pwm: Provide a gpio device for waveform drivers

A PWM is a more general concept than an output-only GPIO. When using
duty_length = period_length the PWM looks like an active GPIO, with
duty_length = 0 like an inactive GPIO. With the waveform abstraction
there is enough control over the configuration to ensure that PWMs that
cannot generate a constant signal at both levels error out.

The pwm-pca9685 driver already provides a gpio chip. When this driver is
converted to the waveform callbacks, the gpio part can just be dropped.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://lore.kernel.org/r/20250717151117.1828585-2-u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>

drivers/pwm/Kconfig

@@ -38,6 +38,15 @@ config PWM_DEBUG
 	  It is expected to introduce some runtime overhead and diagnostic
 	  output to the kernel log, so only enable while working on a driver.
 
+config PWM_PROVIDE_GPIO
+	bool "Provide a GPIO chip for each PWM chip"
+	depends on GPIOLIB
+	help
+	  Most PWMs can emit both a constant active high and a constant active
+	  low signal and so they can be used as GPIO. Say Y here to let each
+	  PWM chip provide a GPIO chip and so be easily plugged into consumers
+	  that know how to handle GPIOs but not PWMs.
+
 config PWM_AB8500
 	tristate "AB8500 PWM support"
 	depends on AB8500_CORE && ARCH_U8500

drivers/pwm/core.c

@@ -2391,6 +2391,51 @@ static const struct file_operations pwm_cdev_fileops = {
 
 static dev_t pwm_devt;
 
+static int pwm_gpio_request(struct gpio_chip *gc, unsigned int offset)
+{
+	struct pwm_chip *chip = gpiochip_get_data(gc);
+	struct pwm_device *pwm;
+
+	pwm = pwm_request_from_chip(chip, offset, "pwm-gpio");
+	if (IS_ERR(pwm))
+		return PTR_ERR(pwm);
+
+	return 0;
+}
+
+static void pwm_gpio_free(struct gpio_chip *gc, unsigned int offset)
+{
+	struct pwm_chip *chip = gpiochip_get_data(gc);
+
+	pwm_put(&chip->pwms[offset]);
+}
+
+static int pwm_gpio_get_direction(struct gpio_chip *gc, unsigned int offset)
+{
+	return GPIO_LINE_DIRECTION_OUT;
+}
+
+static int pwm_gpio_set(struct gpio_chip *gc, unsigned int offset, int value)
+{
+	struct pwm_chip *chip = gpiochip_get_data(gc);
+	struct pwm_device *pwm = &chip->pwms[offset];
+	int ret;
+	struct pwm_waveform wf = {
+		.period_length_ns = 1,
+	};
+
+	ret = pwm_round_waveform_might_sleep(pwm, &wf);
+	if (ret < 0)
+		return ret;
+
+	if (value)
+		wf.duty_length_ns = wf.period_length_ns;
+	else
+		wf.duty_length_ns = 0;
+
+	return pwm_set_waveform_might_sleep(pwm, &wf, true);
+}
+
 /**
  * __pwmchip_add() - register a new PWM chip
  * @chip: the PWM chip to add
@@ -2457,9 +2502,33 @@ int __pwmchip_add(struct pwm_chip *chip, struct module *owner)
 	if (ret)
 		goto err_device_add;
 
+	if (IS_ENABLED(CONFIG_PWM_PROVIDE_GPIO) && chip->ops->write_waveform) {
+		struct device *parent = pwmchip_parent(chip);
+
+		chip->gpio = (typeof(chip->gpio)){
+			.label = dev_name(parent),
+			.parent = parent,
+			.request = pwm_gpio_request,
+			.free = pwm_gpio_free,
+			.get_direction = pwm_gpio_get_direction,
+			.set = pwm_gpio_set,
+			.base = -1,
+			.ngpio = chip->npwm,
+			.can_sleep = true,
+		};
+
+		ret = gpiochip_add_data(&chip->gpio, chip);
+		if (ret)
+			goto err_gpiochip_add;
+	}
+
 	return 0;
 
+err_gpiochip_add:
+
+	cdev_device_del(&chip->cdev, &chip->dev);
 err_device_add:
+
 	scoped_guard(pwmchip, chip)
 		chip->operational = false;
 
@@ -2480,6 +2549,9 @@ EXPORT_SYMBOL_GPL(__pwmchip_add);
  */
 void pwmchip_remove(struct pwm_chip *chip)
 {
+	if (IS_ENABLED(CONFIG_PWM_PROVIDE_GPIO) && chip->ops->write_waveform)
+		gpiochip_remove(&chip->gpio);
+
 	pwmchip_sysfs_unexport(chip);
 
 	scoped_guard(mutex, &pwm_lock) {

include/linux/pwm.h

@@ -5,6 +5,7 @@
 #include <linux/cdev.h>
 #include <linux/device.h>
 #include <linux/err.h>
+#include <linux/gpio/driver.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/of.h>
@@ -321,6 +322,7 @@ struct pwm_ops {
  * @npwm: number of PWMs controlled by this chip
  * @of_xlate: request a PWM device given a device tree PWM specifier
  * @atomic: can the driver's ->apply() be called in atomic context
+ * @gpio: &struct gpio_chip to operate this PWM chip's lines as GPO
  * @uses_pwmchip_alloc: signals if pwmchip_allow was used to allocate this chip
  * @operational: signals if the chip can be used (or is already deregistered)
  * @nonatomic_lock: mutex for nonatomic chips
@@ -340,6 +342,7 @@ struct pwm_chip {
 	bool atomic;
 
 	/* only used internally by the PWM framework */
+	struct gpio_chip gpio;
 	bool uses_pwmchip_alloc;
 	bool operational;
 	union {