gpiolib: cdev: support setting debounce

Add support for setting debounce on a line via the GPIO uAPI.
Where debounce is not supported by hardware, a software debounce is
provided.

The implementation of the software debouncer waits for the line to be
stable for the debounce period before determining if a level change,
and a corresponding edge event, has occurred.  This provides maximum
protection against glitches, but also introduces a debounce_period
latency to edge events.

The software debouncer is integrated with the edge detection as it
utilises the line interrupt, and integration is simpler than getting
the two to interwork.  Where software debounce AND edge detection is
required, the debouncer provides both.
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

drivers/gpio/gpiolib.c

@@ -2097,6 +2097,9 @@ static bool gpiod_free_commit(struct gpio_desc *desc)
 		clear_bit(FLAG_IS_HOGGED, &desc->flags);
 #ifdef CONFIG_OF_DYNAMIC
 		desc->hog = NULL;
+#endif
+#ifdef CONFIG_GPIO_CDEV
+		WRITE_ONCE(desc->debounce_period_us, 0);
 #endif
 		ret = true;
 	}

drivers/gpio/gpiolib.h

@@ -124,6 +124,10 @@ struct gpio_desc {
 #ifdef CONFIG_OF_DYNAMIC
 	struct device_node	*hog;
 #endif
+#ifdef CONFIG_GPIO_CDEV
+	/* debounce period in microseconds */
+	unsigned int		debounce_period_us;
+#endif
 };
 
 int gpiod_request(struct gpio_desc *desc, const char *label);