+ * Validate the sequenceID sent by the server.
+ * Return success if the sequenceID is one more than what we last saw on
+ * this slot, accounting for wraparound. Increments the slot's sequence.
+ *
+ * We don't yet implement a duplicate request cache, so at this time
+ * we will log replays, and process them as if we had not seen them before,
+ * but we don't bump the sequence in the slot. Not too worried about it,
+ * since we only currently implement idempotent callbacks anyway.
+ *
+ * We have a single slot backchannel at this time, so we don't bother
+ * checking the used_slots bit array on the table. The lower layer guarantees
+ * a single outstanding callback request at a time.
+ */
+static int
+validate_seqid(struct nfs4_slot_table *tbl, u32 slotid, u32 seqid)
+{
+ struct nfs4_slot *slot;
+
+ dprintk("%s enter. slotid %d seqid %d\n",
+ __func__, slotid, seqid);
+
+ if (slotid > NFS41_BC_MAX_CALLBACKS)
+ return htonl(NFS4ERR_BADSLOT);
+
+ slot = tbl->slots + slotid;
+ dprintk("%s slot table seqid: %d\n", __func__, slot->seq_nr);
+
+ /* Normal */
+ if (likely(seqid == slot->seq_nr + 1)) {
+ slot->seq_nr++;
+ return htonl(NFS4_OK);
+ }
+
+ /* Replay */
+ if (seqid == slot->seq_nr) {
+ dprintk("%s seqid %d is a replay - no DRC available\n",
+ __func__, seqid);
+ return htonl(NFS4_OK);
+ }
+
+ /* Wraparound */
+ if (seqid == 1 && (slot->seq_nr + 1) == 0) {
+ slot->seq_nr = 1;
+ return htonl(NFS4_OK);
+ }
+
+ /* Misordered request */
+ return htonl(NFS4ERR_SEQ_MISORDERED);
+}
+
+/*