[PATCH v2] net/nfp: fix the Tx descriptor free logic of nfd3

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

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")
Cc: stable@dpdk.org

Signed-off-by: Chaoyong He <chaoyong.he@corigine.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@corigine.com>
---
v2:
* Rebase to the latest main branch.
---
 drivers/net/nfp/nfd3/nfp_nfd3.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/net/nfp/nfd3/nfp_nfd3.h b/drivers/net/nfp/nfd3/nfp_nfd3.h
index 7bf2349904..e772bc4711 100644
--- a/drivers/net/nfp/nfd3/nfp_nfd3.h
+++ b/drivers/net/nfp/nfd3/nfp_nfd3.h
@@ -52,10 +52,14 @@ struct nfp_net_nfd3_tx_desc {
 static inline uint32_t
 nfp_net_nfd3_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;
+
+	return (free_desc > 8) ? (free_desc - 8) : 0;
 }
 
 /*
-- 
2.39.1