Message-ID: <20201124082019.vpkr3xnp55arjpnp@pengutronix.de>
Date:   Tue, 24 Nov 2020 09:20:19 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Jonathan Neuschäfer <j.neuschaefer@....net>,
        Thierry Reding <thierry.reding@...il.com>
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>,
        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 v4 4/7] pwm: ntxec: Add driver for PWM function in
 Netronix EC

Hello,

On Sun, Nov 22, 2020 at 11:27:36PM +0100, Jonathan Neuschäfer wrote:
> The Netronix EC provides a PWM output which is used for the backlight
> on some ebook readers. This patches adds a driver for the PWM output.
> 
> The .get_state callback is not implemented, because the PWM state can't
> be read back from the hardware.
> 
> Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@....net>
> ---
> 
> v4:
> - Document hardware/driver limitations
> - Only accept normal polarity
> - Fix a typo ("zone" -> "zero")
> - change MAX_PERIOD_NS to 0xffff * 125
> - Clamp period to the maximum rather than returning an error
> - Rename private struct pointer to priv
> - Rearrage control flow in _probe to save a few lines and a temporary variable
> - Add missing MODULE_ALIAS line
> - Spell out ODM
> 
> v3:
> - https://lore.kernel.org/lkml/20200924192455.2484005-5-j.neuschaefer@gmx.net/
> - Relicense as GPLv2 or later
> - Add email address to copyright line
> - Remove OF compatible string and don't include linux/of_device.h
> - Fix bogus ?: in return line
> - Don't use a comma after sentinels
> - Avoid ret |= ... pattern
> - Move 8-bit register conversion to ntxec.h
> 
> v2:
> - https://lore.kernel.org/lkml/20200905133230.1014581-6-j.neuschaefer@gmx.net/
> - Various grammar and style improvements, as suggested by Uwe Kleine-König,
>   Lee Jones, and Alexandre Belloni
> - Switch to regmap
> - Prefix registers with NTXEC_REG_
> - Add help text to the Kconfig option
> - Use the .apply callback instead of the old API
> - Add a #define for the time base (125ns)
> - Don't change device state in .probe; this avoids multiple problems
> - Rework division and overflow check logic to perform divisions in 32 bits
> - Avoid setting duty cycle to zero, to work around a hardware quirk
> ---
>  drivers/pwm/Kconfig     |   8 ++
>  drivers/pwm/Makefile    |   1 +
>  drivers/pwm/pwm-ntxec.c | 166 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 175 insertions(+)
>  create mode 100644 drivers/pwm/pwm-ntxec.c
> 
> diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
> index 63be5362fd3a5..815f329ed5b46 100644
> --- a/drivers/pwm/Kconfig
> +++ b/drivers/pwm/Kconfig
> @@ -350,6 +350,14 @@ config PWM_MXS
>  	  To compile this driver as a module, choose M here: the module
>  	  will be called pwm-mxs.
> 
> +config PWM_NTXEC
> +	tristate "Netronix embedded controller PWM support"
> +	depends on MFD_NTXEC
> +	help
> +	  Say yes here if you want to support the PWM output of the embedded
> +	  controller found in certain e-book readers designed by the original
> +	  design manufacturer Netronix.
> +
>  config PWM_OMAP_DMTIMER
>  	tristate "OMAP Dual-Mode Timer PWM support"
>  	depends on OF
> diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
> index cbdcd55d69eef..1deb29e6ae8e5 100644
> --- a/drivers/pwm/Makefile
> +++ b/drivers/pwm/Makefile
> @@ -32,6 +32,7 @@ obj-$(CONFIG_PWM_MESON)		+= pwm-meson.o
>  obj-$(CONFIG_PWM_MEDIATEK)	+= pwm-mediatek.o
>  obj-$(CONFIG_PWM_MTK_DISP)	+= pwm-mtk-disp.o
>  obj-$(CONFIG_PWM_MXS)		+= pwm-mxs.o
> +obj-$(CONFIG_PWM_NTXEC)		+= pwm-ntxec.o
>  obj-$(CONFIG_PWM_OMAP_DMTIMER)	+= pwm-omap-dmtimer.o
>  obj-$(CONFIG_PWM_PCA9685)	+= pwm-pca9685.o
>  obj-$(CONFIG_PWM_PXA)		+= pwm-pxa.o
> diff --git a/drivers/pwm/pwm-ntxec.c b/drivers/pwm/pwm-ntxec.c
> new file mode 100644
> index 0000000000000..4f4f736d71aba
> --- /dev/null
> +++ b/drivers/pwm/pwm-ntxec.c
> @@ -0,0 +1,166 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * The Netronix embedded controller is a microcontroller found in some
> + * e-book readers designed by the original design manufacturer Netronix, Inc.
> + * It contains RTC, battery monitoring, system power management, and PWM
> + * functionality.
> + *
> + * This driver implements PWM output.
> + *
> + * Copyright 2020 Jonathan Neuschäfer <j.neuschaefer@....net>
> + *
> + * Limitations:
> + * - The get_state callback is not implemented, because the current state of
> + *   the PWM output can't be read back from the hardware.
> + * - The hardware can only generate normal polarity output.
> + */
> +
> +#include <linux/mfd/ntxec.h>
> +#include <linux/module.h>
> +#include <linux/platform_device.h>
> +#include <linux/pwm.h>
> +#include <linux/regmap.h>
> +#include <linux/types.h>
> +
> +struct ntxec_pwm {
> +	struct device *dev;
> +	struct ntxec *ec;
> +	struct pwm_chip chip;
> +};
> +
> +static struct ntxec_pwm *pwmchip_to_priv(struct pwm_chip *chip)
> +{
> +	return container_of(chip, struct ntxec_pwm, chip);
> +}
> +
> +#define NTXEC_REG_AUTO_OFF_HI	0xa1
> +#define NTXEC_REG_AUTO_OFF_LO	0xa2
> +#define NTXEC_REG_ENABLE	0xa3
> +#define NTXEC_REG_PERIOD_LOW	0xa4
> +#define NTXEC_REG_PERIOD_HIGH	0xa5
> +#define NTXEC_REG_DUTY_LOW	0xa6
> +#define NTXEC_REG_DUTY_HIGH	0xa7
> +
> +/*
> + * The time base used in the EC is 8MHz, or 125ns. Period and duty cycle are
> + * measured in this unit.
> + */
> +#define TIME_BASE_NS 125
> +
> +/*
> + * The maximum input value (in nanoseconds) is determined by the time base and
> + * the range of the hardware registers that hold the converted value.
> + * It fits into 32 bits, so we can do our calculations in 32 bits as well.
> + */
> +#define MAX_PERIOD_NS (TIME_BASE_NS * 0xffff)
> +
> +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 duty = state->duty_cycle;
> +	unsigned int period = state->period;

