gpio: provide gpiod_is_shared()

Provide an interface allowing consumers to check if a GPIO descriptor
represents a GPIO that can potentially be shared by multiple consumers
at the same time. This is exposed to allow subsystems that already
work around the limitations of the current non-exclusive GPIO handling
in some ways, to gradually convert to relying on the new shared GPIO
feature of GPIOLIB.

Extend the gpiolib-shared module to mark the GPIO shared proxy
descriptors with a flag checked by the new interface.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20251112-gpio-shared-v4-6-b51f97b1abd8@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

3 files changed, 39 insertions, 0 deletions

diff --git a/drivers/gpio/gpiolib-shared.c b/drivers/gpio/gpiolib-shared.c
index 1fcbf53b6eca..fa1d16635ea7 100644
--- a/drivers/gpio/gpiolib-shared.c
+++ b/drivers/gpio/gpiolib-shared.c
@@ -315,6 +315,24 @@ int gpio_device_setup_shared(struct gpio_device *gdev)
 	guard(mutex)(&gpio_shared_lock);
 
 	list_for_each_entry(entry, &gpio_shared_list, list) {
+		list_for_each_entry(ref, &entry->refs, list) {
+			if (gdev->dev.parent == &ref->adev.dev) {
+				/*
+				 * This is a shared GPIO proxy. Mark its
+				 * descriptor as such and return here.
+				 */
+				__set_bit(GPIOD_FLAG_SHARED_PROXY,
+					  &gdev->descs[0].flags);
+				return 0;
+			}
+		}
+	}
+
+	/*
+	 * This is not a shared GPIO proxy but it still may be the device
+	 * exposing shared pins. Find them and create the proxy devices.
+	 */
+	list_for_each_entry(entry, &gpio_shared_list, list) {
 		if (!device_match_fwnode(&gdev->dev, entry->fwnode))
 			continue;
 
diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 1e4c99179712..678d07dc768c 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -3991,6 +3991,26 @@ int gpiod_set_consumer_name(struct gpio_desc *desc, const char *name)
 EXPORT_SYMBOL_GPL(gpiod_set_consumer_name);
 
 /**
+ * gpiod_is_shared() - check if this GPIO can be shared by multiple consumers
+ * @desc: GPIO to inspect
+ *
+ * Returns:
+ * True if this GPIO can be shared by multiple consumers at once. False if it's
+ * a regular, exclusive GPIO.
+ *
+ * Note:
+ * This function returning true does not mean that this GPIO is currently being
+ * shared. It means the GPIO core has registered the fact that the firmware
+ * configuration indicates that it can be shared by multiple consumers and is
+ * in charge of arbitrating the access.
+ */
+bool gpiod_is_shared(const struct gpio_desc *desc)
+{
+	return test_bit(GPIOD_FLAG_SHARED_PROXY, &desc->flags);
+}
+EXPORT_SYMBOL_GPL(gpiod_is_shared);
+
+/**
  * gpiod_to_irq() - return the IRQ corresponding to a GPIO
  * @desc: gpio whose IRQ will be returned (already requested)
  *
diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
index 09ac92ed0853..abd870fb4a3b 100644
--- a/drivers/gpio/gpiolib.h
+++ b/drivers/gpio/gpiolib.h
@@ -205,6 +205,7 @@ struct gpio_desc {
 #define GPIOD_FLAG_EVENT_CLOCK_REALTIME	18 /* GPIO CDEV reports REALTIME timestamps in events */
 #define GPIOD_FLAG_EVENT_CLOCK_HTE	19 /* GPIO CDEV reports hardware timestamps in events */
 #define GPIOD_FLAG_SHARED		20 /* GPIO is shared by multiple consumers */
+#define GPIOD_FLAG_SHARED_PROXY		21 /* GPIO is a virtual proxy to a physically shared pin. */
 
 	/* Connection label */
 	struct gpio_desc_label __rcu *label;