[PATCH net v2] net: dsa: motorcomm: Publish a consistent MIB sample

[PATCH net v2] net: dsa: motorcomm: Publish a consistent MIB sample

[David Yang]

get_stats64() derived rx_bytes and tx_bytes by subtracting ETH_FCS_LEN
per packet from the published byte counters, but u64_stats_sync gives
neither ordering nor retry across a multi-field read on 64-bit kernels,
and a failed register read published a partially refreshed buffer as
well. A fresh packet count could then pair with a stale byte counter,
and the subtraction underflowed on unsigned arithmetic, reporting
rx_bytes or tx_bytes as about 2^64.

Do the arithmetic in the writer, on the complete sample collected under
priv->reg_lock, and publish the adjusted byte counters next to the
frame totals, so that get_stats64() only copies published values. On a
failed register read, keep the last complete sample.

Found by LLM. The issue is transient and abnormal values will be flushed
by the next function call.

Fixes: 186623f4aa72 ("net: dsa: yt921x: Add support for Motorcomm YT921x")
Signed-off-by: David Yang <mmyangfl@gmail.com>
---
v2:
 - more issues found by sashiko
v1: https://lore.kernel.org/r/20261102101334.3525485-1-mmyangfl@gmail.com
 drivers/net/dsa/motorcomm/mib.c | 23 +++++++++++++++--------
 drivers/net/dsa/motorcomm/mib.h |  2 ++
 2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/drivers/net/dsa/motorcomm/mib.c b/drivers/net/dsa/motorcomm/mib.c
index 6b170fc6e187..d47aafc468a3 100644
--- a/drivers/net/dsa/motorcomm/mib.c
+++ b/drivers/net/dsa/motorcomm/mib.c
@@ -95,6 +95,8 @@ static int yt921x_mib_read(struct yt921x_priv *priv, int port)
 	unsigned long flags;
 	u64 rx_frames;
 	u64 tx_frames;
+	u64 rx_bytes;
+	u64 tx_bytes;
 	int res = 0;
 
 	mib = &pm->stats;
@@ -127,6 +129,12 @@ 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]);
@@ -144,14 +152,15 @@ static int yt921x_mib_read(struct yt921x_priv *priv, int port)
 		    u64_stats_read(&mib->tx_512_1023byte) +
 		    u64_stats_read(&mib->tx_1024_1518byte) +
 		    u64_stats_read(&mib->tx_jumbo);
+	rx_bytes = u64_stats_read(&mib->rx_good_bytes) - ETH_FCS_LEN * rx_frames;
+	tx_bytes = u64_stats_read(&mib->tx_good_bytes) - ETH_FCS_LEN * tx_frames;
 	u64_stats_set(&pm->rx_frames, rx_frames);
 	u64_stats_set(&pm->tx_frames, tx_frames);
+	u64_stats_set(&pm->rx_bytes, rx_bytes);
+	u64_stats_set(&pm->tx_bytes, tx_bytes);
 	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)
@@ -391,10 +400,8 @@ yt921x_dsa_get_stats64(struct dsa_switch *ds, int port,
 
 		stats->rx_packets = u64_stats_read(&pm->rx_frames);
 		stats->tx_packets = u64_stats_read(&pm->tx_frames);
-		stats->rx_bytes = u64_stats_read(&mib->rx_good_bytes) -
-				  ETH_FCS_LEN * stats->rx_packets;
-		stats->tx_bytes = u64_stats_read(&mib->tx_good_bytes) -
-				  ETH_FCS_LEN * stats->tx_packets;
+		stats->rx_bytes = u64_stats_read(&pm->rx_bytes);
+		stats->tx_bytes = u64_stats_read(&pm->tx_bytes);
 		stats->rx_dropped = u64_stats_read(&mib->rx_dropped);
 		/* stats->tx_dropped */
 		stats->multicast = u64_stats_read(&mib->rx_multicast);
diff --git a/drivers/net/dsa/motorcomm/mib.h b/drivers/net/dsa/motorcomm/mib.h
index 9c24a52d46cc..2e2b832cb245 100644
--- a/drivers/net/dsa/motorcomm/mib.h
+++ b/drivers/net/dsa/motorcomm/mib.h
@@ -136,6 +136,8 @@ struct yt921x_mib {
 	struct yt921x_mib_stats stats;
 	u64_stats_t rx_frames;
 	u64_stats_t tx_frames;
+	u64_stats_t rx_bytes;
+	u64_stats_t tx_bytes;
 	/* protected by priv->reg_lock */
 	u64 data[YT921X_MIB_STAT_NUM];
 };
-- 
2.53.0

Re: [PATCH net v2] net: dsa: motorcomm: Publish a consistent MIB sample

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - Whether the issue was actually triggered, or is only theoretical
   (e.g. found by code inspection). If it was triggered please include
   the symptoms, like the stack trace or error messages.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.

Re: [PATCH net v2] net: dsa: motorcomm: Publish a consistent MIB sample

[David Yang]

On Tue, Nov 3, 2026 at 9:53 PM <netdev-bot+sinfo@kernel.org> wrote:
>
> Hi!
>
> This is an automated message. This series looks like a fix, but its
> commit messages seem to be missing some information:
>
>  - Whether the issue was actually triggered, or is only theoretical
>    (e.g. found by code inspection). If it was triggered please include
>    the symptoms, like the stack trace or error messages.
>
> Please do not repost the series just to address the above. Instead,
> reply to this email with the missing information, so that reviewers
> can take it into account. If the series needs another revision for
> other reasons, please include the information in the commit messages
> then.
>
> The evaluation is done by an LLM so it may be wrong, if you think
> that is the case please reply and explain.

Sequence related, cannot reliably reproduce.