[PATCH net-next v2 5/6] net: dsa: motorcomm: Fix MIB synchronization

[David Yang <mmyangfl@gmail.com>]

The previous MIB sync was based on the false assumption that 64-bit
variables are always atomic. To fix that:

  * .get_stats64() uses u64_stats_sync, together with u64_stats_t;
  * others take the free mutex lock if possible.

To minimize time of the write side of u64_stats_sync, a per-port buffer
is used so that all the stat can be written back altogether after IO
operations.

Signed-off-by: David Yang <mmyangfl@gmail.com>
---
 drivers/net/dsa/motorcomm/chip.c |  2 +
 drivers/net/dsa/motorcomm/mib.c  | 90 +++++++++++++++++++-------------
 drivers/net/dsa/motorcomm/mib.h  |  6 +++
 3 files changed, 63 insertions(+), 35 deletions(-)

diff --git a/drivers/net/dsa/motorcomm/chip.c b/drivers/net/dsa/motorcomm/chip.c
index d8520c38f9c7..b4059053ef15 100644
--- a/drivers/net/dsa/motorcomm/chip.c
+++ b/drivers/net/dsa/motorcomm/chip.c
@@ -19,6 +19,7 @@
 #include <linux/of_mdio.h>
 #include <linux/of_net.h>
 #include <linux/sort.h>
+#include <linux/u64_stats_sync.h>
 
 #include <net/dsa.h>
 #include <net/dscp.h>
@@ -3778,6 +3779,7 @@ static int yt921x_dsa_port_setup(struct dsa_switch *ds, int port)
 		pp->mib = pm;
 
 		pm->port = pp;
+		u64_stats_init(&pm->syncp);
 		INIT_DELAYED_WORK(&pm->work, yt921x_mib_poll);
 	}
 
diff --git a/drivers/net/dsa/motorcomm/mib.c b/drivers/net/dsa/motorcomm/mib.c
index d8a564d83bad..31d86c3122c7 100644
--- a/drivers/net/dsa/motorcomm/mib.c
+++ b/drivers/net/dsa/motorcomm/mib.c
@@ -92,6 +92,7 @@ static int yt921x_mib_read(struct yt921x_priv *priv, int port)
 	struct device *dev = to_device(priv);
 	struct yt921x_mib *pm = pp->mib;
 	struct yt921x_mib_stats *mib;
+	u64 *buf = pm->data;
 	u64 rx_frames;
 	u64 tx_frames;
 	int res = 0;
@@ -101,7 +102,6 @@ static int yt921x_mib_read(struct yt921x_priv *priv, int port)
 	for (size_t i = 0; i < ARRAY_SIZE(yt921x_mib_descs); i++) {
 		const struct yt921x_mib_desc *desc = &yt921x_mib_descs[i];
 		u32 reg = YT921X_MIBn_DATA0(port) + desc->offset;
-		u64_stats_t *valp = &((u64_stats_t *)mib)[i];
 		u32 val0;
 		u64 val;
 
@@ -110,7 +110,7 @@ static int yt921x_mib_read(struct yt921x_priv *priv, int port)
 			break;
 
 		if (desc->size <= 1) {
-			u64 old_val = u64_stats_read(valp);
+			u64 old_val = buf[i];
 
 			val = (old_val & ~(u64)U32_MAX) | val0;
 			if (val < old_val)
@@ -124,9 +124,12 @@ static int yt921x_mib_read(struct yt921x_priv *priv, int port)
 			val = ((u64)val1 << 32) | val0;
 		}
 
-		u64_stats_set(valp, val);
+		buf[i] = val;
 	}
 
+	u64_stats_update_begin(&pm->syncp);
+	for (size_t i = 0; i < ARRAY_SIZE(yt921x_mib_descs); i++)
+		u64_stats_set(&((u64_stats_t *)mib)[i], buf[i]);
 	rx_frames = u64_stats_read(&mib->rx_64byte) +
 		    u64_stats_read(&mib->rx_65_127byte) +
 		    u64_stats_read(&mib->rx_128_255byte) +
@@ -143,6 +146,7 @@ static int yt921x_mib_read(struct yt921x_priv *priv, int port)
 		    u64_stats_read(&mib->tx_jumbo);
 	u64_stats_set(&pm->rx_frames, rx_frames);
 	u64_stats_set(&pm->tx_frames, tx_frames);
+	u64_stats_update_end(&pm->syncp);
 
