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net: infrastructure for hardware time stamping

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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>
This commit is contained in:
Patrick Ohly
2009-02-12 05:03:37 +00:00
committed by David S. Miller
parent cb9eff0978
commit ac45f602ee
3 changed files with 161 additions and 3 deletions
Download Patch File Download Diff File Expand all files Collapse all files

32
net/core/dev.c
View File

@@ -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);