e1000e: fix the use of magic numbers for buffer overrun issue

This is a follow on to commit b10effb ("fix buffer overrun while the
 I219 is processing DMA transactions") to address David Laights concerns
about the use of "magic" numbers.  So define masks as well as add
additional code comments to give a better understanding of what needs to
be done to avoid a buffer overrun.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Reviewed-by: Alexander H Duyck <alexander.h.duyck@intel.com>
Reviewed-by: Dima Ruinskiy <dima.ruinskiy@intel.com>
Reviewed-by: Raanan Avargil <raanan.avargil@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/e1000e/ich8lan.h

@@ -113,7 +113,8 @@
 #define NVM_SIZE_MULTIPLIER 4096	/*multiplier for NVMS field */
 #define E1000_FLASH_BASE_ADDR 0xE000	/*offset of NVM access regs */
 #define E1000_CTRL_EXT_NVMVS 0x3	/*NVM valid sector */
-#define E1000_TARC0_CB_MULTIQ_3_REQ	(1 << 28 | 1 << 29)
+#define E1000_TARC0_CB_MULTIQ_3_REQ	0x30000000
+#define E1000_TARC0_CB_MULTIQ_2_REQ	0x20000000
 #define PCIE_ICH8_SNOOP_ALL	PCIE_NO_SNOOP_ALL
 
 #define E1000_ICH_RAR_ENTRIES	7

drivers/net/ethernet/intel/e1000e/netdev.c

@@ -3034,9 +3034,12 @@ static void e1000_configure_tx(struct e1000_adapter *adapter)
 		ew32(IOSFPC, reg_val);
 
 		reg_val = er32(TARC(0));
-		/* SPT and KBL Si errata workaround to avoid Tx hang */
-		reg_val &= ~BIT(28);
-		reg_val |= BIT(29);
+		/* SPT and KBL Si errata workaround to avoid Tx hang.
+		 * Dropping the number of outstanding requests from
+		 * 3 to 2 in order to avoid a buffer overrun.
+		 */
+		reg_val &= ~E1000_TARC0_CB_MULTIQ_3_REQ;
+		reg_val |= E1000_TARC0_CB_MULTIQ_2_REQ;
 		ew32(TARC(0), reg_val);
 	}
 }