apple_bl: Add register/unregister functions

Add functions to allow other modules to enable or disable apple_bl. This
will be used by the gmux driver to disable apple_bl when the gmux is
present, as it is a better and more reliable option for brightness
control.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>

drivers/video/backlight/apple_bl.c

@@ -24,6 +24,7 @@
 #include <linux/io.h>
 #include <linux/pci.h>
 #include <linux/acpi.h>
+#include <linux/atomic.h>
 
 static struct backlight_device *apple_backlight_device;
 
@@ -221,14 +222,32 @@ static struct acpi_driver apple_bl_driver = {
 	},
 };
 
+static atomic_t apple_bl_registered = ATOMIC_INIT(0);
+
+int apple_bl_register(void)
+{
+	if (atomic_xchg(&apple_bl_registered, 1) == 0)
+		return acpi_bus_register_driver(&apple_bl_driver);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(apple_bl_register);
+
+void apple_bl_unregister(void)
+{
+	if (atomic_xchg(&apple_bl_registered, 0) == 1)
+		acpi_bus_unregister_driver(&apple_bl_driver);
+}
+EXPORT_SYMBOL_GPL(apple_bl_unregister);
+
 static int __init apple_bl_init(void)
 {
-	return acpi_bus_register_driver(&apple_bl_driver);
+	return apple_bl_register();
 }
 
 static void __exit apple_bl_exit(void)
 {
-	acpi_bus_unregister_driver(&apple_bl_driver);
+	apple_bl_unregister();
 }
 
 module_init(apple_bl_init);

include/linux/apple_bl.h

+/*
+ * apple_bl exported symbols
+ */
+
+#ifndef _LINUX_APPLE_BL_H
+#define _LINUX_APPLE_BL_H
+
+#ifdef CONFIG_BACKLIGHT_APPLE
+
+extern int apple_bl_register(void);
+extern void apple_bl_unregister(void);
+
+#else /* !CONFIG_BACKLIGHT_APPLE */
+
+static inline int apple_bl_register(void)
+{
+	return 0;
+}
+
+static inline void apple_bl_unregister(void)
+{
+}
+
+#endif /* !CONFIG_BACKLIGHT_APPLE */
+
+#endif /* _LINUX_APPLE_BL_H */