[PATCH 21.11] net/nfp: fix Tx descriptor free logic of NFD3

[Chaoyong He <chaoyong.he@corigine.com>]

[ upstream commit e97738919c2315e07c2e98b6a9cc3912c335364a ]

In the Tx descriptor free logic of nfd3, the former logic might force
cast a negative number into a very big unsigned number, and which will
cause potential problem in the xmit loop.

The xmit loop will continue in the place where it should break, and will
overwrite the Tx descriptor which is not free to use by the PMD.

Fixes: 74a640dac864 ("net/nfp: avoid modulo operations for handling ring wrapping")

Signed-off-by: Chaoyong He <chaoyong.he@corigine.com>
---
 drivers/net/nfp/nfp_rxtx.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/net/nfp/nfp_rxtx.c b/drivers/net/nfp/nfp_rxtx.c
index 639c1c925b..4fa608d417 100644
--- a/drivers/net/nfp/nfp_rxtx.c
+++ b/drivers/net/nfp/nfp_rxtx.c
@@ -796,10 +796,14 @@ nfp_net_tx_queue_setup(struct rte_eth_dev *dev, uint16_t queue_idx,
 static inline
 uint32_t nfp_free_tx_desc(struct nfp_net_txq *txq)
 {
+	uint32_t free_desc;
+
 	if (txq->wr_p >= txq->rd_p)
-		return txq->tx_count - (txq->wr_p - txq->rd_p) - 8;
+		free_desc = txq->tx_count - (txq->wr_p - txq->rd_p);
 	else
-		return txq->rd_p - txq->wr_p - 8;
+		free_desc = txq->rd_p - txq->wr_p - 8;
+
+	return (free_desc > 8) ? (free_desc - 8) : 0;
 }
 
 /*
-- 
2.39.1