net-sysfs: expose number of carrier on/off changes

This allows to monitor carrier on/off transitions and detect link
flapping issues:
 - new /sys/class/net/X/carrier_changes
 - new rtnetlink IFLA_CARRIER_CHANGES (getlink)

Tested:
  - grep . /sys/class/net/*/carrier_changes
    + ip link set dev X down/up
    + plug/unplug cable
  - updated iproute2: prints IFLA_CARRIER_CHANGES
  - iproute2 20121211-2 (debian): unchanged behavior

Signed-off-by: David Decotigny <decot@googlers.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 34cae3ee74f1..45537ed7a5b3 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1308,6 +1308,9 @@ struct net_device {
         atomic_long_t           rx_dropped;
         atomic_long_t           tx_dropped;
 
+        /* Stats to monitor carrier on<->off transitions */
+        atomic_t                carrier_changes;
+
 #ifdef CONFIG_WIRELESS_EXT
         /* List of functions to handle Wireless Extensions (instead of ioctl).
          * See <net/iw_handler.h> for details. Jean II */