af_packet: don't enable global timestamps

[Stephen Hemminger <shemminger@linux-foundation.org>]

Andi mentioned he did something like this already, but never
submitted it.

The dhcp client application uses AF_PACKET with a packet filter to
receive data. The application doesn't even use timestamps, but because
the AF_PACKET API has timestamps, they get turned on globally which
causes an expensive time of day lookup for every packet received
on any system that uses the standard DHCP client.

The fix is to not enable the timestamp (but use if if available).
This causes the time lookup to only occur on those packets
that are destined for the AF_PACKET socket. 
The timestamping occurs after packet filtering
so all packets dropped by filtering to not cause a clock call.

The one downside of this a a few microseconds additional delay
added from the normal timestamping location (netif_rx) until the
receive callback in AF_PACKET. But since the offset is fairly consistent
it should not upset applications that do want really use timestamps,
like wireshark.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>


--- a/net/packet/af_packet.c	2007-07-23 09:31:26.000000000 +0100
+++ b/net/packet/af_packet.c	2007-09-03 14:55:00.000000000 +0100
@@ -640,11 +640,10 @@ static int tpacket_rcv(struct sk_buff *s
 	h->tp_snaplen = snaplen;
 	h->tp_mac = macoff;
 	h->tp_net = netoff;
-	if (skb->tstamp.tv64 == 0) {
-		__net_timestamp(skb);
-		sock_enable_timestamp(sk);
-	}
-	tv = ktime_to_timeval(skb->tstamp);
+	if (skb->tstamp.tv64)
+		tv = ktime_to_timeval(skb->tstamp);
+	else
+		do_gettimeofday(&tv);
 	h->tp_sec = tv.tv_sec;
 	h->tp_usec = tv.tv_usec;