drivers/gpio: Add a driver that wraps the PWM API as a GPIO controller

For cases where spare PWM outputs are available, but are desired
to be addressed a standard outputs instead.
Wraps a PWM channel as a new GPIO chip with the one output.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>

drivers/gpio/Kconfig

@@ -473,6 +473,14 @@ config GPIO_PMIC_EIC_SPRD
 	help
 	  Say yes here to support Spreadtrum PMIC EIC device.
 
+config GPIO_PWM
+	tristate "PWM chip GPIO"
+	depends on OF_GPIO
+	depends on PWM
+	help
+	  Turn on support for exposing a PWM chip as a GPIO
+	  driver.
+
 config GPIO_PXA
 	bool "PXA GPIO support"
 	depends on ARCH_PXA || ARCH_MMP || COMPILE_TEST

drivers/gpio/Makefile

@@ -120,6 +120,7 @@ obj-$(CONFIG_GPIO_PCI_IDIO_16)		+= gpio-pci-idio-16.o
 obj-$(CONFIG_GPIO_PISOSR)		+= gpio-pisosr.o
 obj-$(CONFIG_GPIO_PL061)		+= gpio-pl061.o
 obj-$(CONFIG_GPIO_PMIC_EIC_SPRD)	+= gpio-pmic-eic-sprd.o
+obj-$(CONFIG_GPIO_PWM)			+= gpio-pwm.o
 obj-$(CONFIG_GPIO_PXA)			+= gpio-pxa.o
 obj-$(CONFIG_GPIO_RASPBERRYPI_EXP)	+= gpio-raspberrypi-exp.o
 obj-$(CONFIG_GPIO_RC5T583)		+= gpio-rc5t583.o

drivers/gpio/gpio-pwm.c

@@ -0,0 +1,144 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * GPIO driver wrapping PWM API
+ *
+ * PWM 0% and PWM 100% are equivalent to digital GPIO
+ * outputs, and there are times where it is useful to use
+ * PWM outputs as straight GPIOs (eg outputs of NXP PCA9685
+ * I2C PWM chip). This driver wraps the PWM API as a GPIO
+ * controller.
+ *
+ * Copyright (C) 2021 Raspberry Pi (Trading) Ltd.
+ */
+
+#include <linux/err.h>
+#include <linux/gpio/driver.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/pwm.h>
+
+struct pwm_gpio {
+	struct gpio_chip gc;
+	struct pwm_device **pwm;
+};
+
+static int pwm_gpio_get_direction(struct gpio_chip *gc, unsigned int off)
+{
+	return GPIO_LINE_DIRECTION_OUT;
+}
+
+static void pwm_gpio_set(struct gpio_chip *gc, unsigned int off, int val)
+{
+	struct pwm_gpio *pwm_gpio = gpiochip_get_data(gc);
+	struct pwm_state state;
+
+	pwm_get_state(pwm_gpio->pwm[off], &state);
+	state.duty_cycle = val ? state.period : 0;
+	pwm_apply_state(pwm_gpio->pwm[off], &state);
+}
+
+static int pwm_gpio_parse_dt(struct pwm_gpio *pwm_gpio,
+			     struct device *dev)
+{
+	struct device_node *node = dev->of_node;
+	struct pwm_state state;
+	int ret = 0, i, num_gpios;
+	const char *pwm_name;
+
+	if (!node)
+		return -ENODEV;
+
+	num_gpios = of_property_count_strings(node, "pwm-names");
+	if (num_gpios <= 0)
+		return 0;
+
+	pwm_gpio->pwm = devm_kzalloc(dev,
+				     sizeof(*pwm_gpio->pwm) * num_gpios,
+				     GFP_KERNEL);
+	if (!pwm_gpio->pwm)
+		return -ENOMEM;
+
+	for (i = 0; i < num_gpios; i++) {
+		ret = of_property_read_string_index(node, "pwm-names", i,
+						    &pwm_name);
+		if (ret) {
+			dev_err(dev, "unable to get pwm device index %d, name %s",
+				i, pwm_name);
+			goto error;
+		}
+
+		pwm_gpio->pwm[i] = devm_pwm_get(dev, pwm_name);
+		if (IS_ERR(pwm_gpio->pwm[i])) {
+			ret = PTR_ERR(pwm_gpio->pwm[i]);
+			if (ret != -EPROBE_DEFER)
+				dev_err(dev, "unable to request PWM\n");
+			goto error;
+		}
+
+		/* Sync up PWM state. */
+		pwm_init_state(pwm_gpio->pwm[i], &state);
+
+		state.duty_cycle = 0;
+		pwm_apply_state(pwm_gpio->pwm[i], &state);
+	}
+
+	pwm_gpio->gc.ngpio = num_gpios;
+
+error:
+	return ret;
+}
+
+static int pwm_gpio_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct pwm_gpio *pwm_gpio;
+	int ret;
+
+	pwm_gpio = devm_kzalloc(dev, sizeof(*pwm_gpio), GFP_KERNEL);
+	if (!pwm_gpio)
+		return -ENOMEM;
+
+	pwm_gpio->gc.parent = dev;
+	pwm_gpio->gc.label = "pwm-gpio";
+	pwm_gpio->gc.owner = THIS_MODULE;
+	pwm_gpio->gc.of_node = dev->of_node;
+	pwm_gpio->gc.base = -1;
+
+	pwm_gpio->gc.get_direction = pwm_gpio_get_direction;
+	pwm_gpio->gc.set = pwm_gpio_set;
+	pwm_gpio->gc.can_sleep = true;
+
+	ret = pwm_gpio_parse_dt(pwm_gpio, dev);
+	if (ret)
+		return ret;
+
+	if (!pwm_gpio->gc.ngpio)
+		return 0;
+
+	return devm_gpiochip_add_data(dev, &pwm_gpio->gc, pwm_gpio);
+}
+
+static int pwm_gpio_remove(struct platform_device *pdev)
+{
+	return 0;
+}
+
+static const struct of_device_id pwm_gpio_of_match[] = {
+	{ .compatible = "pwm-gpio" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, pwm_gpio_of_match);
+
+static struct platform_driver pwm_gpio_driver = {
+	.driver	= {
+		.name		= "pwm-gpio",
+		.of_match_table	= of_match_ptr(pwm_gpio_of_match),
+	},
+	.probe	= pwm_gpio_probe,
+	.remove	= pwm_gpio_remove,
+};
+module_platform_driver(pwm_gpio_driver);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Dave Stevenson <dave.stevenson@raspberrypi.com>");
+MODULE_DESCRIPTION("PWM GPIO driver");