driver core: create class_is_registered()

Some classes (i.e. gpio), want to know if they have been registered or
not, and poke around in the class's internal structures to try to figure
this out.  Because this is not really a good idea, provide a function
for classes to call to try to figure this out.

Note, this is racy as the state of the class could change at any moment
in time after the call is made, but as usually a class only wants to
know if it has been registered yet or not, it should be fairly safe to
use, and is just as safe as the previous "poke at the class internals"
check was.

Move the gpiolib code to use this function as proof that it works
properly.

Cc: Bartosz Golaszewski <brgl@bgdev.pl>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: linux-gpio@vger.kernel.org
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230331093318.82288-2-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/class.c

@@ -634,6 +634,31 @@ void class_compat_remove_link(struct class_compat *cls, struct device *dev,
 }
 EXPORT_SYMBOL_GPL(class_compat_remove_link);
 
+/**
+ * class_is_registered - determine if at this moment in time, a class is
+ *			 registered in the driver core or not.
+ * @class: the class to check
+ *
+ * Returns a boolean to state if the class is registered in the driver core
+ * or not.  Note that the value could switch right after this call is made,
+ * so only use this in places where you "know" it is safe to do so (usually
+ * to determine if the specific class has been registered yet or not).
+ *
+ * Be careful in using this.
+ */
+bool class_is_registered(const struct class *class)
+{
+	struct subsys_private *sp = class_to_subsys(class);
+	bool is_initialized = false;
+
+	if (sp) {
+		is_initialized = true;
+		subsys_put(sp);
+	}
+	return is_initialized;
+}
+EXPORT_SYMBOL_GPL(class_is_registered);
+
 int __init classes_init(void)
 {
 	class_kset = kset_create_and_add("class", NULL, NULL);

drivers/gpio/gpiolib-sysfs.c

@@ -554,7 +554,7 @@ int gpiod_export(struct gpio_desc *desc, bool direction_may_change)
 	int			offset;
 
 	/* can't export until sysfs is available ... */
-	if (!gpio_class.p) {
+	if (!class_is_registered(&gpio_class)) {
 		pr_debug("%s: called too early!\n", __func__);
 		return -ENOENT;
 	}
@@ -728,7 +728,7 @@ int gpiochip_sysfs_register(struct gpio_device *gdev)
 	 * register later, in gpiolib_sysfs_init() ... here we just
 	 * verify that _some_ field of gpio_class got initialized.
 	 */
-	if (!gpio_class.p)
+	if (!class_is_registered(&gpio_class))
 		return 0;
 
 	/*

include/linux/device/class.h

@@ -84,6 +84,7 @@ extern struct kobject *sysfs_dev_block_kobj;
 
 int __must_check class_register(struct class *class);
 void class_unregister(const struct class *class);
+bool class_is_registered(const struct class *class);
 
 struct class_compat;
 struct class_compat *class_compat_register(const char *name);