Commit 7f61257c authored by Vincent Whitchurch's avatar Vincent Whitchurch Committed by Uwe Kleine-König
Browse files

pwm: Add GPIO PWM driver 



Add a software PWM which toggles a GPIO from a high-resolution timer.

This will naturally not be as accurate or as efficient as a hardware
PWM, but it is useful in some cases.  I have for example used it for
evaluating LED brightness handling (via leds-pwm) on a board where the
LED was just hooked up to a GPIO, and for a simple verification of the
timer frequency on another platform.

Since high-resolution timers are used, sleeping GPIO chips are not
supported and are rejected in the probe function.

Signed-off-by: default avatarVincent Whitchurch <vincent.whitchurch@axis.com>
Co-developed-by: default avatarStefan Wahren <wahrenst@gmx.net>
Signed-off-by: default avatarStefan Wahren <wahrenst@gmx.net>
Co-developed-by: default avatarLinus Walleij <linus.walleij@linaro.org>
Reviewed-by: default avatarAndy Shevchenko <andy@kernel.org>
Signed-off-by: default avatarLinus Walleij <linus.walleij@linaro.org>
Reviewed-by: default avatarDhruva Gole <d-gole@ti.com>
Link: https://lore.kernel.org/r/20240604-pwm-gpio-v7-2-6b67cf60db92@linaro.org


Signed-off-by: default avatarUwe Kleine-König <ukleinek@kernel.org>
parent 1577ddaa

Documentation/driver-api/gpio/drivers-on-gpio.rst

+6 −1
Original line number Diff line number Diff line
@@ -27,7 +27,12 @@ hardware descriptions such as device tree or ACPI: 
  to the lines for a more permanent solution of this type.



- gpio-beeper: drivers/input/misc/gpio-beeper.c is used to provide a beep from

  an external speaker connected to a GPIO line.

  an external speaker connected to a GPIO line. (If the beep is controlled by

  off/on, for an actual PWM waveform, see pwm-gpio below.)



- pwm-gpio: drivers/pwm/pwm-gpio.c is used to toggle a GPIO with a high

  resolution timer producing a PWM waveform on the GPIO line, as well as

  Linux high resolution timers can do.



- extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an

  external connector status, such as a headset line for an audio driver or an

drivers/pwm/Kconfig

+11 −0
Original line number Diff line number Diff line
@@ -236,6 +236,17 @@ config PWM_FSL_FTM 
	  To compile this driver as a module, choose M here: the module

	  will be called pwm-fsl-ftm.



config PWM_GPIO

	tristate "GPIO PWM support"

	depends on GPIOLIB

	depends on HIGH_RES_TIMERS

	help

	  Generic PWM framework driver for software PWM toggling a GPIO pin

	  from kernel high-resolution timers.



	  To compile this driver as a module, choose M here: the module

	  will be called pwm-gpio.



config PWM_HIBVT

	tristate "HiSilicon BVT PWM support"

	depends on ARCH_HISI || COMPILE_TEST

drivers/pwm/Makefile

+1 −0
Original line number Diff line number Diff line
@@ -19,6 +19,7 @@ obj-$(CONFIG_PWM_DWC_CORE) += pwm-dwc-core.o 
obj-$(CONFIG_PWM_DWC)		+= pwm-dwc.o

obj-$(CONFIG_PWM_EP93XX)	+= pwm-ep93xx.o

obj-$(CONFIG_PWM_FSL_FTM)	+= pwm-fsl-ftm.o

obj-$(CONFIG_PWM_GPIO)		+= pwm-gpio.o

obj-$(CONFIG_PWM_HIBVT)		+= pwm-hibvt.o

obj-$(CONFIG_PWM_IMG)		+= pwm-img.o

obj-$(CONFIG_PWM_IMX1)		+= pwm-imx1.o

drivers/pwm/pwm-gpio.c

0 → 100644
+241 −0
Original line number Diff line number Diff line 
// SPDX-License-Identifier: GPL-2.0-only

/*

 * Generic software PWM for modulating GPIOs

 *

 * Copyright (C) 2020 Axis Communications AB

 * Copyright (C) 2020 Nicola Di Lieto

 * Copyright (C) 2024 Stefan Wahren

 * Copyright (C) 2024 Linus Walleij

 */



#include <linux/cleanup.h>

#include <linux/container_of.h>

#include <linux/device.h>

#include <linux/err.h>

#include <linux/gpio/consumer.h>

#include <linux/hrtimer.h>

#include <linux/math.h>

#include <linux/module.h>

#include <linux/mod_devicetable.h>

#include <linux/platform_device.h>

#include <linux/property.h>

#include <linux/pwm.h>

#include <linux/spinlock.h>

#include <linux/time.h>

#include <linux/types.h>



struct pwm_gpio {

	struct hrtimer gpio_timer;

	struct gpio_desc *gpio;

	struct pwm_state state;

	struct pwm_state next_state;



	/* Protect internal state between pwm_ops and hrtimer */

	spinlock_t lock;



	bool changing;

	bool running;

	bool level;

};



static void pwm_gpio_round(struct pwm_state *dest, const struct pwm_state *src)

{

	u64 dividend;

	u32 remainder;



	*dest = *src;



	/* Round down to hrtimer resolution */

	dividend = dest->period;

	remainder = do_div(dividend, hrtimer_resolution);

	dest->period -= remainder;



	dividend = dest->duty_cycle;

	remainder = do_div(dividend, hrtimer_resolution);

	dest->duty_cycle -= remainder;

}



static u64 pwm_gpio_toggle(struct pwm_gpio *gpwm, bool level)

