lan78xx: Set flow control threshold to prevent packet loss

Set threshold at which flow control is triggered to 3/4 full of
the internal Rx packet FIFO to prevent packet drops at high data
rates. The new setting reduces the number of dropped UDP frames
and TCP retransmit requests especially on less capable CPUs.
Signed-off-by: John Efstathiades <john.efstathiades@pebblebay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/lan78xx.c

@@ -46,6 +46,19 @@
 
 #define MAX_RX_FIFO_SIZE		(12 * 1024)
 #define MAX_TX_FIFO_SIZE		(12 * 1024)
+
+#define FLOW_THRESHOLD(n)		((((n) + 511) / 512) & 0x7F)
+#define FLOW_CTRL_THRESHOLD(on, off)	((FLOW_THRESHOLD(on)  << 0) | \
+					 (FLOW_THRESHOLD(off) << 8))
+
+/* Flow control turned on when Rx FIFO level rises above this level (bytes) */
+#define FLOW_ON_SS			9216
+#define FLOW_ON_HS			8704
+
+/* Flow control turned off when Rx FIFO level falls below this level (bytes) */
+#define FLOW_OFF_SS			4096
+#define FLOW_OFF_HS			1024
+
 #define DEFAULT_BURST_CAP_SIZE		(MAX_TX_FIFO_SIZE)
 #define DEFAULT_BULK_IN_DELAY		(0x0800)
 #define MAX_SINGLE_PACKET_SIZE		(9000)
@@ -1135,9 +1148,9 @@ static int lan78xx_update_flowcontrol(struct lan78xx_net *dev, u8 duplex,
 		flow |= FLOW_CR_RX_FCEN_;
 
 	if (dev->udev->speed == USB_SPEED_SUPER)
-		fct_flow = 0x817;
+		fct_flow = FLOW_CTRL_THRESHOLD(FLOW_ON_SS, FLOW_OFF_SS);
 	else if (dev->udev->speed == USB_SPEED_HIGH)
-		fct_flow = 0x211;
+		fct_flow = FLOW_CTRL_THRESHOLD(FLOW_ON_HS, FLOW_OFF_HS);
 
 	netif_dbg(dev, link, dev->net, "rx pause %s, tx pause %s",
 		  (cap & FLOW_CTRL_RX ? "enabled" : "disabled"),