sis900: come alive after temporary memory shortage

1) Forgotten counter incrementation in sis900_rx() in case
     it doesn't get memory for skb, that leads to whole interface failure.
     Problem is accompanied with messages:
    eth0: Memory squeeze,deferring packet.
    eth0: NULL pointer encountered in Rx ring, skipping

2) If counter cur_rx overflows and there'll be temporary memory problems
     buffer can't be recreated later, when memory IS available.

3) Limit the work in handler to prevent the endless packets processing
   if new packets are generated faster then handled.

Signed-off-by: Konstantin Khorenko <khorenko@sw.ru>
Signed-off-by: Vasily Averin <vvs@sw.ru>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>

drivers/net/sis900.c

@@ -1696,15 +1696,20 @@ static int sis900_rx(struct net_device *net_dev)
 	long ioaddr = net_dev->base_addr;
 	unsigned int entry = sis_priv->cur_rx % NUM_RX_DESC;
 	u32 rx_status = sis_priv->rx_ring[entry].cmdsts;
+	int rx_work_limit;
 
 	if (netif_msg_rx_status(sis_priv))
 		printk(KERN_DEBUG "sis900_rx, cur_rx:%4.4d, dirty_rx:%4.4d "
 		       "status:0x%8.8x\n",
 		       sis_priv->cur_rx, sis_priv->dirty_rx, rx_status);
+	rx_work_limit = sis_priv->dirty_rx + NUM_RX_DESC - sis_priv->cur_rx;
 
 	while (rx_status & OWN) {
 		unsigned int rx_size;
 
+		if (--rx_work_limit < 0)
+			break;
+
 		rx_size = (rx_status & DSIZE) - CRC_SIZE;
 
 		if (rx_status & (ABORT|OVERRUN|TOOLONG|RUNT|RXISERR|CRCERR|FAERR)) {
@@ -1732,9 +1737,11 @@ static int sis900_rx(struct net_device *net_dev)
 			   we are working on NULL sk_buff :-( */
 			if (sis_priv->rx_skbuff[entry] == NULL) {
 				if (netif_msg_rx_err(sis_priv))
-					printk(KERN_INFO "%s: NULL pointer " 
-						"encountered in Rx ring, skipping\n",
-						net_dev->name);
+					printk(KERN_WARNING "%s: NULL pointer " 
+					      "encountered in Rx ring\n"
+					      "cur_rx:%4.4d, dirty_rx:%4.4d\n",
+					      net_dev->name, sis_priv->cur_rx,
+					      sis_priv->dirty_rx);
 				break;
 			}
 
@@ -1770,6 +1777,7 @@ static int sis900_rx(struct net_device *net_dev)
 				sis_priv->rx_ring[entry].cmdsts = 0;
 				sis_priv->rx_ring[entry].bufptr = 0;
 				sis_priv->stats.rx_dropped++;
+				sis_priv->cur_rx++;
 				break;
 			}
 			skb->dev = net_dev;
@@ -1787,7 +1795,7 @@ static int sis900_rx(struct net_device *net_dev)
 
 	/* refill the Rx buffer, what if the rate of refilling is slower
 	 * than consuming ?? */
-	for (;sis_priv->cur_rx - sis_priv->dirty_rx > 0; sis_priv->dirty_rx++) {
+	for (; sis_priv->cur_rx != sis_priv->dirty_rx; sis_priv->dirty_rx++) {
 		struct sk_buff *skb;
 
 		entry = sis_priv->dirty_rx % NUM_RX_DESC;