3 files changed, 28 insertions, 10 deletions

diff --git a/drivers/net/ethernet/sfc/falcon_xmac.c b/drivers/net/ethernet/sfc/falcon_xmac.c
index 6106ef15dee3..8333865d4c95 100644
--- a/drivers/net/ethernet/sfc/falcon_xmac.c
+++ b/drivers/net/ethernet/sfc/falcon_xmac.c
@@ -341,12 +341,12 @@ void falcon_update_stats_xmac(struct efx_nic *efx)
         FALCON_STAT(efx, XgTxIpSrcErrPkt, tx_ip_src_error);
 
         /* Update derived statistics */
-        mac_stats->tx_good_bytes =
-                (mac_stats->tx_bytes - mac_stats->tx_bad_bytes -
-                 mac_stats->tx_control * 64);
-        mac_stats->rx_bad_bytes =
-                (mac_stats->rx_bytes - mac_stats->rx_good_bytes -
-                 mac_stats->rx_control * 64);
+        efx_update_diff_stat(&mac_stats->tx_good_bytes,
+                             mac_stats->tx_bytes - mac_stats->tx_bad_bytes -
+                             mac_stats->tx_control * 64);
+        efx_update_diff_stat(&mac_stats->rx_bad_bytes,
+                             mac_stats->rx_bytes - mac_stats->rx_good_bytes -
+                             mac_stats->rx_control * 64);
 }
 
 void falcon_poll_xmac(struct efx_nic *efx)
diff --git a/drivers/net/ethernet/sfc/nic.h b/drivers/net/ethernet/sfc/nic.h
index f48ccf6bb3b9..bab5cd9f5740 100644
--- a/drivers/net/ethernet/sfc/nic.h
+++ b/drivers/net/ethernet/sfc/nic.h
@@ -294,6 +294,24 @@ extern bool falcon_xmac_check_fault(struct efx_nic *efx);
 extern int falcon_reconfigure_xmac(struct efx_nic *efx);
 extern void falcon_update_stats_xmac(struct efx_nic *efx);
 
+/* Some statistics are computed as A - B where A and B each increase
+ * linearly with some hardware counter(s) and the counters are read
+ * asynchronously.  If the counters contributing to B are always read
+ * after those contributing to A, the computed value may be lower than
+ * the true value by some variable amount, and may decrease between
+ * subsequent computations.
+ *
+ * We should never allow statistics to decrease or to exceed the true
+ * value.  Since the computed value will never be greater than the
+ * true value, we can achieve this by only storing the computed value
+ * when it increases.
+ */
+static inline void efx_update_diff_stat(u64 *stat, u64 diff)
+{
+        if ((s64)(diff - *stat) > 0)
+                *stat = diff;
+}
+
 /* Interrupts and test events */
 extern int efx_nic_init_interrupt(struct efx_nic *efx);
 extern void efx_nic_enable_interrupts(struct efx_nic *efx);
diff --git a/drivers/net/ethernet/sfc/siena.c b/drivers/net/ethernet/sfc/siena.c
index 2354886293db..6bafd216e55e 100644
--- a/drivers/net/ethernet/sfc/siena.c
+++ b/drivers/net/ethernet/sfc/siena.c
@@ -458,8 +458,8 @@ static int siena_try_update_nic_stats(struct efx_nic *efx)
 
         MAC_STAT(tx_bytes, TX_BYTES);
         MAC_STAT(tx_bad_bytes, TX_BAD_BYTES);
-        mac_stats->tx_good_bytes = (mac_stats->tx_bytes -
-                                    mac_stats->tx_bad_bytes);
+        efx_update_diff_stat(&mac_stats->tx_good_bytes,
+                             mac_stats->tx_bytes - mac_stats->tx_bad_bytes);
         MAC_STAT(tx_packets, TX_PKTS);
         MAC_STAT(tx_bad, TX_BAD_FCS_PKTS);
         MAC_STAT(tx_pause, TX_PAUSE_PKTS);
@@ -492,8 +492,8 @@ static int siena_try_update_nic_stats(struct efx_nic *efx)
         MAC_STAT(tx_ip_src_error, TX_IP_SRC_ERR_PKTS);
         MAC_STAT(rx_bytes, RX_BYTES);
         MAC_STAT(rx_bad_bytes, RX_BAD_BYTES);
-        mac_stats->rx_good_bytes = (mac_stats->rx_bytes -
-                                    mac_stats->rx_bad_bytes);
+        efx_update_diff_stat(&mac_stats->rx_good_bytes,
+                             mac_stats->rx_bytes - mac_stats->rx_bad_bytes);
         MAC_STAT(rx_packets, RX_PKTS);
         MAC_STAT(rx_good, RX_GOOD_PKTS);
         MAC_STAT(rx_bad, RX_BAD_FCS_PKTS);