netdev - [RFC PATCH 2/2] ixgbe: Add functionality for delaying the MMIO write for Tx

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Date:	Wed, 11 Jul 2012 17:26:08 -0700
From:	Alexander Duyck <alexander.h.duyck@...el.com>
To:	netdev@...r.kernel.org
Cc:	davem@...emloft.net, jeffrey.t.kirsher@...el.com,
	edumazet@...gle.com, bhutchings@...arflare.com,
	therbert@...gle.com, alexander.duyck@...il.com
Subject: [RFC PATCH 2/2] ixgbe: Add functionality for delaying the MMIO write
	for Tx

This change makes it so that ixgbe can use the new framework for delaying
the MMIO writes in the transmit path.  With this change in place we see a
significant reduction in CPU utilization and increase in overall packets
per second throughput for bulk traffic tests.  In addition I have not seen
any increase in latency as a result of this patch.

Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
---

 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c |   22 ++++++++++++++++++++--
 1 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index 9ec65ee..e9b71b8 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -884,6 +884,8 @@ static bool ixgbe_clean_tx_irq(struct ixgbe_q_vector *q_vector,
 		return true;
 	}
 
+	netif_tx_dispatch_queue(txring_txq(tx_ring));
+
 	netdev_tx_completed_queue(txring_txq(tx_ring),
 				  total_packets, total_bytes);
 
@@ -5825,6 +5827,22 @@ static void ixgbe_service_task(struct work_struct *work)
 	ixgbe_service_event_complete(adapter);
 }
 
+static void ixgbe_complete_xmit_frame(struct net_device *dev,
+				      unsigned int index)
+{
+	struct ixgbe_adapter *adapter = netdev_priv(dev);
+	struct ixgbe_ring *tx_ring = adapter->tx_ring[index];
+
+	/* notify HW of packet */
+	writel(tx_ring->next_to_use, tx_ring->tail);
+
+	/*
+	 * we need this if more than one processor can write to our tail
+	 * at a time, it synchronizes IO on IA64/Altix systems
+	 */
+	mmiowb();
+}
+
 static int ixgbe_tso(struct ixgbe_ring *tx_ring,
 		     struct ixgbe_tx_buffer *first,
 		     u8 *hdr_len)
@@ -6150,8 +6168,7 @@ static void ixgbe_tx_map(struct ixgbe_ring *tx_ring,
 
 	tx_ring->next_to_use = i;
 
-	/* notify HW of packet */
-	writel(i, tx_ring->tail);
+	netdev_complete_xmit(txring_txq(tx_ring));
 
 	return;
 dma_error:
@@ -6961,6 +6978,7 @@ static const struct net_device_ops ixgbe_netdev_ops = {
 	.ndo_open		= ixgbe_open,
 	.ndo_stop		= ixgbe_close,
 	.ndo_start_xmit		= ixgbe_xmit_frame,
+	.ndo_complete_xmit	= ixgbe_complete_xmit_frame,
 #ifdef IXGBE_FCOE
 	.ndo_select_queue	= ixgbe_select_queue,
 #endif

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