gianfar: Add macros for stepping through BDs

This code is based strongly on code from Dai Haruki <Dai.Haruki@freescale.com>.

The gianfar Buffer Descriptors are arranged in a circular array, the end of
which is denoted by setting the "WRAP" bit in the descriptor.  However, the
software knows the end of the ring because it knows how many descriptors are
there.  Rather than check each descriptor for whether the WRAP bit is set,
use pointer math to determine where the next BD is.  This is also useful for
when we want to look at BDs other than the very next one (for Scatter-Gather).
Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/gianfar.c

@@ -1215,7 +1215,7 @@ static int gfar_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct gfar_private *priv = netdev_priv(dev);
 	struct txfcb *fcb = NULL;
-	struct txbd8 *txbdp;
+	struct txbd8 *txbdp, *base;
 	u16 status;
 	unsigned long flags;
 
@@ -1227,6 +1227,7 @@ static int gfar_start_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	/* Point at the first free tx descriptor */
 	txbdp = priv->cur_tx;
+	base = priv->tx_bd_base;
 
 	/* Clear all but the WRAP status flags */
 	status = txbdp->status & TXBD_WRAP;
@@ -1279,12 +1280,7 @@ static int gfar_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	eieio();
 	txbdp->status = status;
 
-	/* If this was the last BD in the ring, the next one */
-	/* is at the beginning of the ring */
-	if (txbdp->status & TXBD_WRAP)
-		txbdp = priv->tx_bd_base;
-	else
-		txbdp++;
+	txbdp = next_bd(txbdp, base, priv->tx_ring_size);
 
 	/* If the next BD still needs to be cleaned up, then the bds
 	   are full.  We need to tell the kernel to stop sending us stuff. */
@@ -1470,11 +1466,12 @@ static void gfar_timeout(struct net_device *dev)
 /* Interrupt Handler for Transmit complete */
 static int gfar_clean_tx_ring(struct net_device *dev)
 {
-	struct txbd8 *bdp;
+	struct txbd8 *bdp, *base;
 	struct gfar_private *priv = netdev_priv(dev);
 	int howmany = 0;
 
 	bdp = priv->dirty_tx;
+	base = priv->tx_bd_base;
 	while ((bdp->status & TXBD_READY) == 0) {
 		/* If dirty_tx and cur_tx are the same, then either the */
 		/* ring is empty or full now (it could only be full in the beginning, */
@@ -1504,11 +1501,7 @@ static int gfar_clean_tx_ring(struct net_device *dev)
 		/* Clean BD length for empty detection */
 		bdp->length = 0;
 
-		/* update bdp to point at next bd in the ring (wrapping if necessary) */
-		if (bdp->status & TXBD_WRAP)
-			bdp = priv->tx_bd_base;
-		else
-			bdp++;
+		bdp = next_bd(bdp, base, priv->tx_ring_size);
 
 		/* Move dirty_tx to be the next bd */
 		priv->dirty_tx = bdp;
@@ -1712,7 +1705,7 @@ static int gfar_process_frame(struct net_device *dev, struct sk_buff *skb,
  */
 int gfar_clean_rx_ring(struct net_device *dev, int rx_work_limit)
 {
-	struct rxbd8 *bdp;
+	struct rxbd8 *bdp, *base;
 	struct sk_buff *skb;
 	int pkt_len;
 	int amount_pull;
@@ -1721,6 +1714,7 @@ int gfar_clean_rx_ring(struct net_device *dev, int rx_work_limit)
 
 	/* Get the first full descriptor */
 	bdp = priv->cur_rx;
+	base = priv->rx_bd_base;
 
 	amount_pull = (gfar_uses_fcb(priv) ? GMAC_FCB_LEN : 0) +
 		priv->padding;
@@ -1776,10 +1770,7 @@ int gfar_clean_rx_ring(struct net_device *dev, int rx_work_limit)
 		gfar_new_rxbdp(dev, bdp, newskb);
 
 		/* Update to the next pointer */
-		if (bdp->status & RXBD_WRAP)
-			bdp = priv->rx_bd_base;
-		else
-			bdp++;
+		bdp = next_bd(bdp, base, priv->rx_ring_size);
 
 		/* update to point at the next skb */
 		priv->skb_currx =

drivers/net/gianfar.h

@@ -196,6 +196,12 @@ extern const char gfar_driver_version[];
 #define DEFAULT_TXIC mk_ic_value(DEFAULT_TXCOUNT, DEFAULT_TXTIME)
 #define DEFAULT_RXIC mk_ic_value(DEFAULT_RXCOUNT, DEFAULT_RXTIME)
 
+#define skip_bd(bdp, stride, base, ring_size) ({ \
+	typeof(bdp) new_bd = (bdp) + (stride); \
+	(new_bd >= (base) + (ring_size)) ? (new_bd - (ring_size)) : new_bd; })
+
+#define next_bd(bdp, base, ring_size) skip_bd(bdp, 1, base, ring_size)
+
 #define RCTRL_PAL_MASK		0x001f0000
 #define RCTRL_VLEX		0x00002000
 #define RCTRL_FILREN		0x00001000