bnxt: count packets discarded because of netpoll

bnxt may discard packets if Rx completions are consumed
in an attempt to let netpoll make progress. It should be
extremely rare in practice but nonetheless such events
should be counted.

Since completion ring memory is allocated dynamically use
a similar scheme to what is done for HW stats to save them.

Report the stats in rx_dropped and per-netdev ethtool
counter. Chances that users care which ring dropped are
very low.

v3: only save the stat to rx_dropped on reset,
rx_total_netpoll_discards will now only show drops since
last reset, similar to other "total_discard" counters.
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/broadcom/bnxt/bnxt.c

@@ -2003,6 +2003,7 @@ static int bnxt_force_rx_discard(struct bnxt *bp,
 	struct rx_cmp *rxcmp;
 	u16 cp_cons;
 	u8 cmp_type;
+	int rc;
 
 	cp_cons = RING_CMP(tmp_raw_cons);
 	rxcmp = (struct rx_cmp *)
@@ -2031,7 +2032,10 @@ static int bnxt_force_rx_discard(struct bnxt *bp,
 		tpa_end1->rx_tpa_end_cmp_errors_v2 |=
 			cpu_to_le32(RX_TPA_END_CMP_ERRORS);
 	}
-	return bnxt_rx_pkt(bp, cpr, raw_cons, event);
+	rc = bnxt_rx_pkt(bp, cpr, raw_cons, event);
+	if (rc && rc != -EBUSY)
+		cpr->sw_stats.rx.rx_netpoll_discards += 1;
+	return rc;
 }
 
 u32 bnxt_fw_health_readl(struct bnxt *bp, int reg_idx)
@@ -10646,6 +10650,8 @@ static void bnxt_get_ring_stats(struct bnxt *bp,
 		stats->multicast += BNXT_GET_RING_STATS64(sw, rx_mcast_pkts);
 
 		stats->tx_dropped += BNXT_GET_RING_STATS64(sw, tx_error_pkts);
+
+		stats->rx_dropped += cpr->sw_stats.rx.rx_netpoll_discards;
 	}
 }
 
@@ -10660,6 +10666,7 @@ static void bnxt_add_prev_stats(struct bnxt *bp,
 	stats->tx_bytes += prev_stats->tx_bytes;
 	stats->rx_missed_errors += prev_stats->rx_missed_errors;
 	stats->multicast += prev_stats->multicast;
+	stats->rx_dropped += prev_stats->rx_dropped;
 	stats->tx_dropped += prev_stats->tx_dropped;
 }
 

drivers/net/ethernet/broadcom/bnxt/bnxt.h

@@ -939,6 +939,7 @@ struct bnxt_rx_sw_stats {
 	u64			rx_l4_csum_errors;
 	u64			rx_resets;
 	u64			rx_buf_errors;
+	u64			rx_netpoll_discards;
 };
 
 struct bnxt_cmn_sw_stats {

drivers/net/ethernet/broadcom/bnxt/bnxt_ethtool.c

@@ -307,6 +307,7 @@ static const char * const bnxt_cmn_sw_stats_str[] = {
 enum {
 	RX_TOTAL_DISCARDS,
 	TX_TOTAL_DISCARDS,
+	RX_NETPOLL_DISCARDS,
 };
 
 static struct {
@@ -315,6 +316,7 @@ static struct {
 } bnxt_sw_func_stats[] = {
 	{0, "rx_total_discard_pkts"},
 	{0, "tx_total_discard_pkts"},
+	{0, "rx_total_netpoll_discards"},
 };
 
 #define NUM_RING_RX_SW_STATS		ARRAY_SIZE(bnxt_rx_sw_stats_str)
@@ -603,6 +605,8 @@ static void bnxt_get_ethtool_stats(struct net_device *dev,
 			BNXT_GET_RING_STATS64(sw_stats, rx_discard_pkts);
 		bnxt_sw_func_stats[TX_TOTAL_DISCARDS].counter +=
 			BNXT_GET_RING_STATS64(sw_stats, tx_discard_pkts);
+		bnxt_sw_func_stats[RX_NETPOLL_DISCARDS].counter +=
+			cpr->sw_stats.rx.rx_netpoll_discards;
 	}
 
 	for (i = 0; i < BNXT_NUM_SW_FUNC_STATS; i++, j++)