driver core: faux: only create the device if probe() succeeds

It's really hard to know if a faux device properly passes the callback
to probe() without having to poke around in the faux_device structure
and then clean up.  Instead of having to have every user of the api do
this logic, just do it in the faux device core itself.

This makes the use of a custom probe() callback for a faux device much
simpler overall.

Suggested-by: Kurt Borja <kuurtb@gmail.com>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Reviewed-by: Kurt Borja <kuurtb@gmail.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/2025022545-unroasted-common-fa0e@gregkh
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: gregkh

drivers/base/faux.c

@@ -102,7 +102,9 @@ static void faux_device_release(struct device *dev)
  *
  * Note, when this function is called, the functions specified in struct
  * faux_ops can be called before the function returns, so be prepared for
- * everything to be properly initialized before that point in time.
+ * everything to be properly initialized before that point in time.  If the
+ * probe callback (if one is present) does NOT succeed, the creation of the
+ * device will fail and NULL will be returned.
  *
  * Return:
  * * NULL if an error happened with creating the device
@@ -147,6 +149,17 @@ struct faux_device *faux_device_create_with_groups(const char *name,
 		return NULL;
 	}
 
+	/*
+	 * Verify that we did bind the driver to the device (i.e. probe worked),
+	 * if not, let's fail the creation as trying to guess if probe was
+	 * successful is almost impossible to determine by the caller.
+	 */
+	if (!dev->driver) {
+		dev_err(dev, "probe did not succeed, tearing down the device\n");
+		faux_device_destroy(faux_dev);
+		faux_dev = NULL;
+	}
+
 	return faux_dev;
 }
 EXPORT_SYMBOL_GPL(faux_device_create_with_groups);