[PATCH net-next v2 2/2] net: stmmac: fix transmit interrupt coalescing

[Russell King (Oracle)]

The accounting for transmit frames does not count the descriptors
correctly. It uses:

	tx_packets = (tx_q->cur_tx + 1) - first_tx;

however, these are indexes into a circular buffer, so cur_tx can be
less than first_tx, and when that happens, tx_packets becomes a very
large unsigned integer. When this is added to tx_q->tx_count_frames,
it has the effect of reducing the count of frames, possibly causing
it to also wrap to a very large unsigned integer.

Fix this by using CIRC_CNT() to calculate the number of descriptors
used.

Signed-off-by: Russell King (Oracle) <rmk+kernel at armlinux.org.uk>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 2d74fe98ad61..baf2ff577f45 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -4503,7 +4503,8 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
 	tx_q->tx_skbuff_dma[tx_q->cur_tx].buf_type = STMMAC_TXBUF_T_SKB;
 
 	/* Manage tx mitigation */
-	tx_packets = (tx_q->cur_tx + 1) - first_tx;
+	tx_packets = CIRC_CNT(tx_q->cur_tx + 1, first_tx,
+			      priv->dma_conf.dma_tx_size);
 	tx_q->tx_count_frames += tx_packets;
 
 	if ((skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP) && priv->hwts_tx_en)
@@ -4781,7 +4782,7 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 	 * This approach takes care about the fragments: desc is the first
 	 * element in case of no SG.
 	 */
-	tx_packets = (entry + 1) - first_tx;
+	tx_packets = CIRC_CNT(entry + 1, first_tx, priv->dma_conf.dma_tx_size);
 	tx_q->tx_count_frames += tx_packets;
 
 	if ((skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP) && priv->hwts_tx_en)
-- 
2.47.3