ixgbe: Fix FCRTH value in VM-to-VM loopback mode

The 82599 and X540 datasheets require that FCRTH be "set" for Tx
switching (VM-to-VM loopback) but it did not previously specify what
the value should be set to. It has now been determined that
the correct value is RXPBSIZE - (24*1024).

This setting is also required for later devices.
Signed-off-by: NMark Rustad <mark.d.rustad@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

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

@@ -2164,10 +2164,11 @@ s32 ixgbe_fc_enable_generic(struct ixgbe_hw *hw)
 			/*
 			 * In order to prevent Tx hangs when the internal Tx
 			 * switch is enabled we must set the high water mark
-			 * to the maximum FCRTH value.  This allows the Tx
-			 * switch to function even under heavy Rx workloads.
+			 * to the Rx packet buffer size - 24KB.  This allows
+			 * the Tx switch to function even under heavy Rx
+			 * workloads.
 			 */
-			fcrth = IXGBE_READ_REG(hw, IXGBE_RXPBSIZE(i)) - 32;
+			fcrth = IXGBE_READ_REG(hw, IXGBE_RXPBSIZE(i)) - 24576;
 		}
 
 		IXGBE_WRITE_REG(hw, IXGBE_FCRTH_82599(i), fcrth);

drivers/net/ethernet/intel/ixgbe/ixgbe_dcb_82599.c

@@ -259,7 +259,13 @@ s32 ixgbe_dcb_config_pfc_82599(struct ixgbe_hw *hw, u8 pfc_en, u8 *prio_tc)
 			fcrtl = (hw->fc.low_water[i] << 10) | IXGBE_FCRTL_XONE;
 			IXGBE_WRITE_REG(hw, IXGBE_FCRTL_82599(i), fcrtl);
 		} else {
-			reg = IXGBE_READ_REG(hw, IXGBE_RXPBSIZE(i)) - 32;
+			/* In order to prevent Tx hangs when the internal Tx
+			 * switch is enabled we must set the high water mark
+			 * to the Rx packet buffer size - 24KB.  This allows
+			 * the Tx switch to function even under heavy Rx
+			 * workloads.
+			 */
+			reg = IXGBE_READ_REG(hw, IXGBE_RXPBSIZE(i)) - 24576;
 			IXGBE_WRITE_REG(hw, IXGBE_FCRTL_82599(i), 0);
 		}