 	if (res)
 		dev_err(dev, "Failed to %s port %d: %i\n", "read stats for",
@@ -191,16 +195,16 @@ yt921x_dsa_get_ethtool_stats(struct dsa_switch *ds, int port, uint64_t *data)
 	struct yt921x_priv *priv = to_yt921x_priv(ds);
 	struct yt921x_port *pp = &priv->ports[port];
 	struct yt921x_mib *pm = pp->mib;
-	struct yt921x_mib_stats *mib;
+	u64 *buf;
 	size_t j;
 
 	if (!pm)
 		return;
-	mib = &pm->stats;
+	buf = pm->data;
 
 	mutex_lock(&priv->reg_lock);
+
 	yt921x_mib_read(priv, port);
-	mutex_unlock(&priv->reg_lock);
 
 	j = 0;
 	for (size_t i = 0; i < ARRAY_SIZE(yt921x_mib_descs); i++) {
@@ -209,9 +213,11 @@ yt921x_dsa_get_ethtool_stats(struct dsa_switch *ds, int port, uint64_t *data)
 		if (!desc->name)
 			continue;
 
-		data[j] = u64_stats_read(&((u64_stats_t *)mib)[i]);
+		data[j] = buf[i];
 		j++;
 	}
+
+	mutex_unlock(&priv->reg_lock);
 }
 
 int yt921x_dsa_get_sset_count(struct dsa_switch *ds, int port, int sset)
@@ -245,8 +251,8 @@ yt921x_dsa_get_eth_mac_stats(struct dsa_switch *ds, int port,
 	mib = &pm->stats;
 
 	mutex_lock(&priv->reg_lock);
+
 	yt921x_mib_read(priv, port);
-	mutex_unlock(&priv->reg_lock);
 
 	mac_stats->FramesTransmittedOK = u64_stats_read(&pm->tx_frames);
 	mac_stats->SingleCollisionFrames = u64_stats_read(&mib->tx_single_collisions);
@@ -270,6 +276,8 @@ yt921x_dsa_get_eth_mac_stats(struct dsa_switch *ds, int port,
 	/* mac_stats->InRangeLengthErrors */
 	/* mac_stats->OutOfRangeLengthField */
 	mac_stats->FrameTooLongErrors = u64_stats_read(&mib->rx_oversize_errors);
+
+	mutex_unlock(&priv->reg_lock);
 }
 
 void
@@ -286,12 +294,14 @@ yt921x_dsa_get_eth_ctrl_stats(struct dsa_switch *ds, int port,
 	mib = &pm->stats;
 
 	mutex_lock(&priv->reg_lock);
+
 	yt921x_mib_read(priv, port);
-	mutex_unlock(&priv->reg_lock);
 
 	ctrl_stats->MACControlFramesTransmitted = u64_stats_read(&mib->tx_pause);
 	ctrl_stats->MACControlFramesReceived = u64_stats_read(&mib->rx_pause);
 	/* ctrl_stats->UnsupportedOpcodesReceived */
+
+	mutex_unlock(&priv->reg_lock);
 }
 
 const struct ethtool_rmon_hist_range yt921x_rmon_ranges[] = {
@@ -320,8 +330,8 @@ yt921x_dsa_get_rmon_stats(struct dsa_switch *ds, int port,
 	mib = &pm->stats;
 
 	mutex_lock(&priv->reg_lock);
+
 	yt921x_mib_read(priv, port);
-	mutex_unlock(&priv->reg_lock);
 
 	*ranges = yt921x_rmon_ranges;
 
@@ -345,6 +355,8 @@ yt921x_dsa_get_rmon_stats(struct dsa_switch *ds, int port,
 	rmon_stats->hist_tx[4] = u64_stats_read(&mib->tx_512_1023byte);
 	rmon_stats->hist_tx[5] = u64_stats_read(&mib->tx_1024_1518byte);
 	rmon_stats->hist_tx[6] = u64_stats_read(&mib->tx_jumbo);
+
+	mutex_unlock(&priv->reg_lock);
 }
 
 void
@@ -355,39 +367,45 @@ yt921x_dsa_get_stats64(struct dsa_switch *ds, int port,
 	struct yt921x_port *pp = &priv->ports[port];
 	struct yt921x_mib *pm = pp->mib;
 	struct yt921x_mib_stats *mib;
+	unsigned int start;
 
 	if (!pm)
 		return;
 	mib = &pm->stats;
 
