net: infrastructure for hardware time stamping
The additional per-packet information (16 bytes for time stamps, 1
byte for flags) is stored for all packets in the skb_shared_info
struct. This implementation detail is hidden from users of that
information via skb_* accessor functions. A separate struct resp.
union is used for the additional information so that it can be
stored/copied easily outside of skb_shared_info.
Compared to previous implementations (reusing the tstamp field
depending on the context, optional additional structures) this
is the simplest solution. It does not extend sk_buff itself.
TX time stamping is implemented in software if the device driver
doesn't support hardware time stamping.
The new semantic for hardware/software time stamping around
ndo_start_xmit() is based on two assumptions about existing
network device drivers which don't support hardware time
stamping and know nothing about it:
- they leave the new skb_shared_tx unmodified
- the keep the connection to the originating socket in skb->sk
alive, i.e., don't call skb_orphan()
Given that skb_shared_tx is new, the first assumption is safe.
The second is only true for some drivers. As a result, software
TX time stamping currently works with the bnx2 driver, but not
with the unmodified igb driver (the two drivers this patch series
was tested with).
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/dev.c b/net/core/dev.c
index 1e27a67..d20c28e 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1672,10 +1672,21 @@
return 0;
}
+static void tstamp_tx(struct sk_buff *skb)
+{
+ union skb_shared_tx *shtx =
+ skb_tx(skb);
+ if (unlikely(shtx->software &&
+ !shtx->in_progress)) {
+ skb_tstamp_tx(skb, NULL);
+ }
+}
+
int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
struct netdev_queue *txq)
{
const struct net_device_ops *ops = dev->netdev_ops;
+ int rc;
prefetch(&dev->netdev_ops->ndo_start_xmit);
if (likely(!skb->next)) {
@@ -1689,13 +1700,29 @@
goto gso;
}
- return ops->ndo_start_xmit(skb, dev);
+ rc = ops->ndo_start_xmit(skb, dev);
+ /*
+ * TODO: if skb_orphan() was called by
+ * dev->hard_start_xmit() (for example, the unmodified
+ * igb driver does that; bnx2 doesn't), then
+ * skb_tx_software_timestamp() will be unable to send
+ * back the time stamp.
+ *
+ * How can this be prevented? Always create another
+ * reference to the socket before calling
+ * dev->hard_start_xmit()? Prevent that skb_orphan()
+ * does anything in dev->hard_start_xmit() by clearing
+ * the skb destructor before the call and restoring it
+ * afterwards, then doing the skb_orphan() ourselves?
+ */
+ if (likely(!rc))
+ tstamp_tx(skb);
+ return rc;
}
gso:
do {
struct sk_buff *nskb = skb->next;
- int rc;
skb->next = nskb->next;
nskb->next = NULL;
@@ -1705,6 +1732,7 @@
skb->next = nskb;
return rc;
}
+ tstamp_tx(skb);
if (unlikely(netif_tx_queue_stopped(txq) && skb->next))
return NETDEV_TX_BUSY;
} while (skb->next);