state->duty_cycle and state->period are u64, so you're losing
information here. Consider state->duty_cycle = 0x100000001 and
state->period = 0x200000001.

> +	int res = 0;
> +
> +	if (state->polarity != PWM_POLARITY_NORMAL)
> +		return -EINVAL;
> +
> +	if (period > MAX_PERIOD_NS) {
> +		period = MAX_PERIOD_NS;
> +
> +		if (duty > period)
> +			duty = period;
> +	}
> +
> +	period /= TIME_BASE_NS;
> +	duty /= TIME_BASE_NS;
> +
> +	res = regmap_write(priv->ec->regmap, NTXEC_REG_PERIOD_HIGH, ntxec_reg8(period >> 8));
> +	if (res)
> +		return res;

I wonder if you can add some logic to the regmap in the mfd driver such
that ntxec_reg8 isn't necessary for all users.

> +	res = regmap_write(priv->ec->regmap, NTXEC_REG_PERIOD_LOW, ntxec_reg8(period));
> +	if (res)
> +		return res;
> +
> +	res = regmap_write(priv->ec->regmap, NTXEC_REG_DUTY_HIGH, ntxec_reg8(duty >> 8));
> +	if (res)
> +		return res;
> +
> +	res = regmap_write(priv->ec->regmap, NTXEC_REG_DUTY_LOW, ntxec_reg8(duty));
> +	if (res)
> +		return res;

I think I already asked, but I don't remember the reply: What happens to
the output between these writes? A comment here about this would be
suitable.

> +
> +	/*
> +	 * 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 (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));
> +	} else {
> +		return regmap_write(priv->ec->regmap, NTXEC_REG_ENABLE, ntxec_reg8(0));
> +	}
> +}
> +
> +static struct pwm_ops ntxec_pwm_ops = {

This can be const.

> +	.apply = ntxec_pwm_apply,

/*
 * The current state cannot be read out, so there is no .get_state
 * callback.
 */

Hmm, at least you could provice a .get_state() callback that reports the
setting that was actually implemented for in the last call to .apply()?

@Thierry: Do you have concerns here? Actually it would be more effective
to have a callback (like .apply()) that modfies its pwm_state
accordingly. (Some drivers did that in the past, but I changed that to
get an uniform behaviour in 71523d1812aca61e32e742e87ec064e3d8c615e1.)
The downside is that people have to understand that concept to properly
use it. I'm torn about the right approach.

> +	.owner = THIS_MODULE,
> +};
> +
> +static int ntxec_pwm_probe(struct platform_device *pdev)
> +{
> +	struct ntxec *ec = dev_get_drvdata(pdev->dev.parent);
> +	struct ntxec_pwm *priv;
> +	struct pwm_chip *chip;
> +
> +	priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
> +	if (!priv)
> +		return -ENOMEM;
> +
> +	priv->ec = ec;
> +	priv->dev = &pdev->dev;
> +
> +	platform_set_drvdata(pdev, priv);
> +
> +	chip = &priv->chip;
> +	chip->dev = &pdev->dev;
> +	chip->ops = &ntxec_pwm_ops;
> +	chip->base = -1;
> +	chip->npwm = 1;
> +
> +	return pwmchip_add(chip);
> +}
> +
> +static int ntxec_pwm_remove(struct platform_device *pdev)
> +{
> +	struct ntxec_pwm *priv = platform_get_drvdata(pdev);
> +	struct pwm_chip *chip = &priv->chip;
> +
> +	return pwmchip_remove(chip);
> +}
> +
> +static struct platform_driver ntxec_pwm_driver = {
> +	.driver = {
> +		.name = "ntxec-pwm",
> +	},
> +	.probe = ntxec_pwm_probe,
> +	.remove = ntxec_pwm_remove,
> +};
> +module_platform_driver(ntxec_pwm_driver);
> +
> +MODULE_AUTHOR("Jonathan Neuschäfer <j.neuschaefer@....net>");
> +MODULE_DESCRIPTION("PWM driver for Netronix EC");
> +MODULE_LICENSE("GPL");
> +MODULE_ALIAS("platform:ntxec-pwm");

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

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