[PATCH net] net: dsa: motorcomm: Do not publish partial MIB sample

[David Yang <mmyangfl@gmail.com>]

When a register read failed midway, the buffer was published anyway,
mixing fresh packet counters with stale byte counters; get_stats64 then
subtracts ETH_FCS_LEN per packet from the stale byte counter and
underflows on unsigned arithmetic.

Keep the last complete sample instead.

Fixes: 186623f4aa72 ("net: dsa: yt921x: Add support for Motorcomm YT921x")
Signed-off-by: David Yang <mmyangfl@gmail.com>
---
 drivers/net/dsa/motorcomm/mib.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/net/dsa/motorcomm/mib.c b/drivers/net/dsa/motorcomm/mib.c
index 6b170fc6e187..bbcc418fcc87 100644
--- a/drivers/net/dsa/motorcomm/mib.c
+++ b/drivers/net/dsa/motorcomm/mib.c
@@ -127,6 +127,12 @@ static int yt921x_mib_read(struct yt921x_priv *priv, int port)
 		buf[i] = val;
 	}
 
+	if (res) {
+		dev_err(dev, "Failed to %s port %d: %i\n", "read stats for",
+			port, res);
+		return res;
+	}
+
 	flags = u64_stats_update_begin_irqsave(&pm->syncp);
 	for (size_t i = 0; i < ARRAY_SIZE(yt921x_mib_descs); i++)
 		u64_stats_set(&((u64_stats_t *)mib)[i], buf[i]);
@@ -148,10 +154,7 @@ static int yt921x_mib_read(struct yt921x_priv *priv, int port)
 	u64_stats_set(&pm->tx_frames, tx_frames);
 	u64_stats_update_end_irqrestore(&pm->syncp, flags);
 
-	if (res)
-		dev_err(dev, "Failed to %s port %d: %i\n", "read stats for",
-			port, res);
-	return res;
+	return 0;
 }
 
 void yt921x_mib_poll(struct work_struct *work)
-- 
2.53.0