cpuidle: move to use bus_get_dev_root()

Direct access to the struct bus_type dev_root pointer is going away soon
so replace that with a call to bus_get_dev_root() instead, which is what
it is there for.

This allows us to clean up the cpuidle_add_interface() call a bit as it
was only called in one place, with the same argument so just put that
into the function itself.  Note that cpuidle_remove_interface() should
also probably be removed in the future as there are no callers of it for
some reason.

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: linux-pm@vger.kernel.org
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230322090557.2943479-1-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/cpuidle/cpuidle.c

@@ -808,7 +808,7 @@ static int __init cpuidle_init(void)
 	if (cpuidle_disabled())
 		return -ENODEV;
 
-	return cpuidle_add_interface(cpu_subsys.dev_root);
+	return cpuidle_add_interface();
 }
 
 module_param(off, int, 0444);

drivers/cpuidle/cpuidle.h

@@ -30,7 +30,7 @@ extern int cpuidle_switch_governor(struct cpuidle_governor *gov);
 
 struct device;
 
-extern int cpuidle_add_interface(struct device *dev);
+extern int cpuidle_add_interface(void);
 extern void cpuidle_remove_interface(struct device *dev);
 extern int cpuidle_add_device_sysfs(struct cpuidle_device *device);
 extern void cpuidle_remove_device_sysfs(struct cpuidle_device *device);

drivers/cpuidle/sysfs.c

@@ -119,11 +119,18 @@ static struct attribute_group cpuidle_attr_group = {
 
 /**
  * cpuidle_add_interface - add CPU global sysfs attributes
- * @dev: the target device
  */
-int cpuidle_add_interface(struct device *dev)
+int cpuidle_add_interface(void)
 {
-	return sysfs_create_group(&dev->kobj, &cpuidle_attr_group);
+	struct device *dev_root = bus_get_dev_root(&cpu_subsys);
+	int retval;
+
+	if (!dev_root)
+		return -EINVAL;
+
+	retval = sysfs_create_group(&dev_root->kobj, &cpuidle_attr_group);
+	put_device(dev_root);
+	return retval;
 }
 
 /**