From: Vincent Whitchurch <redacted>

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 <redacted>
Co-developed-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
---
 drivers/pwm/Kconfig    |  11 ++
 drivers/pwm/Makefile   |   1 +
 drivers/pwm/pwm-gpio.c | 221 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 233 insertions(+)
 create mode 100644 drivers/pwm/pwm-gpio.c

diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
index 4b956d661755..7cfda2cde130 100644
--- a/drivers/pwm/Kconfig
+++ b/drivers/pwm/Kconfig

@@ -227,6 +227,17 @@ 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 a 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

diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
index c5ec9e168ee7..59d1a46bb1af 100644
--- a/drivers/pwm/Makefile
+++ b/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

diff --git a/drivers/pwm/pwm-gpio.c b/drivers/pwm/pwm-gpio.c
new file mode 100644
index 000000000000..14637abe465b
--- /dev/null
+++ b/drivers/pwm/pwm-gpio.c

@@ -0,0 +1,221 @@
+// 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
+ */
+
+#include <linux/device.h>
+#include <linux/err.h>
+#include <linux/gpio/consumer.h>
+#include <linux/hrtimer.h>
+#include <linux/module.h>
+#include <linux/mod_devicetable.h>
+#include <linux/platform_device.h>
+#include <linux/pwm.h>
+#include <linux/spinlock.h>
+
+struct pwm_gpio {
+	struct pwm_chip chip;
+	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 unsigned long 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);
+	unsigned long next_toggle;
+	unsigned long flags;
+	bool new_level;
+
+	spin_lock_irqsave(&gpwm->lock, flags);
+
+	/* 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));
+	}
+
+	spin_unlock_irqrestore(&gpwm->lock, flags);
+
+	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 = container_of(chip, struct pwm_gpio, chip);
+	bool invert = state->polarity == PWM_POLARITY_INVERSED;
+	unsigned long flags;