net: ravb: Count packets instead of descriptors in R-Car RX path

The units of "work done" in the RX path should be packets instead of
descriptors.

Descriptors which are used by the hardware to record error conditions or
are empty in the case of a DMA mapping error should not count towards
our RX work budget.

Also make the limit variable unsigned as it can never be negative.

Fixes: c156633 ("Renesas Ethernet AVB driver proper")
Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/ethernet/renesas/ravb_main.c

@@ -892,29 +892,24 @@ static bool ravb_rx_rcar(struct net_device *ndev, int *quota, int q)
 	struct ravb_private *priv = netdev_priv(ndev);
 	const struct ravb_hw_info *info = priv->info;
 	int entry = priv->cur_rx[q] % priv->num_rx_ring[q];
-	int boguscnt = (priv->dirty_rx[q] + priv->num_rx_ring[q]) -
-			priv->cur_rx[q];
 	struct net_device_stats *stats = &priv->stats[q];
 	struct ravb_ex_rx_desc *desc;
+	unsigned int limit, i;
 	struct sk_buff *skb;
 	dma_addr_t dma_addr;
 	struct timespec64 ts;
+	int rx_packets = 0;
 	u8  desc_status;
 	u16 pkt_len;
-	int limit;
 
-	boguscnt = min(boguscnt, *quota);
-	limit = boguscnt;
+	limit = priv->dirty_rx[q] + priv->num_rx_ring[q] - priv->cur_rx[q];
 	desc = &priv->rx_ring[q].ex_desc[entry];
-	while (desc->die_dt != DT_FEMPTY) {
+	for (i = 0; i < limit && rx_packets < *quota && desc->die_dt != DT_FEMPTY; i++) {
 		/* Descriptor type must be checked before all other reads */
 		dma_rmb();
 		desc_status = desc->msc;
 		pkt_len = le16_to_cpu(desc->ds_cc) & RX_DS;
 
-		if (--boguscnt < 0)
-			break;
-
 		/* We use 0-byte descriptors to mark the DMA mapping errors */
 		if (!pkt_len)
 			continue;
@@ -960,7 +955,7 @@ static bool ravb_rx_rcar(struct net_device *ndev, int *quota, int q)
 			if (ndev->features & NETIF_F_RXCSUM)
 				ravb_rx_csum(skb);
 			napi_gro_receive(&priv->napi[q], skb);
-			stats->rx_packets++;
+			rx_packets++;
 			stats->rx_bytes += pkt_len;
 		}
 
@@ -995,9 +990,9 @@ static bool ravb_rx_rcar(struct net_device *ndev, int *quota, int q)
 		desc->die_dt = DT_FEMPTY;
 	}
 
-	*quota -= limit - (++boguscnt);
-
-	return boguscnt <= 0;
+	stats->rx_packets += rx_packets;
+	*quota -= rx_packets;
+	return *quota == 0;
 }
 
 /* Packet receive function for Ethernet AVB */