{

	const struct pwm_state *state = &gpwm->state;

	bool invert = state->polarity == PWM_POLARITY_INVERSED;



	gpwm->level = level;

	gpiod_set_value(gpwm->gpio, gpwm->level ^ invert);



	if (!state->duty_cycle || state->duty_cycle == state->period) {

		gpwm->running = false;

		return 0;

	}



	gpwm->running = true;

	return level ? state->duty_cycle : state->period - state->duty_cycle;

}



static enum hrtimer_restart pwm_gpio_timer(struct hrtimer *gpio_timer)

{

	struct pwm_gpio *gpwm = container_of(gpio_timer, struct pwm_gpio,

					     gpio_timer);

	u64 next_toggle;

	bool new_level;



	guard(spinlock_irqsave)(&gpwm->lock);



	/* Apply new state at end of current period */

	if (!gpwm->level && gpwm->changing) {

		gpwm->changing = false;

		gpwm->state = gpwm->next_state;

		new_level = !!gpwm->state.duty_cycle;

	} else {

		new_level = !gpwm->level;

	}



	next_toggle = pwm_gpio_toggle(gpwm, new_level);

	if (next_toggle)

		hrtimer_forward(gpio_timer, hrtimer_get_expires(gpio_timer),

				ns_to_ktime(next_toggle));



	return next_toggle ? HRTIMER_RESTART : HRTIMER_NORESTART;

}



static int pwm_gpio_apply(struct pwm_chip *chip, struct pwm_device *pwm,

			  const struct pwm_state *state)

{

	struct pwm_gpio *gpwm = pwmchip_get_drvdata(chip);

	bool invert = state->polarity == PWM_POLARITY_INVERSED;



	if (state->duty_cycle && state->duty_cycle < hrtimer_resolution)

		return -EINVAL;



	if (state->duty_cycle != state->period &&

	    (state->period - state->duty_cycle < hrtimer_resolution))

		return -EINVAL;



	if (!state->enabled) {

		hrtimer_cancel(&gpwm->gpio_timer);

	} else if (!gpwm->running) {

		int ret;



		/*

		 * This just enables the output, but pwm_gpio_toggle()

		 * really starts the duty cycle.

		 */

		ret = gpiod_direction_output(gpwm->gpio, invert);

		if (ret)

			return ret;

	}



	guard(spinlock_irqsave)(&gpwm->lock);



	if (!state->enabled) {

		pwm_gpio_round(&gpwm->state, state);

		gpwm->running = false;

		gpwm->changing = false;



		gpiod_set_value(gpwm->gpio, invert);

	} else if (gpwm->running) {

		pwm_gpio_round(&gpwm->next_state, state);

		gpwm->changing = true;

	} else {

		unsigned long next_toggle;



		pwm_gpio_round(&gpwm->state, state);

		gpwm->changing = false;



		next_toggle = pwm_gpio_toggle(gpwm, !!state->duty_cycle);

		if (next_toggle)

			hrtimer_start(&gpwm->gpio_timer, next_toggle,

				      HRTIMER_MODE_REL);

	}



	return 0;

}



static int pwm_gpio_get_state(struct pwm_chip *chip, struct pwm_device *pwm,

			       struct pwm_state *state)

{

	struct pwm_gpio *gpwm = pwmchip_get_drvdata(chip);



	guard(spinlock_irqsave)(&gpwm->lock);



	if (gpwm->changing)

		*state = gpwm->next_state;

	else

		*state = gpwm->state;



	return 0;

}



static const struct pwm_ops pwm_gpio_ops = {

	.apply = pwm_gpio_apply,

	.get_state = pwm_gpio_get_state,

};



static void pwm_gpio_disable_hrtimer(void *data)

{

	struct pwm_gpio *gpwm = data;



	hrtimer_cancel(&gpwm->gpio_timer);

}



static int pwm_gpio_probe(struct platform_device *pdev)

{

	struct device *dev = &pdev->dev;

	struct pwm_chip *chip;

	struct pwm_gpio *gpwm;

	int ret;



	chip = devm_pwmchip_alloc(dev, 1, sizeof(*gpwm));

	if (IS_ERR(chip))

		return PTR_ERR(chip);



	gpwm = pwmchip_get_drvdata(chip);



	spin_lock_init(&gpwm->lock);



	gpwm->gpio = devm_gpiod_get(dev, NULL, GPIOD_ASIS);

	if (IS_ERR(gpwm->gpio))

		return dev_err_probe(dev, PTR_ERR(gpwm->gpio),

				     "%pfw: could not get gpio\n",

				     dev_fwnode(dev));



	if (gpiod_cansleep(gpwm->gpio))

		return dev_err_probe(dev, -EINVAL,

				     "%pfw: sleeping GPIO not supported\n",

				     dev_fwnode(dev));



	chip->ops = &pwm_gpio_ops;

	chip->atomic = true;



	hrtimer_init(&gpwm->gpio_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);

	ret = devm_add_action_or_reset(dev, pwm_gpio_disable_hrtimer, gpwm);

	if (ret)

		return ret;



	gpwm->gpio_timer.function = pwm_gpio_timer;



	ret = pwmchip_add(chip);

	if (ret < 0)

		return dev_err_probe(dev, ret, "could not add pwmchip\n");



	return 0;

}



static const struct of_device_id pwm_gpio_dt_ids[] = {

	{ .compatible = "pwm-gpio" },

	{ /* sentinel */ }

};

MODULE_DEVICE_TABLE(of, pwm_gpio_dt_ids);



static struct platform_driver pwm_gpio_driver = {

	.driver = {

		.name = "pwm-gpio",

		.of_match_table = pwm_gpio_dt_ids,

	},

	.probe = pwm_gpio_probe,

};

module_platform_driver(pwm_gpio_driver);



MODULE_DESCRIPTION("PWM GPIO driver");

MODULE_AUTHOR("Vincent Whitchurch");

MODULE_LICENSE("GPL");