igb: change descriptor control thresholds

This change simplifies the code by setting RX_PTHRESH to 8 for
all devices, as it was unlikely that there was any advantage to
set it at 16 for earlier cards. Additionally TX_WTHRESH is
set to 1 for the 82576 NIC to improve performance by enabling
a minimal amount of write combining when writing back descriptors.

Signed-off-by: Nicholas Nunley <nicholasx.d.nunley@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/igb/igb.h

@@ -95,13 +95,13 @@ struct vf_data_storage {
  *           descriptors until either it has this many to write back, or the
  *           ITR timer expires.
  */
-#define IGB_RX_PTHRESH                    (hw->mac.type <= e1000_82576 ? 16 : 8)
+#define IGB_RX_PTHRESH                     8
 #define IGB_RX_HTHRESH                     8
 #define IGB_RX_WTHRESH                     1
 #define IGB_TX_PTHRESH                     8
 #define IGB_TX_HTHRESH                     1
 #define IGB_TX_WTHRESH                     ((hw->mac.type == e1000_82576 && \
-                                             adapter->msix_entries) ? 0 : 16)
+                                             adapter->msix_entries) ? 1 : 16)
 
 /* this is the size past which hardware will drop packets when setting LPE=0 */
 #define MAXIMUM_ETHERNET_VLAN_SIZE 1522