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: Vincent Whitchurch <vincent.whitchurch@axis.com>
Co-developed-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Co-developed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Link: https://lore.kernel.org/r/20240604-pwm-gpio-v7-2-6b67cf60db92@linaro.org
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>

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

@@ -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

@@ -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

@@ -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

@@ -0,0 +1,241 @@
+// 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");