[PATCH v2] net/ixgbe: fix min Rx/Tx descriptors

[Mingjin Ye <mingjinx.ye@intel.com>]

The minimum free packet threshold (tx_free_thresh) and the minimum RS bit
threshold (tx_rs_thresh) both have a default value of 32. Therefore, the
default minimum number of ring descriptors value is 64.

For reference, see "Configuration of Transmit Queues" in
doc/guides/prog_guide/ethdev/ethdev.rst

Fixes: dee5f1fd5fc7 ("ixgbe: get queue info and descriptor limits")
Cc: stable@dpdk.org

Signed-off-by: Mingjin Ye <mingjinx.ye@intel.com>
---
v2: Change doc.
---
 doc/guides/nics/ixgbe.rst            | 5 +++--
 drivers/net/intel/ixgbe/ixgbe_rxtx.h | 2 +-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/doc/guides/nics/ixgbe.rst b/doc/guides/nics/ixgbe.rst
index c5c6a6c34b..aaed324e15 100644
--- a/doc/guides/nics/ixgbe.rst
+++ b/doc/guides/nics/ixgbe.rst
@@ -68,11 +68,12 @@ Ensure that the following pre-conditions are satisfied:
 
 *   (rxq->nb_rx_desc % rxq->rx_free_thresh) == 0
 
-*   rxq->nb_rx_desc  < (IXGBE_MAX_RING_DESC - RTE_PMD_IXGBE_RX_MAX_BURST)
+*   rxq->nb_rx_desc >= IXGBE_MIN_RING_DESC
+
+*   rxq->nb_rx_desc <= IXGBE_MAX_RING_DESC
 
 These conditions are checked in the code.
 
-Scattered packets are not supported in this mode.
 If an incoming packet is greater than the maximum acceptable length of one "mbuf" data size (by default, the size is 2 KB),
 vPMD for RX would be disabled.
 
diff --git a/drivers/net/intel/ixgbe/ixgbe_rxtx.h b/drivers/net/intel/ixgbe/ixgbe_rxtx.h
index 278f665108..54569c7ade 100644
--- a/drivers/net/intel/ixgbe/ixgbe_rxtx.h
+++ b/drivers/net/intel/ixgbe/ixgbe_rxtx.h
@@ -26,7 +26,7 @@
  * descriptors should meet the following condition:
  *      (num_ring_desc * sizeof(rx/tx descriptor)) % 128 == 0
  */
-#define	IXGBE_MIN_RING_DESC	32
+#define	IXGBE_MIN_RING_DESC	64
 #define	IXGBE_MAX_RING_DESC	8192
 
 #define RTE_PMD_IXGBE_TX_MAX_BURST 32
-- 
2.25.1