[PATCH net-next v6 1/2] net: axienet: Be more careful about updating tx_bd_tail

[Robert Hancock <robert.hancock@calian.com>]

The axienet_start_xmit function was updating the tx_bd_tail variable
multiple times, with potential rollbacks on error or invalid
intermediate positions, even though this variable is also used in the
TX completion path. Use READ_ONCE and WRITE_ONCE to make this update
more atomic, and move the write before the MMIO write to start the
transfer, so it is protected by that implicit write barrier.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
---
 .../net/ethernet/xilinx/xilinx_axienet_main.c | 23 +++++++++++--------
 1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
index d6fc3f7acdf0..2f39eb4de249 100644
--- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
+++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
@@ -807,12 +807,15 @@ axienet_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 	u32 csum_index_off;
 	skb_frag_t *frag;
 	dma_addr_t tail_p, phys;
+	u32 orig_tail_ptr, new_tail_ptr;
 	struct axienet_local *lp = netdev_priv(ndev);
 	struct axidma_bd *cur_p;
-	u32 orig_tail_ptr = lp->tx_bd_tail;
+
+	orig_tail_ptr = READ_ONCE(lp->tx_bd_tail);
+	new_tail_ptr = orig_tail_ptr;
 
 	num_frag = skb_shinfo(skb)->nr_frags;
-	cur_p = &lp->tx_bd_v[lp->tx_bd_tail];
+	cur_p = &lp->tx_bd_v[orig_tail_ptr];
 
 	if (axienet_check_tx_bd_space(lp, num_frag + 1)) {
 		/* Should not happen as last start_xmit call should have
@@ -852,9 +855,9 @@ axienet_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 	cur_p->cntrl = skb_headlen(skb) | XAXIDMA_BD_CTRL_TXSOF_MASK;
 
 	for (ii = 0; ii < num_frag; ii++) {
-		if (++lp->tx_bd_tail >= lp->tx_bd_num)
-			lp->tx_bd_tail = 0;
-		cur_p = &lp->tx_bd_v[lp->tx_bd_tail];
+		if (++new_tail_ptr >= lp->tx_bd_num)
+			new_tail_ptr = 0;
+		cur_p = &lp->tx_bd_v[new_tail_ptr];
 		frag = &skb_shinfo(skb)->frags[ii];
 		phys = dma_map_single(lp->dev,
 				      skb_frag_address(frag),
@@ -866,8 +869,6 @@ axienet_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 			ndev->stats.tx_dropped++;
 			axienet_free_tx_chain(ndev, orig_tail_ptr, ii + 1,
 					      NULL);
-			lp->tx_bd_tail = orig_tail_ptr;
-
 			return NETDEV_TX_OK;
 		}
 		desc_set_phys_addr(lp, phys, cur_p);
@@ -877,11 +878,13 @@ axienet_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 	cur_p->cntrl |= XAXIDMA_BD_CTRL_TXEOF_MASK;
 	cur_p->skb = skb;
 
-	tail_p = lp->tx_bd_p + sizeof(*lp->tx_bd_v) * lp->tx_bd_tail;
+	tail_p = lp->tx_bd_p + sizeof(*lp->tx_bd_v) * new_tail_ptr;
+	if (++new_tail_ptr >= lp->tx_bd_num)
+		new_tail_ptr = 0;
+	WRITE_ONCE(lp->tx_bd_tail, new_tail_ptr);
+
 	/* Start the transfer */
 	axienet_dma_out_addr(lp, XAXIDMA_TX_TDESC_OFFSET, tail_p);
-	if (++lp->tx_bd_tail >= lp->tx_bd_num)
-		lp->tx_bd_tail = 0;
 
 	/* Stop queue if next transmit may not have space */
 	if (axienet_check_tx_bd_space(lp, MAX_SKB_FRAGS + 1)) {
-- 
2.31.1