net: macb: Fix race between HW and driver

Under "heavy" RX load, the driver cannot handle the descriptors fast
enough. In detail, when a descriptor is consumed, its used flag is
cleared and once the RX budget is consumed all descriptors with a
cleared used flag are prepared to receive more data. Under load though,
the HW may constantly receive more data and use those descriptors with a
cleared used flag before they are actually prepared for next usage.

The head and tail pointers into the RX-ring should always be valid and
we can omit clearing and checking of the used flag.
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/cadence/macb.c

@@ -599,25 +599,16 @@ static void gem_rx_refill(struct macb *bp)
 {
 	unsigned int		entry;
 	struct sk_buff		*skb;
-	struct macb_dma_desc	*desc;
 	dma_addr_t		paddr;
 
 	while (CIRC_SPACE(bp->rx_prepared_head, bp->rx_tail, RX_RING_SIZE) > 0) {
-		u32 addr, ctrl;
-
 		entry = macb_rx_ring_wrap(bp->rx_prepared_head);
-		desc = &bp->rx_ring[entry];
 
 		/* Make hw descriptor updates visible to CPU */
 		rmb();
 
-		addr = desc->addr;
-		ctrl = desc->ctrl;
 		bp->rx_prepared_head++;
 
-		if ((addr & MACB_BIT(RX_USED)))
-			continue;
-
 		if (bp->rx_skbuff[entry] == NULL) {
 			/* allocate sk_buff for this free entry in ring */
 			skb = netdev_alloc_skb(bp->dev, bp->rx_buffer_size);
@@ -698,7 +689,6 @@ static int gem_rx(struct macb *bp, int budget)
 		if (!(addr & MACB_BIT(RX_USED)))
 			break;
 
-		desc->addr &= ~MACB_BIT(RX_USED);
 		bp->rx_tail++;
 		count++;