ixgbe: Fix RSC completion delay causing Rx interrupts to stop

When a user disables interrupt throttling with ethtool on 82599 devices,
the interrupt timer may not be re-enabled if hardware RSC is running.  The
RSC completions in hardware don't complete before the next ITR event tries
to fire, so the ITR timer never gets re-armed.  This patch increases the
amount of time between interrupts when throttling is disabled (rx-usecs =
0) when the hardware RSC deature is enabled.
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ixgbe/ixgbe.h

@@ -96,6 +96,8 @@
 #define IXGBE_TX_FLAGS_VLAN_PRIO_MASK   0x0000e000
 #define IXGBE_TX_FLAGS_VLAN_SHIFT	16
 
+#define IXGBE_MAX_RSC_INT_RATE          162760
+
 /* wrapper around a pointer to a socket buffer,
  * so a DMA handle can be stored along with the buffer */
 struct ixgbe_tx_buffer {

drivers/net/ixgbe/ixgbe_ethtool.c

@@ -1975,6 +1975,9 @@ static int ixgbe_set_coalesce(struct net_device *netdev,
 		 * any other value means disable eitr, which is best
 		 * served by setting the interrupt rate very high
 		 */
+		if (adapter->flags2 & IXGBE_FLAG2_RSC_ENABLED)
+			adapter->eitr_param = IXGBE_MAX_RSC_INT_RATE;
+		else
 			adapter->eitr_param = IXGBE_MAX_INT_RATE;
 		adapter->itr_setting = 0;
 	}