netdev - [PATCH net-next 33/40] net: fec: shrink the window for 'tx int lost'

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Date:	Thu, 28 Jan 2016 14:25:57 -0700
From:	Troy Kisky <troy.kisky@...ndarydevices.com>
To:	netdev@...r.kernel.org, davem@...emloft.net, B38611@...escale.com
Cc:	fabio.estevam@...escale.com, l.stach@...gutronix.de,
	andrew@...n.ch, tremyfr@...il.com, linux@....linux.org.uk,
	linux-arm-kernel@...ts.infradead.org, laci@...ndarydevices.com,
	shawnguo@...nel.org, Troy Kisky <troy.kisky@...ndarydevices.com>
Subject: [PATCH net-next 33/40] net: fec: shrink the window for 'tx int lost'

Perform skb_tx_timestamp before transferring ownership

This shrinks the window for 'tx int lost'
If the packet is transmitted and napi clears the tx interrupt
before the txq->bd.cur assignment, then the tx interrupt is
effectively lost.

Signed-off-by: Troy Kisky <troy.kisky@...ndarydevices.com>
---
 drivers/net/ethernet/freescale/fec_main.c | 29 +++++++++++++----------------
 1 file changed, 13 insertions(+), 16 deletions(-)

diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
index 9da2c6d..512b1ad 100644
--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c
@@ -510,27 +510,24 @@ static int fec_enet_txq_submit_skb(struct fec_enet_priv_tx_q *txq,
 	index = fec_enet_get_bd_index(last_bdp, &txq->bd);
 	/* Save skb pointer */
 	txq->tx_skbuff[index] = skb;
+	last_bdp = fec_enet_get_nextdesc(last_bdp, &txq->bd);
+	skb_tx_timestamp(skb);
 
 	/* Make sure the updates to rest of the descriptor are performed before
 	 * transferring ownership.
 	 */
 	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.
+	/* Send it on its way.  Transfer ownership
+	 * Make sure the update to bdp and tx_skbuff are performed before
+	 * the update to txq->bd.cur.
+	 * Keep window for interrupt between the next 3 lines as small as
+	 * possible, to avoid "tx int lost" in case no more tx interrupts
+	 * happen within 2 seconds.
 	 */
 	bdp->cbd_sc = status;
-
-	/* If this was the last BD in the ring, start at the beginning again. */
-	bdp = fec_enet_get_nextdesc(last_bdp, &txq->bd);
-
-	skb_tx_timestamp(skb);
-
-	/* Make sure the update to bdp and tx_skbuff are performed before
-	 * txq->bd.cur.
-	 */
 	wmb();
-	txq->bd.cur = bdp;
+	txq->bd.cur = last_bdp;
 
 	/* Trigger transmission start */
 	writel(0, txq->bd.reg_desc_active);
@@ -719,12 +716,12 @@ static int fec_enet_txq_submit_tso(struct fec_enet_priv_tx_q *txq,
 	 * transferring ownership.
 	 */
 	wmb();
-	/* Send it on its way.  Tell FEC it's ready, interrupt when done,
+	/* Send it on its way.  Transfer ownership
+	 * Keep window for interrupt between the next 3 lines as small as
+	 * possible, to avoid "tx int lost" in case no more tx interrupts
+	 * happen within 2 seconds.
 	 */
 	first_bdp->cbd_sc = status;
-	/* Make sure ownership is transferred before the
-	 * update to txq->bd.cur.
-	 */
 	wmb();
 	txq->bd.cur = bdp;
 
-- 
2.5.0