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>

1 files changed, 30 insertions, 2 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 1e27a67df24..d20c28e839d 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1672,10 +1672,21 @@ static int dev_gso_segment(struct sk_buff *skb)
         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 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
                                 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 @@ gso:
                         skb->next = nskb;
                         return rc;
                 }
+                tstamp_tx(skb);
                 if (unlikely(netif_tx_queue_stopped(txq) && skb->next))
                         return NETDEV_TX_BUSY;
         } while (skb->next);