r8169: disable tx interrupt coalescing on RTL8168

In contrast to switching rx irq coalescing off what fixed an issue,
switching tx irq coalescing off is merely a latency optimization,
therefore net-next. As part of this change:

- Remove INTT_0 .. INTT_3 constants, they aren't used.

- Remove comment in rtl_hw_start_8169(), we now have a detailed
  description by the code in rtl_set_coalesce().

- Due to switching irq coalescing off per default we don't need the
  initialization in rtl_hw_start_8168(). If ethtool is used to switch
  on coalescing then rtl_set_coalesce() will configure this register.

For the sake of completeness: This patch just changes the default.
Users still have the option to configure irq coalescing via ethtool.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/realtek/r8169.c

@@ -490,10 +490,6 @@ enum rtl_register_content {
 	PCIDAC		= (1 << 4),
 	PCIMulRW	= (1 << 3),
 #define INTT_MASK	GENMASK(1, 0)
-	INTT_0		= 0x0000,	// 8168
-	INTT_1		= 0x0001,	// 8168
-	INTT_2		= 0x0002,	// 8168
-	INTT_3		= 0x0003,	// 8168
 
 	/* rtl8169_PHYstatus */
 	TBI_Enable	= 0x80,
@@ -4702,6 +4698,8 @@ static void rtl_hw_start(struct  rtl8169_private *tp)
 	rtl_set_rx_tx_desc_registers(tp);
 	rtl_lock_config_regs(tp);
 
+	/* disable interrupt coalescing */
+	RTL_W16(tp, IntrMitigate, 0x0000);
 	/* Initially a 10 us delay. Turned it into a PCI commit. - FR */
 	RTL_R8(tp, IntrMask);
 	RTL_W8(tp, ChipCmd, CmdTxEnb | CmdRxEnb);
@@ -4734,12 +4732,6 @@ static void rtl_hw_start_8169(struct rtl8169_private *tp)
 
 	rtl8169_set_magic_reg(tp, tp->mac_version);
 
-	/*
-	 * Undocumented corner. Supposedly:
-	 * (TxTimer << 12) | (TxPackets << 8) | (RxTimer << 4) | RxPackets
-	 */
-	RTL_W16(tp, IntrMitigate, 0x0000);
-
 	RTL_W32(tp, RxMissed, 0);
 }
 
@@ -5456,12 +5448,6 @@ static void rtl_hw_start_8168(struct rtl8169_private *tp)
 {
 	RTL_W8(tp, MaxTxPacketSize, TxPacketMax);
 
-	tp->cp_cmd &= ~INTT_MASK;
-	tp->cp_cmd |= PktCntrDisable | INTT_1;
-	RTL_W16(tp, CPlusCmd, tp->cp_cmd);
-
-	RTL_W16(tp, IntrMitigate, 0x5100);
-
 	/* Work around for RxFIFO overflow. */
 	if (tp->mac_version == RTL_GIGA_MAC_VER_11) {
 		tp->irq_mask |= RxFIFOOver;
@@ -5749,8 +5735,6 @@ static void rtl_hw_start_8101(struct rtl8169_private *tp)
 		rtl_hw_start_8168h_1(tp);
 		break;
 	}
-
-	RTL_W16(tp, IntrMitigate, 0x0000);
 }
 
 static int rtl8169_change_mtu(struct net_device *dev, int new_mtu)