.owner		= THIS_MODULE,

Please don't align the =, just a single space before them is fine.
More important: Please implement .apply() (and .get_state()) instead of
the old API. Also please enable PWM_DEBUG which might save us a review
iteration.

> +};
> +
> +static int ntxec_pwm_probe(struct platform_device *pdev)
> +{
> +	struct ntxec *ec = dev_get_drvdata(pdev->dev.parent);
> +	struct ntxec_pwm *pwm;
> +	struct pwm_chip *chip;
> +	int res;
> +
> +	pwm = devm_kzalloc(&pdev->dev, sizeof(*pwm), GFP_KERNEL);
> +	if (!pwm)
> +		return -ENOMEM;
> +
> +	pwm->ec = ec;
> +	pwm->dev = &pdev->dev;
> +
> +	chip = &pwm->chip;
> +	chip->dev = &pdev->dev;
> +	chip->ops = &ntxec_pwm_ops;
> +	chip->base = -1;
> +	chip->npwm = 1;
> +
> +	res = pwmchip_add(chip);
> +	if (res < 0)
> +		return res;
> +
> +	platform_set_drvdata(pdev, pwm);
> +
> +	res |= ntxec_write8(pwm->ec, NTXEC_ENABLE, 0);
> +	res |= ntxec_write8(pwm->ec, NTXEC_UNK_A, 0xff);
> +	res |= ntxec_write8(pwm->ec, NTXEC_UNK_B, 0xff);
> +
> +	return (res < 0) ? -EIO : 0;

This is broken for several reasons:

 - You're not supposed to modify the output in .probe
 - if ntxec_write8 results in an error you keep the pwm registered.
 - From the moment on pwmchip_add returns the callbacks can be called.
   The calls to ntxec_write8 probably interfere here.

Best regards
Uwe

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

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