igb: always use adapter->itr as EITR value

The igb driver was switching between adapter->itr containing the EITR value
and the number of interrupts per second.  This resulted in high latencies
being seen after brining the interface down and then back up.  To resolve
the issue the itr value will now only contain the value that should be
programmed into EITR.
Signed-off-by: NAlexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/igb/igb_main.c

@@ -2898,13 +2898,13 @@ static void igb_set_itr(struct igb_adapter *adapter)
 	switch (current_itr) {
 	/* counts and packets in update_itr are dependent on these numbers */
 	case lowest_latency:
-		new_itr = 70000;
+		new_itr = 56;  /* aka 70,000 ints/sec */
 		break;
 	case low_latency:
-		new_itr = 20000; /* aka hwitr = ~200 */
+		new_itr = 196; /* aka 20,000 ints/sec */
 		break;
 	case bulk_latency:
-		new_itr = 4000;
+		new_itr = 980; /* aka 4,000 ints/sec */
 		break;
 	default:
 		break;
@@ -2923,7 +2923,8 @@ static void igb_set_itr(struct igb_adapter *adapter)
 		 * by adding intermediate steps when interrupt rate is
 		 * increasing */
 		new_itr = new_itr > adapter->itr ?
-			     min(adapter->itr + (new_itr >> 2), new_itr) :
+			     max((new_itr * adapter->itr) /
+			         (new_itr + (adapter->itr >> 2)), new_itr) :
 			     new_itr;
 		/* Don't write the value here; it resets the adapter's
 		 * internal timer, and causes us to delay far longer than
@@ -2932,7 +2933,7 @@ static void igb_set_itr(struct igb_adapter *adapter)
 		 * ends up being correct.
 		 */
 		adapter->itr = new_itr;
-		adapter->rx_ring->itr_val = 1000000000 / (new_itr * 256);
+		adapter->rx_ring->itr_val = new_itr;
 		adapter->rx_ring->set_itr = 1;
 	}