sh_eth: Mitigate lost statistics updates

The statistics registers have write-clear behaviour, which means we
will lose any increment between the read and write.  Mitigate this by
only clearing when we read a non-zero value, so we will never falsely
report a total of zero.  This also saves time as we only handle
error statistics here and they won't often be incremented.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/ethernet/renesas/sh_eth.c b/drivers/net/ethernet/renesas/sh_eth.c
index c92dd17..7fb244f 100644 (file)
--- a/drivers/net/ethernet/renesas/sh_eth.c
+++ b/drivers/net/ethernet/renesas/sh_eth.c
@@ -2417,6 +2417,22 @@ static int sh_eth_start_xmit(struct sk_buff *skb, struct net_device *ndev)
        return NETDEV_TX_OK;
 }
 
+/* The statistics registers have write-clear behaviour, which means we
+ * will lose any increment between the read and write.  We mitigate
+ * this by only clearing when we read a non-zero value, so we will
+ * never falsely report a total of zero.
+ */
+static void
+sh_eth_update_stat(struct net_device *ndev, unsigned long *stat, int reg)
+{
+       u32 delta = sh_eth_read(ndev, reg);
+
+       if (delta) {
+               *stat += delta;
+               sh_eth_write(ndev, 0, reg);
+       }
+}
+
 static struct net_device_stats *sh_eth_get_stats(struct net_device *ndev)
 {
        struct sh_eth_private *mdp = netdev_priv(ndev);
@@ -2427,21 +2443,18 @@ static struct net_device_stats *sh_eth_get_stats(struct net_device *ndev)
        if (!mdp->is_opened)
                return &ndev->stats;
 
-       ndev->stats.tx_dropped += sh_eth_read(ndev, TROCR);
-       sh_eth_write(ndev, 0, TROCR);   /* (write clear) */
-       ndev->stats.collisions += sh_eth_read(ndev, CDCR);
-       sh_eth_write(ndev, 0, CDCR);    /* (write clear) */
-       ndev->stats.tx_carrier_errors += sh_eth_read(ndev, LCCR);
-       sh_eth_write(ndev, 0, LCCR);    /* (write clear) */
+       sh_eth_update_stat(ndev, &ndev->stats.tx_dropped, TROCR);
+       sh_eth_update_stat(ndev, &ndev->stats.collisions, CDCR);
+       sh_eth_update_stat(ndev, &ndev->stats.tx_carrier_errors, LCCR);
 
        if (sh_eth_is_gether(mdp)) {
-               ndev->stats.tx_carrier_errors += sh_eth_read(ndev, CERCR);
-               sh_eth_write(ndev, 0, CERCR);   /* (write clear) */
-               ndev->stats.tx_carrier_errors += sh_eth_read(ndev, CEECR);
-               sh_eth_write(ndev, 0, CEECR);   /* (write clear) */
+               sh_eth_update_stat(ndev, &ndev->stats.tx_carrier_errors,
+                                  CERCR);
+               sh_eth_update_stat(ndev, &ndev->stats.tx_carrier_errors,
+                                  CEECR);
        } else {
-               ndev->stats.tx_carrier_errors += sh_eth_read(ndev, CNDCR);
-               sh_eth_write(ndev, 0, CNDCR);   /* (write clear) */
+               sh_eth_update_stat(ndev, &ndev->stats.tx_carrier_errors,
+                                  CNDCR);
        }
 
        return &ndev->stats;