netdev: Improve comments on netdev_rxq_recv().

The comment was incomplete in some ways and simply wrong in others.

Also ensure that *cnt is set to 0 if an error is encountered.  It's nice
when callers can rely on this.

Signed-off-by: Ben Pfaff <[email protected]>
Acked-by: Justin Pettit <[email protected]>

lib/netdev-provider.h

@@ -713,23 +713,29 @@ struct netdev_class {
     void (*rxq_destruct)(struct netdev_rxq *);
     void (*rxq_dealloc)(struct netdev_rxq *);
 
-    /* Attempts to receive batch of packets from 'rx' and place array of
-     * pointers into '*pkts'. netdev is responsible for allocating buffers.
-     * '*cnt' points to packet count for given batch. Once packets are returned
-     * to caller, netdev should give up ownership of ofpbuf data.
-     *
-     * Implementations should allocate buffer with DP_NETDEV_HEADROOM headroom
-     * and add a VLAN header which is obtained out-of-band to the packet.
-     *
-     * Caller is expected to pass array of size MAX_RX_BATCH.
-     * This function may be set to null if it would always return EOPNOTSUPP
-     * anyhow. */
+    /* Attempts to receive a batch of packets from 'rx'.  The caller supplies
+     * 'pkts' as the pointer to the beginning of an array of MAX_RX_BATCH
+     * pointers to dp_packet.  If successful, the implementation stores
+     * pointers to up to MAX_RX_BATCH dp_packets into the array, transferring
+     * ownership of the packets to the caller, stores the number of received
+     * packets into '*cnt', and returns 0.
+     *
+     * The implementation does not necessarily initialize any non-data members
+     * of 'pkts'.  That is, the caller must initialize layer pointers and
+     * metadata itself, if desired, e.g. with pkt_metadata_init() and
+     * miniflow_extract().
+     *
+     * Implementations should allocate buffers with DP_NETDEV_HEADROOM bytes of
+     * headroom.
+     *
+     * Returns EAGAIN immediately if no packet is ready to be received or
+     * another positive errno value if an error was encountered. */
     int (*rxq_recv)(struct netdev_rxq *rx, struct dp_packet **pkts,
                     int *cnt);
 
     /* Registers with the poll loop to wake up from the next call to
-     * poll_block() when a packet is ready to be received with netdev_rxq_recv()
-     * on 'rx'. */
+     * poll_block() when a packet is ready to be received with
+     * netdev_rxq_recv() on 'rx'. */
     void (*rxq_wait)(struct netdev_rxq *rx);
 
     /* Discards all packets waiting to be received from 'rx'. */

lib/netdev.c

@@ -639,28 +639,28 @@ netdev_rxq_close(struct netdev_rxq *rx)
     }
 }
 
-/* Attempts to receive batch of packets from 'rx'.
- *
- * Returns EAGAIN immediately if no packet is ready to be received.
- *
- * Returns EMSGSIZE, and discards the packet, if the received packet is longer
- * than 'dp_packet_tailroom(buffer)'.
- *
- * It is advised that the tailroom of 'buffer' should be
- * VLAN_HEADER_LEN bytes longer than the MTU to allow space for an
- * out-of-band VLAN header to be added to the packet.  At the very least,
- * 'buffer' must have at least ETH_TOTAL_MIN bytes of tailroom.
- *
- * This function may be set to null if it would always return EOPNOTSUPP
- * anyhow. */
+/* Attempts to receive a batch of packets from 'rx'.  'pkts' should point to
+ * the beginning of an array of MAX_RX_BATCH pointers to dp_packet.  If
+ * successful, this function stores pointers to up to MAX_RX_BATCH dp_packets
+ * into the array, transferring ownership of the packets to the caller, stores
+ * the number of received packets into '*cnt', and returns 0.
+ *
+ * The implementation does not necessarily initialize any non-data members of
+ * 'pkts'.  That is, the caller must initialize layer pointers and metadata
+ * itself, if desired, e.g. with pkt_metadata_init() and miniflow_extract().
+ *
+ * Returns EAGAIN immediately if no packet is ready to be received or another
+ * positive errno value if an error was encountered. */
 int
-netdev_rxq_recv(struct netdev_rxq *rx, struct dp_packet **buffers, int *cnt)
+netdev_rxq_recv(struct netdev_rxq *rx, struct dp_packet **pkts, int *cnt)
 {
     int retval;
 
-    retval = rx->netdev->netdev_class->rxq_recv(rx, buffers, cnt);
+    retval = rx->netdev->netdev_class->rxq_recv(rx, pkts, cnt);
     if (!retval) {
         COVERAGE_INC(netdev_received);
+    } else {
+        *cnt = 0;
     }
     return retval;
 }