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, 11 insertions, 0 deletions

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index daed9a64c6f6..462396278484 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -253,6 +253,16 @@ static ssize_t operstate_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(operstate);
 
+static ssize_t carrier_changes_show(struct device *dev,
+                                    struct device_attribute *attr,
+                                    char *buf)
+{
+        struct net_device *netdev = to_net_dev(dev);
+        return sprintf(buf, fmt_dec,
+                       atomic_read(&netdev->carrier_changes));
+}
+static DEVICE_ATTR_RO(carrier_changes);
+
 /* read-write attributes */
 
 static int change_mtu(struct net_device *net, unsigned long new_mtu)
@@ -386,6 +396,7 @@ static struct attribute *net_class_attrs[] = {
         &dev_attr_duplex.attr,
         &dev_attr_dormant.attr,
         &dev_attr_operstate.attr,
+        &dev_attr_carrier_changes.attr,
         &dev_attr_ifalias.attr,
         &dev_attr_carrier.attr,
         &dev_attr_mtu.attr,