diff --git a/drivers/staging/lustre/lustre/ptlrpc/layout.c b/drivers/staging/lustre/lustre/ptlrpc/layout.c
index 99d7c667df28..2052848aa59b 100644
--- a/drivers/staging/lustre/lustre/ptlrpc/layout.c
+++ b/drivers/staging/lustre/lustre/ptlrpc/layout.c
@@ -1181,6 +1181,23 @@ struct req_format RQF_FLD_QUERY =
 	DEFINE_REQ_FMT0("FLD_QUERY", fld_query_client, fld_query_server);
 EXPORT_SYMBOL(RQF_FLD_QUERY);
 
+/*
+ * The 'fld_read_server' uses 'RMF_GENERIC_DATA' to hold the 'FLD_QUERY'
+ * RPC reply that is composed of 'struct lu_seq_range_array'. But there
+ * is not registered swabber function for 'RMF_GENERIC_DATA'. So the RPC
+ * peers need to handle the RPC reply with fixed little-endian format.
+ *
+ * In theory, we can define new structure with some swabber registered to
+ * handle the 'FLD_QUERY' RPC reply result automatically. But from the
+ * implementation view, it is not easy to be done within current "struct
+ * req_msg_field" framework. Because the sequence range array in the RPC
+ * reply is not fixed length, instead, its length depends on 'lu_seq_range'
+ * count, that is unknown when prepare the RPC buffer. Generally, for such
+ * flexible length RPC usage, there will be a field in the RPC layout to
+ * indicate the data length. But for the 'FLD_READ' RPC, we have no way to
+ * do that unless we add new length filed that will broken the on-wire RPC
+ * protocol and cause interoperability trouble with old peer.
+ */
 struct req_format RQF_FLD_READ =
 	DEFINE_REQ_FMT0("FLD_READ", fld_read_client, fld_read_server);
 EXPORT_SYMBOL(RQF_FLD_READ);