[PATCH 4/4] wifi: mt76: mt7996: fix potential tx_retries underflow

[Ryder Lee <ryder.lee@mediatek.com>]

When FIELD_GET returns 0 for the retry count, subtracting 1 causes
an unsigned integer underflow, resulting in tx_retries becoming a
very large value (0xFFFFFFFF for u32).

Fix by checking if count is non-zero before subtracting 1.

Fixes: 2461599f835e ("wifi: mt76: mt7996: get tx_retries and tx_failed from txfree")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
---
 drivers/net/wireless/mediatek/mt76/mt7996/mac.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/wireless/mediatek/mt76/mt7996/mac.c b/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
index a59c14c8f..3fad977ba 100644
--- a/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
+++ b/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
@@ -1361,13 +1361,13 @@ mt7996_mac_tx_free(struct mt7996_dev *dev, void *data, int len)
 				cur_info++;
 			continue;
 		} else if (info & MT_TXFREE_INFO_HEADER) {
-			u32 tx_retries = 0, tx_failed = 0;
+			u32 tx_retries = 0, tx_failed = 0, count;
 
 			if (!wcid)
 				continue;
 
-			tx_retries =
-				FIELD_GET(MT_TXFREE_INFO_COUNT, info) - 1;
+			count = FIELD_GET(MT_TXFREE_INFO_COUNT, info);
+			tx_retries = count ? count - 1 : 0;
 			tx_failed = tx_retries +
 				!!FIELD_GET(MT_TXFREE_INFO_STAT, info);
 
-- 
2.45.2