[PATCH 2/2] pwm: Add GPIO PWM driver

[Vincent Whitchurch <vincent.whitchurch@axis.com>]

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>
---
While preparing this driver for posting, I found a pwm-gpio driver posted to
the lists way back in 2015 by Olliver Schinagl:

 https://lore.kernel.org/linux-pwm/1445895161-2317-8-git-send-email-o.schinagl@ultimaker.com/

This driver was developed independently, but since both drivers are trivial
they are quite similar.  The main difference I see (apart from the usage of
newer APIs and DT schemas) is that this driver only supports one PWM per
instance, which makes for simpler code.  I also reject sleeping GPIO chips
explicitly while that driver uses gpio_set_value_cansleep() from a hrtimer,
which is a no-no.

 drivers/pwm/Kconfig    |  10 ++++
 drivers/pwm/Makefile   |   1 +
 drivers/pwm/pwm-gpio.c | 123 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 134 insertions(+)
 create mode 100644 drivers/pwm/pwm-gpio.c

diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
index 7dbcf6973d33..20e4fda82e61 100644
--- a/drivers/pwm/Kconfig
+++ b/drivers/pwm/Kconfig
@@ -181,6 +181,16 @@ 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 OF && GPIOLIB
+	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 2c2ba0a03557..2e045f063cd1 100644
--- a/drivers/pwm/Makefile
+++ b/drivers/pwm/Makefile
@@ -15,6 +15,7 @@ obj-$(CONFIG_PWM_CRC)		+= pwm-crc.o
 obj-$(CONFIG_PWM_CROS_EC)	+= pwm-cros-ec.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..e579aca0f937
--- /dev/null
+++ b/drivers/pwm/pwm-gpio.c
@@ -0,0 +1,123 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (C) 2020 Axis Communications AB */
+
+#include <linux/gpio/consumer.h>
+#include <linux/platform_device.h>
+#include <linux/hrtimer.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/pwm.h>
+#include <linux/err.h>
+#include <linux/of.h>
+
+struct pwm_gpio {
+	struct pwm_chip chip;
+	struct hrtimer hrtimer;
+	struct gpio_desc *gpio;
+	ktime_t on_interval;
+	ktime_t off_interval;
+	bool invert;
+	bool on;
+};
+
+static enum hrtimer_restart pwm_gpio_timer(struct hrtimer *hrtimer)
+{
+	struct pwm_gpio *gpwm = container_of(hrtimer, struct pwm_gpio, hrtimer);
+	bool newon = !gpwm->on;
+
+	gpwm->on = newon;
+	gpiod_set_value(gpwm->gpio, newon ^ gpwm->invert);
+
+	hrtimer_forward_now(hrtimer, newon ? gpwm->on_interval : gpwm->off_interval);
+
+	return HRTIMER_RESTART;
+}
+
+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);
+
+	hrtimer_cancel(&gpwm->hrtimer);
+
+	if (!state->enabled) {
+		gpiod_set_value(gpwm->gpio, 0);
+		return 0;
+	}
+
+	gpwm->on_interval = ns_to_ktime(state->duty_cycle);
+	gpwm->off_interval = ns_to_ktime(state->period - state->duty_cycle);
+	gpwm->invert = state->polarity == PWM_POLARITY_INVERSED;
+
+	gpwm->on = !!gpwm->on_interval;
+	gpiod_set_value(gpwm->gpio, gpwm->on ^ gpwm->invert);
+
+	if (gpwm->on_interval && gpwm->off_interval)
+		hrtimer_start(&gpwm->hrtimer, gpwm->on_interval, HRTIMER_MODE_REL);
+
+	return 0;
+}
+
+static const struct pwm_ops pwm_gpio_ops = {
+	.owner = THIS_MODULE,
+	.apply = pwm_gpio_apply,
+};
+
+static int pwm_gpio_probe(struct platform_device *pdev)
+{
+	struct pwm_gpio *gpwm;
+	int ret;
+
+	gpwm = devm_kzalloc(&pdev->dev, sizeof(*gpwm), GFP_KERNEL);
+	if (!gpwm)
+		return -ENOMEM;
+
+	gpwm->gpio = devm_gpiod_get(&pdev->dev, NULL, GPIOD_OUT_LOW);
+	if (IS_ERR(gpwm->gpio))
+		return PTR_ERR(gpwm->gpio);
+
+	if (gpiod_cansleep(gpwm->gpio))
+		return -EINVAL;
+
+	gpwm->chip.dev = &pdev->dev;
+	gpwm->chip.ops = &pwm_gpio_ops;
+	gpwm->chip.base = pdev->id;
+	gpwm->chip.npwm = 1;
+
+	hrtimer_init(&gpwm->hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	gpwm->hrtimer.function = pwm_gpio_timer;
+
+	ret = pwmchip_add(&gpwm->chip);
+	if (ret < 0)
+		return ret;
+
+	platform_set_drvdata(pdev, gpwm);
+
+	return 0;
+}
+
+static int pwm_gpio_remove(struct platform_device *pdev)
+{
+	struct pwm_gpio *gpwm = platform_get_drvdata(pdev);
+
+	return pwmchip_remove(&gpwm->chip);
+}
+
+static const struct of_device_id pwm_gpio_dt_ids[] = {
+	{ .compatible = "pwm-gpio", },
+	{ }
+};
+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,
+	.remove = pwm_gpio_remove,
+};
+
+module_platform_driver(pwm_gpio_driver);
+
+MODULE_LICENSE("GPL v2");
-- 
2.25.1