bnxt_en: Fix TX timeout when TX ring size is set to the smallest

The smallest TX ring size we support must fit a TX SKB with MAX_SKB_FRAGS
+ 1.  Because the first TX BD for a packet is always a long TX BD, we
need an extra TX BD to fit this packet.  Define BNXT_MIN_TX_DESC_CNT with
this value to make this more clear.  The current code uses a minimum
that is off by 1.  Fix it using this constant.

The tx_wake_thresh to determine when to wake up the TX queue is half the
ring size but we must have at least BNXT_MIN_TX_DESC_CNT for the next
packet which may have maximum fragments.  So the comparison of the
available TX BDs with tx_wake_thresh should be >= instead of > in the
current code.  Otherwise, at the smallest ring size, we will never wake
up the TX queue and will cause TX timeout.

Fixes: c0c050c ("bnxt_en: New Broadcom ethernet driver.")
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadocm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/bnxt/bnxt.c

@@ -391,7 +391,7 @@ static bool bnxt_txr_netif_try_stop_queue(struct bnxt *bp,
 	 * netif_tx_queue_stopped().
 	 */
 	smp_mb();
-	if (bnxt_tx_avail(bp, txr) > bp->tx_wake_thresh) {
+	if (bnxt_tx_avail(bp, txr) >= bp->tx_wake_thresh) {
 		netif_tx_wake_queue(txq);
 		return false;
 	}
@@ -764,7 +764,7 @@ static void bnxt_tx_int(struct bnxt *bp, struct bnxt_napi *bnapi, int nr_pkts)
 	smp_mb();
 
 	if (unlikely(netif_tx_queue_stopped(txq)) &&
-	    bnxt_tx_avail(bp, txr) > bp->tx_wake_thresh &&
+	    bnxt_tx_avail(bp, txr) >= bp->tx_wake_thresh &&
 	    READ_ONCE(txr->dev_state) != BNXT_DEV_STATE_CLOSING)
 		netif_tx_wake_queue(txq);
 }
@@ -2416,7 +2416,7 @@ static int __bnxt_poll_work(struct bnxt *bp, struct bnxt_cp_ring_info *cpr,
 		if (TX_CMP_TYPE(txcmp) == CMP_TYPE_TX_L2_CMP) {
 			tx_pkts++;
 			/* return full budget so NAPI will complete. */
-			if (unlikely(tx_pkts > bp->tx_wake_thresh)) {
+			if (unlikely(tx_pkts >= bp->tx_wake_thresh)) {
 				rx_pkts = budget;
 				raw_cons = NEXT_RAW_CMP(raw_cons);
 				if (budget)
@@ -3640,7 +3640,7 @@ static int bnxt_init_tx_rings(struct bnxt *bp)
 	u16 i;
 
 	bp->tx_wake_thresh = max_t(int, bp->tx_ring_size / 2,
-				   MAX_SKB_FRAGS + 1);
+				   BNXT_MIN_TX_DESC_CNT);
 
 	for (i = 0; i < bp->tx_nr_rings; i++) {
 		struct bnxt_tx_ring_info *txr = &bp->tx_ring[i];

drivers/net/ethernet/broadcom/bnxt/bnxt.h

@@ -629,6 +629,11 @@ struct nqe_cn {
 #define BNXT_MAX_RX_JUM_DESC_CNT	(RX_DESC_CNT * MAX_RX_AGG_PAGES - 1)
 #define BNXT_MAX_TX_DESC_CNT		(TX_DESC_CNT * MAX_TX_PAGES - 1)
 
+/* Minimum TX BDs for a TX packet with MAX_SKB_FRAGS + 1.  We need one extra
+ * BD because the first TX BD is always a long BD.
+ */
+#define BNXT_MIN_TX_DESC_CNT		(MAX_SKB_FRAGS + 2)
+
 #define RX_RING(x)	(((x) & ~(RX_DESC_CNT - 1)) >> (BNXT_PAGE_SHIFT - 4))
 #define RX_IDX(x)	((x) & (RX_DESC_CNT - 1))
 

drivers/net/ethernet/broadcom/bnxt/bnxt_ethtool.c

@@ -798,7 +798,7 @@ static int bnxt_set_ringparam(struct net_device *dev,
 
 	if ((ering->rx_pending > BNXT_MAX_RX_DESC_CNT) ||
 	    (ering->tx_pending > BNXT_MAX_TX_DESC_CNT) ||
-	    (ering->tx_pending <= MAX_SKB_FRAGS))
+	    (ering->tx_pending < BNXT_MIN_TX_DESC_CNT))
 		return -EINVAL;
 
 	if (netif_running(dev))