diff --git a/drivers/arm/mhu/mhu_wrapper_v2_x.c b/drivers/arm/mhu/mhu_wrapper_v2_x.c
index 60de1d38c..54a588126 100644
--- a/drivers/arm/mhu/mhu_wrapper_v2_x.c
+++ b/drivers/arm/mhu/mhu_wrapper_v2_x.c
@@ -308,5 +308,10 @@ size_t mhu_get_max_message_size(void)
 
 	assert(num_channels != 0);
 
-	return num_channels * sizeof(uint32_t);
+	/*
+	 * Returns only usable size of memory. As one channel is specifically
+	 * used to inform about the size of payload, discard it from avialable
+	 * memory size.
+	 */
+	return (num_channels - 1) * sizeof(uint32_t);
 }
diff --git a/drivers/arm/rss/rss_comms.c b/drivers/arm/rss/rss_comms.c
index 4622af980..a905b8831 100644
--- a/drivers/arm/rss/rss_comms.c
+++ b/drivers/arm/rss/rss_comms.c
@@ -59,8 +59,10 @@ static uint8_t select_protocol_version(const psa_invec *in_vec, size_t in_len,
 	 * messages due to ATU configuration costs to allow access to the
 	 * pointers.
 	 */
-	if ((comms_embed_msg_min_size + in_size_total > comms_mhu_msg_size - sizeof(uint32_t))
-	 || (comms_embed_reply_min_size + out_size_total > comms_mhu_msg_size) - sizeof(uint32_t)) {
+	if ((comms_embed_msg_min_size + in_size_total >
+	     comms_mhu_msg_size - sizeof(uint32_t)) ||
+	    (comms_embed_reply_min_size + out_size_total >
+	     comms_mhu_msg_size - sizeof(uint32_t))) {
 		return RSS_COMMS_PROTOCOL_POINTER_ACCESS;
 	} else {
 		return RSS_COMMS_PROTOCOL_EMBED;