ethdev: document Rx packet number requirement for vector Rx

Updated description of rte_eth_rx_burst() to reflect what drivers,
when using vector instructions, expect from nb_pkts.

Also discussed on the mailing list here:
http://inbox.dpdk.org/dev/98CBD80474FA8B44BF855DF32C47DC35C61257@smartserver.smartshare.dk/

Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>

diff --git a/lib/librte_ethdev/rte_ethdev.h b/lib/librte_ethdev/rte_ethdev.h
index d9679a0..69fc973 100644 (file)
--- a/lib/librte_ethdev/rte_ethdev.h
+++ b/lib/librte_ethdev/rte_ethdev.h
@@ -4803,6 +4803,10 @@ int rte_eth_dev_hairpin_capability_get(uint16_t port_id,
  * burst-oriented optimizations in both synchronous and asynchronous
  * packet processing environments with no overhead in both cases.
  *
+ * @note
+ *   Some drivers using vector instructions require that *nb_pkts* is
+ *   divisible by 4 or 8, depending on the driver implementation.
+ *
  * The rte_eth_rx_burst() function does not provide any error
  * notification to avoid the corresponding overhead. As a hint, the
  * upper-level application might check the status of the device link once
@@ -4819,6 +4823,7 @@ int rte_eth_dev_hairpin_capability_get(uint16_t port_id,
  *   must be large enough to store *nb_pkts* pointers in it.
  * @param nb_pkts
  *   The maximum number of packets to retrieve.
+ *   The value must be divisible by 8 in order to work with any driver.
  * @return
  *   The number of packets actually retrieved, which is the number
  *   of pointers to *rte_mbuf* structures effectively supplied to the