ixgbe: Address issues with Tx WHTRESH value not being set correctly

This change is meant to address the fact that the tx_itr_setting was
dropping to 0 when no separate Tx vectors were provided.  This had resulted
in the driver incorrectly configuring the Tx ring with a WTHRESH of 1 in
order to avoid Tx hangs even though that was not necessary. This change
makes it so that we instead take a look at the Tx ring's q_vector to
determine if the ring will have an ITR value less than 8us.
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_main.c

@@ -2595,12 +2595,15 @@ void ixgbe_configure_tx_ring(struct ixgbe_adapter *adapter,
 	 * to or less than the number of on chip descriptors, which is
 	 * currently 40.
 	 */
-	if (!adapter->tx_itr_setting || !adapter->rx_itr_setting)
+	if (!ring->q_vector || (ring->q_vector->itr < 8))
 		txdctl |= (1 << 16);	/* WTHRESH = 1 */
 	else
 		txdctl |= (8 << 16);	/* WTHRESH = 8 */
 
-	/* PTHRESH=32 is needed to avoid a Tx hang with DFP enabled. */
+	/*
+	 * Setting PTHRESH to 32 both improves performance
+	 * and avoids a TX hang with DFP enabled
+	 */
 	txdctl |= (1 << 8) |	/* HTHRESH = 1 */
 		   32;		/* PTHRESH = 32 */