[PATCH net] net: stmmac: fix RX coalesce frames counter

[Pengfei Zhang <zhangfeionline@gmail.com>]

The RX refill path should set the IC bit once every rx_coal_frames
descriptors and leave the rest to the RX watchdog (RIWT), as done by
commit d429b66e2ebc ("net: stmmac: Implement RX Coalesce Frames
setting").

Since commit 6fa9d691b91a ("net: stmmac: Prevent divide-by-zero") the
counter is increased by N+1 for each descriptor and then compared
against N, so it is always reset to 0. With a non-zero rx-frames, IC is
therefore set on every descriptor and every frame raises an interrupt.

Fix this by counting one descriptor at a time in a helper used by both
stmmac_rx_refill() and stmmac_rx_refill_zc(). Platforms without RIWT
(IC on every descriptor) are unaffected.

Fixes: 6fa9d691b91a ("net: stmmac: Prevent divide-by-zero")
Signed-off-by: Pengfei Zhang <zhangfeionline@gmail.com>
---
The default rx-frames is 0 (watchdog only) since commit da2024510031
("net: stmmac: Tune-up default coalesce settings"), so this is only hit
when rx-frames is set explicitly.

Verified with "ethtool -C eth0 rx-frames 16" under UDP load: without
this patch almost every received frame raised an RX interrupt, about
twice as many interrupts as with rx-frames 0.

 .../net/ethernet/stmicro/stmmac/stmmac_main.c | 36 +++++++++----------
 1 file changed, 18 insertions(+), 18 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index b837e8e27..72678760e 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -5071,6 +5071,22 @@ static void stmmac_rx_vlan(struct net_device *dev, struct sk_buff *skb)
 	}
 }
 
+static bool stmmac_rx_need_ic(struct stmmac_priv *priv,
+			      struct stmmac_rx_queue *rx_q, u32 queue)
+{
+	u32 frames = priv->rx_coal_frames[queue];
+
+	/* without the RX watchdog every descriptor needs IC */
+	if (!priv->use_riwt)
+		return true;
+
+	if (!frames || ++rx_q->rx_count_frames < frames)
+		return false;
+
+	rx_q->rx_count_frames = 0;
+	return true;
+}
+
 /**
  * stmmac_rx_refill - refill used skb preallocated buffers
  * @priv: driver private structure
@@ -5116,15 +5132,7 @@ static inline void stmmac_rx_refill(struct stmmac_priv *priv, u32 queue)
 					 stmmac_rx_check_buf2_cap(priv));
 		stmmac_refill_desc3(priv, rx_q, p);
 
-		rx_q->rx_count_frames++;
-		rx_q->rx_count_frames += priv->rx_coal_frames[queue];
-		if (rx_q->rx_count_frames > priv->rx_coal_frames[queue])
-			rx_q->rx_count_frames = 0;
-
-		use_rx_wd = !priv->rx_coal_frames[queue];
-		use_rx_wd |= rx_q->rx_count_frames > 0;
-		if (!priv->use_riwt)
-			use_rx_wd = false;
+		use_rx_wd = !stmmac_rx_need_ic(priv, rx_q, queue);
 
 		dma_wmb();
 		stmmac_set_rx_owner(priv, p, use_rx_wd);
@@ -5492,15 +5500,7 @@ static bool stmmac_rx_refill_zc(struct stmmac_priv *priv, u32 queue, u32 budget)
 		stmmac_set_desc_sec_addr(priv, rx_desc, 0, false);
 		stmmac_refill_desc3(priv, rx_q, rx_desc);
 
-		rx_q->rx_count_frames++;
-		rx_q->rx_count_frames += priv->rx_coal_frames[queue];
-		if (rx_q->rx_count_frames > priv->rx_coal_frames[queue])
-			rx_q->rx_count_frames = 0;
-
-		use_rx_wd = !priv->rx_coal_frames[queue];
-		use_rx_wd |= rx_q->rx_count_frames > 0;
-		if (!priv->use_riwt)
-			use_rx_wd = false;
+		use_rx_wd = !stmmac_rx_need_ic(priv, rx_q, queue);
 
 		dma_wmb();
 		stmmac_set_rx_owner(priv, rx_desc, use_rx_wd);
-- 
2.54.0