i2c: Guard against oopses from bad init sequences

Guard I2C against oopsing because of init sequence problems, by
verifying that i2c_init() has been called before calling any
routines that rely on that initialization.  This specific test
just requires that bus_register(&i2c_bus_type) was called.

Examples of this kind of oopsing come from subystems and drivers
which register I2C drivers in their subsys_initcall code but
which are statically linked before I2C by drivers/Makefile.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>

drivers/i2c/i2c-core.c

@@ -437,6 +437,10 @@ static int i2c_register_adapter(struct i2c_adapter *adap)
 {
 	int res = 0, dummy;
 
+	/* Can't register until after driver model init */
+	if (unlikely(WARN_ON(!i2c_bus_type.p)))
+		return -EAGAIN;
+
 	mutex_init(&adap->bus_lock);
 	mutex_init(&adap->clist_lock);
 	INIT_LIST_HEAD(&adap->clients);
@@ -696,6 +700,10 @@ int i2c_register_driver(struct module *owner, struct i2c_driver *driver)
 {
 	int res;
 
+	/* Can't register until after driver model init */
+	if (unlikely(WARN_ON(!i2c_bus_type.p)))
+		return -EAGAIN;
+
 	/* new style driver methods can't mix with legacy ones */
 	if (is_newstyle_driver(driver)) {
 		if (driver->attach_adapter || driver->detach_adapter