Merge branch 'ndo_xmit_flush'

Basic deferred TX queue flushing infrastructure.

Over time, and specifically and more recently at the Networking
Workshop during Kernel SUmmit in Chicago, we have discussed the idea
of having some way to optimize transmits of multiple TX packets at
a time.

There are several areas of overhead that could be amortized with such
schemes.  One has to do with locking and transactional overhead, the
other has to do with device specific costs.

This patch set here is more aimed at device specific costs.

Typically a device queues up a packet in the TX queue and then has to
do something to have the device start processing that new entry.
Sometimes this is composed of doing an MMIO write to a "tail"
register, and in other cases it can involve something as expensive as
a hypervisor call.

The basic setup defined here is that when the driver supports deferred
TX queue flushing, ndo_start_xmit should no longer perform that
operation.  Instead a new operation, ndo_xmit_flush, should do it.

I have converted IGB and virtio_net as example initial users.  The IGB
conversion is tested, virtio_net is not but it does compile :-)

All ndo_start_xmit call sites have been abstracted behind a new helper
called netdev_start_xmit().

This just adds the infrastructure, it does not actually add any
instances of actually doing multiple ndo_start_xmit calls per
ndo_xmit_flush invocation.

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 24 insertions, 11 deletions

diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index cb14bbdfb05..b9c020a05fb 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -136,6 +136,7 @@ static void igb_update_phy_info(unsigned long);
 static void igb_watchdog(unsigned long);
 static void igb_watchdog_task(struct work_struct *);
 static netdev_tx_t igb_xmit_frame(struct sk_buff *skb, struct net_device *);
+static void igb_xmit_flush(struct net_device *netdev, u16 queue);
 static struct rtnl_link_stats64 *igb_get_stats64(struct net_device *dev,
 					  struct rtnl_link_stats64 *stats);
 static int igb_change_mtu(struct net_device *, int);
@@ -2075,6 +2076,7 @@ static const struct net_device_ops igb_netdev_ops = {
 	.ndo_open		= igb_open,
 	.ndo_stop		= igb_close,
 	.ndo_start_xmit		= igb_xmit_frame,
+	.ndo_xmit_flush		= igb_xmit_flush,
 	.ndo_get_stats64	= igb_get_stats64,
 	.ndo_set_rx_mode	= igb_set_rx_mode,
 	.ndo_set_mac_address	= igb_set_mac,
@@ -4915,13 +4917,6 @@ static void igb_tx_map(struct igb_ring *tx_ring,
 
 	tx_ring->next_to_use = i;
 
-	writel(i, 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();
-
 	return;
 
 dma_error:
@@ -5057,17 +5052,20 @@ out_drop:
 	return NETDEV_TX_OK;
 }
 
-static inline struct igb_ring *igb_tx_queue_mapping(struct igb_adapter *adapter,
-						    struct sk_buff *skb)
+static struct igb_ring *__igb_tx_queue_mapping(struct igb_adapter *adapter, unsigned int r_idx)
 {
-	unsigned int r_idx = skb->queue_mapping;
-
 	if (r_idx >= adapter->num_tx_queues)
 		r_idx = r_idx % adapter->num_tx_queues;
 
 	return adapter->tx_ring[r_idx];
 }
 
+static inline struct igb_ring *igb_tx_queue_mapping(struct igb_adapter *adapter,
+						    struct sk_buff *skb)
+{
+	return __igb_tx_queue_mapping(adapter, skb->queue_mapping);
+}
+
 static netdev_tx_t igb_xmit_frame(struct sk_buff *skb,
 				  struct net_device *netdev)
 {
@@ -5096,6 +5094,21 @@ static netdev_tx_t igb_xmit_frame(struct sk_buff *skb,
 	return igb_xmit_frame_ring(skb, igb_tx_queue_mapping(adapter, skb));
 }
 
+static void igb_xmit_flush(struct net_device *netdev, u16 queue)
+{
+	struct igb_adapter *adapter = netdev_priv(netdev);
+	struct igb_ring *tx_ring;
+
+	tx_ring = __igb_tx_queue_mapping(adapter, queue);
+
+	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();
+}
+
 /**
  *  igb_tx_timeout - Respond to a Tx Hang
  *  @netdev: network interface device structure