[net-next,PATCH] net: dsa: microchip: Do not count RX/TX Bytes and discards on KSZ87xx

[Marek Vasut <marek.vasut@mailbox.org>]

Unlike KSZ94xx, the KSZ87xx is missing the last four MIB counters at
address 0x20..0x23, namely rx_total, tx_total, rx_discards, tx_discards.

Using values from rx_total / tx_total in rx_bytes / tx_bytes calculation
results in an underflow, because rx_total / tx_total returns values 0,
and the "raw->rx_total - stats->rx_packets * ETH_FCS_LEN;" calculation
undeflows if rx_packets / tx_packets is > 0 .

Stop using the missing MIB counters on KSZ87xx .

Fixes: c6101dd7ffb8 ("net: dsa: ksz9477: move get_stats64 to ksz_common.c")
Signed-off-by: Marek Vasut <marek.vasut@mailbox.org>
---
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Arun Ramadoss <arun.ramadoss@microchip.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Tristram Ha <tristram.ha@microchip.com>
Cc: UNGLinuxDriver@microchip.com
Cc: Vladimir Oltean <olteanv@gmail.com>
Cc: Woojung Huh <woojung.huh@microchip.com>
Cc: netdev@vger.kernel.org
---
 drivers/net/dsa/microchip/ksz_common.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/net/dsa/microchip/ksz_common.c b/drivers/net/dsa/microchip/ksz_common.c
index 7292bfe2f7cac..9c01526779a6d 100644
--- a/drivers/net/dsa/microchip/ksz_common.c
+++ b/drivers/net/dsa/microchip/ksz_common.c
@@ -2239,20 +2239,23 @@ void ksz_r_mib_stats64(struct ksz_device *dev, int port)
 	/* HW counters are counting bytes + FCS which is not acceptable
 	 * for rtnl_link_stats64 interface
 	 */
-	stats->rx_bytes = raw->rx_total - stats->rx_packets * ETH_FCS_LEN;
-	stats->tx_bytes = raw->tx_total - stats->tx_packets * ETH_FCS_LEN;
-
+	if (!ksz_is_ksz87xx(dev)) {
+		stats->rx_bytes = raw->rx_total - stats->rx_packets * ETH_FCS_LEN;
+		stats->tx_bytes = raw->tx_total - stats->tx_packets * ETH_FCS_LEN;
+	}
 	stats->rx_length_errors = raw->rx_undersize + raw->rx_fragments +
 		raw->rx_oversize;
 
 	stats->rx_crc_errors = raw->rx_crc_err;
 	stats->rx_frame_errors = raw->rx_align_err;
-	stats->rx_dropped = raw->rx_discards;
+	if (!ksz_is_ksz87xx(dev))
+		stats->rx_dropped = raw->rx_discards;
 	stats->rx_errors = stats->rx_length_errors + stats->rx_crc_errors +
 		stats->rx_frame_errors  + stats->rx_dropped;
 
 	stats->tx_window_errors = raw->tx_late_col;
-	stats->tx_fifo_errors = raw->tx_discards;
+	if (!ksz_is_ksz87xx(dev))
+		stats->tx_fifo_errors = raw->tx_discards;
 	stats->tx_aborted_errors = raw->tx_exc_col;
 	stats->tx_errors = stats->tx_window_errors + stats->tx_fifo_errors +
 		stats->tx_aborted_errors;
-- 
2.47.2