-	stats->rx_length_errors = u64_stats_read(&mib->rx_undersize_errors) +
-				  u64_stats_read(&mib->rx_fragment_errors);
-	stats->rx_over_errors = u64_stats_read(&mib->rx_oversize_errors);
-	stats->rx_crc_errors = u64_stats_read(&mib->rx_crc_errors);
-	stats->rx_frame_errors = u64_stats_read(&mib->rx_alignment_errors);
-	/* stats->rx_fifo_errors */
-	/* stats->rx_missed_errors */
-
-	stats->tx_aborted_errors = u64_stats_read(&mib->tx_aborted_errors);
-	/* stats->tx_carrier_errors */
-	stats->tx_fifo_errors = u64_stats_read(&mib->tx_undersize_errors);
-	/* stats->tx_heartbeat_errors */
-	stats->tx_window_errors = u64_stats_read(&mib->tx_late_collisions);
-
-	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;
+	do {
+		start = u64_stats_fetch_begin(&pm->syncp);
+
+		stats->rx_length_errors = u64_stats_read(&mib->rx_undersize_errors) +
+					  u64_stats_read(&mib->rx_fragment_errors);
+		stats->rx_over_errors = u64_stats_read(&mib->rx_oversize_errors);
+		stats->rx_crc_errors = u64_stats_read(&mib->rx_crc_errors);
+		stats->rx_frame_errors = u64_stats_read(&mib->rx_alignment_errors);
+		/* stats->rx_fifo_errors */
+		/* stats->rx_missed_errors */
+
+		stats->tx_aborted_errors = u64_stats_read(&mib->tx_aborted_errors);
+		/* stats->tx_carrier_errors */
+		stats->tx_fifo_errors = u64_stats_read(&mib->tx_undersize_errors);
+		/* stats->tx_heartbeat_errors */
+		stats->tx_window_errors = u64_stats_read(&mib->tx_late_collisions);
+
+		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_dropped = u64_stats_read(&mib->rx_dropped);
+		/* stats->tx_dropped */
+		stats->multicast = u64_stats_read(&mib->rx_multicast);
+		stats->collisions = u64_stats_read(&mib->tx_collisions);
+	} while (u64_stats_fetch_retry(&pm->syncp, start));
+
 	stats->rx_errors = stats->rx_length_errors + stats->rx_over_errors +
 			   stats->rx_crc_errors + stats->rx_frame_errors;
 	stats->tx_errors = stats->tx_aborted_errors + stats->tx_fifo_errors +
 			   stats->tx_window_errors;
-	stats->rx_dropped = u64_stats_read(&mib->rx_dropped);
-	/* stats->tx_dropped */
-	stats->multicast = u64_stats_read(&mib->rx_multicast);
-	stats->collisions = u64_stats_read(&mib->tx_collisions);
 }
 
 void
@@ -404,9 +422,11 @@ yt921x_dsa_get_pause_stats(struct dsa_switch *ds, int port,
 	mib = &pm->stats;
 
 	mutex_lock(&priv->reg_lock);
+
 	yt921x_mib_read(priv, port);
-	mutex_unlock(&priv->reg_lock);
 
 	pause_stats->tx_pause_frames = u64_stats_read(&mib->tx_pause);
 	pause_stats->rx_pause_frames = u64_stats_read(&mib->rx_pause);
+
+	mutex_unlock(&priv->reg_lock);
 }
diff --git a/drivers/net/dsa/motorcomm/mib.h b/drivers/net/dsa/motorcomm/mib.h
index d84ae2f449ac..33992a5f9bdc 100644
--- a/drivers/net/dsa/motorcomm/mib.h
+++ b/drivers/net/dsa/motorcomm/mib.h
@@ -125,13 +125,19 @@ struct yt921x_mib_stats {
 	u64_stats_t tx_oam;
 };
 
+#define YT921X_MIB_NUM	(sizeof(struct yt921x_mib_stats) / sizeof(u64_stats_t))
+
 struct yt921x_mib {
 	struct yt921x_port *port;
 
 	struct delayed_work work;
+	struct u64_stats_sync syncp;
+	/* protected by syncp OR priv->reg_lock */
 	struct yt921x_mib_stats stats;
 	u64_stats_t rx_frames;
 	u64_stats_t tx_frames;
+	/* protected by priv->reg_lock */
+	u64 data[YT921X_MIB_NUM];
 };
 
 void yt921x_mib_poll(struct work_struct *work);
-- 
2.53.0