greybus: rework synchronous operation completion

The only time we need a completion signaled on a request is when the
request provided no callback function.  In that case, we wait for
a completion on behalf of the caller.

If an interrupt occurs, we attempt to cancel the message that's
been sent, but we don't actually complete the operation as required.

Instead of simply waiting for the completion, put in place a
special callback function for the synchronous operation.  The
only job the callback has is to signal completion, allowing the
waiter to know it's done.

This means gb_operation_complete() will always have a non-null
callback pointer, so it becomes a simple wrapper, and we can get rid
of it and invoke the callback directly, in gb_operation_work().

Be defensive by checking for a null callback pointer, and reset
it to NULL once it's been called.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/operation.c

@@ -145,21 +145,6 @@ static void gb_message_cancel(struct gb_message *message)
 	hd->driver->buffer_cancel(message->cookie);
 }
 
-/*
- * An operation's response message has arrived.  If no callback was
- * supplied it was submitted for asynchronous completion, so we notify
- * any waiters.  Otherwise we assume calling the completion is enough
- * and nobody else will be waiting.
- */
-static void gb_operation_complete(struct gb_operation *operation)
-{
-	if (operation->callback)
-		operation->callback(operation);
-	else
-		complete(&operation->completion);
-	gb_operation_put(operation);
-}
-
 #if 0
 static void gb_operation_request_handle(struct gb_operation *operation)
 {
@@ -192,7 +177,12 @@ static void gb_operation_work(struct work_struct *work)
 	struct gb_operation *operation;
 
 	operation = container_of(work, struct gb_operation, work);
-	gb_operation_complete(operation);
+	if (WARN_ON(!operation->callback))
+		return;
+
+	operation->callback(operation);
+	operation->callback = NULL;
+	gb_operation_put(operation);
 }
 
 /*
@@ -423,14 +413,22 @@ void gb_operation_put(struct gb_operation *operation)
 		kref_put(&operation->kref, _gb_operation_destroy);
 }
 
+/* Tell the requester we're done */
+static void gb_operation_sync_callback(struct gb_operation *operation)
+{
+	complete(&operation->completion);
+}
+
 /*
  * Send an operation request message.  The caller has filled in
  * any payload so the request message is ready to go.  If non-null,
  * the callback function supplied will be called when the response
- * message has arrived indicating the operation is complete.  A null
+ * message has arrived indicating the operation is complete.  In
+ * that case, the callback function is responsible for extracting
+ * the result of the operation from operation->errno if desired,
+ * and dropping the final reference to the operation.  A null
  * callback function is used for a synchronous request; return from
- * this function won't occur until the operation is complete (or an
- * interrupt occurs).
+ * this function won't occur until the operation is complete.
  */
 int gb_operation_request_send(struct gb_operation *operation,
 				gb_operation_callback callback)
@@ -447,7 +445,11 @@ int gb_operation_request_send(struct gb_operation *operation,
 	 */
 	gb_operation_get(operation);
 
-	operation->callback = callback;
+	/* A null callback pointer means synchronous return */
+	if (callback)
+		operation->callback = callback;
+	else
+		operation->callback = gb_operation_sync_callback;
 	gb_pending_operation_insert(operation);
 
 	/*