gpio: don't set label from irq helpers

We will soon serialize access to the descriptor label using SRCU. The
write-side of the protection will require calling synchronize_srcu()
which must not be called from atomic context. We have two irq helpers:
gpiochip_lock_as_irq() and gpiochip_unlock_as_irq() that set the label
if the GPIO is not requested but is being used as interrupt. They are
called with a spinlock held from the interrupt subsystem.

They must not do it if we are to use SRCU so instead let's move the
special corner case to a dedicated getter.

Don't actually set the label to "interrupt" in the above case but rather
use the newly added gpiod_get_label() helper to hide the logic that
atomically checks the descriptor flags and returns the address of a
static "interrupt" string.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/gpio/gpiolib.c

@@ -107,7 +107,14 @@ static bool gpiolib_initialized;
 
 const char *gpiod_get_label(struct gpio_desc *desc)
 {
-	return desc->label;
+	unsigned long flags;
+
+	flags = READ_ONCE(desc->flags);
+	if (test_bit(FLAG_USED_AS_IRQ, &flags) &&
+	    !test_bit(FLAG_REQUESTED, &flags))
+		return "interrupt";
+
+	return test_bit(FLAG_REQUESTED, &flags) ? desc->label : NULL;
 }
 
 static inline void desc_set_label(struct gpio_desc *d, const char *label)
@@ -3599,14 +3606,6 @@ int gpiochip_lock_as_irq(struct gpio_chip *gc, unsigned int offset)
 	set_bit(FLAG_USED_AS_IRQ, &desc->flags);
 	set_bit(FLAG_IRQ_IS_ENABLED, &desc->flags);
 
-	/*
-	 * If the consumer has not set up a label (such as when the
-	 * IRQ is referenced from .to_irq()) we set up a label here
-	 * so it is clear this is used as an interrupt.
-	 */
-	if (!desc->label)
-		desc_set_label(desc, "interrupt");
-
 	return 0;
 }
 EXPORT_SYMBOL_GPL(gpiochip_lock_as_irq);
@@ -3629,10 +3628,6 @@ void gpiochip_unlock_as_irq(struct gpio_chip *gc, unsigned int offset)
 
 	clear_bit(FLAG_USED_AS_IRQ, &desc->flags);
 	clear_bit(FLAG_IRQ_IS_ENABLED, &desc->flags);
-
-	/* If we only had this marking, erase it */
-	if (desc->label && !strcmp(desc->label, "interrupt"))
-		desc_set_label(desc, NULL);
 }
 EXPORT_SYMBOL_GPL(gpiochip_unlock_as_irq);