greybus: operation: add helper to retrieve max payload size

Add helper to retrieve the maximum payload size for operations on a
specific connection.

Note that the helper is not inlined due to how the header files are
currently organised, but it is not expected to be called after a
connection has been initialised either.
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/operation.c

@@ -504,6 +504,14 @@ struct gb_operation *gb_operation_create(struct gb_connection *connection,
 }
 EXPORT_SYMBOL_GPL(gb_operation_create);
 
+size_t gb_operation_get_payload_size_max(struct gb_connection *connection)
+{
+	struct greybus_host_device *hd = connection->hd;
+
+	return hd->buffer_size_max - sizeof(struct gb_operation_msg_hdr);
+}
+EXPORT_SYMBOL_GPL(gb_operation_get_payload_size_max);
+
 static struct gb_operation *
 gb_operation_create_incoming(struct gb_connection *connection, u16 id,
 				u8 type, void *data, size_t size)

drivers/staging/greybus/operation.h

@@ -130,6 +130,7 @@ void gb_connection_recv(struct gb_connection *connection,
 
 int gb_operation_result(struct gb_operation *operation);
 
+size_t gb_operation_get_payload_size_max(struct gb_connection *connection);
 struct gb_operation *gb_operation_create(struct gb_connection *connection,
 					u8 type, size_t request_size,
 					size_t response_size);