[PATCH v3 net] net: fec: add dma_wmb to ensure correct descriptor values

[Shenwei Wang <shenwei.wang@nxp.com>]

Two dma_wmb() are added in the XDP TX path to ensure proper ordering of
descriptor and buffer updates:
1. A dma_wmb() is added after updating the last BD to make sure
   the updates to rest of the descriptor are visible before
   transferring ownership to FEC.
2. A dma_wmb() is also added after updating the bdp to ensure these
   updates are visible before updating txq->bd.cur.
3. Start the xmit of the frame immediately right after configuring the
   tx descriptor.

Fixes: 6d6b39f180b8 ("net: fec: add initial XDP support")
Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
---
 v3:
  - use the lightweight memory barrier dma_wmb.

 v2:
  - update the inline comments for 2nd wmb per Wei Fang's review.

 drivers/net/ethernet/freescale/fec_main.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
index 6d0b46c76924..a5096c3cac01 100644
--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c
@@ -3834,6 +3834,11 @@ static int fec_enet_txq_xmit_frame(struct fec_enet_private *fep,
 	index = fec_enet_get_bd_index(last_bdp, &txq->bd);
 	txq->tx_skbuff[index] = NULL;

+	/* Make sure the updates to rest of the descriptor are performed before
+	 * transferring ownership.
+	 */
+	dma_wmb();
+
 	/* Send it on its way.  Tell FEC it's ready, interrupt when done,
 	 * it's the last BD of the frame, and to put the CRC on the end.
 	 */
@@ -3843,8 +3848,14 @@ static int fec_enet_txq_xmit_frame(struct fec_enet_private *fep,
 	/* If this was the last BD in the ring, start at the beginning again. */
 	bdp = fec_enet_get_nextdesc(last_bdp, &txq->bd);

+	/* Make sure the update to bdp are performed before txq->bd.cur. */
+	dma_wmb();
+
 	txq->bd.cur = bdp;

+	/* Trigger transmission start */
+	writel(0, txq->bd.reg_desc_active);
+
 	return 0;
 }

@@ -3873,12 +3884,6 @@ static int fec_enet_xdp_xmit(struct net_device *dev,
 		sent_frames++;
 	}

-	/* Make sure the update to bdp and tx_skbuff are performed. */
-	wmb();
-
-	/* Trigger transmission start */
-	writel(0, txq->bd.reg_desc_active);
-
 	__netif_tx_unlock(nq);

 	return sent_frames;
--
2.34.1