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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/netdevice.h

···
 	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 */

include/uapi/linux/if_link.h

···
 	IFLA_NUM_RX_QUEUES,
 	IFLA_CARRIER,
 	IFLA_PHYS_PORT_ID,
+	IFLA_CARRIER_CHANGES,
 	__IFLA_MAX
 };
 

net/core/net-sysfs.c

···
 }
 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)
···
 	&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,

net/core/rtnetlink.c

···
 	       + nla_total_size(4) /* IFLA_NUM_RX_QUEUES */
 	       + nla_total_size(1) /* IFLA_OPERSTATE */
 	       + nla_total_size(1) /* IFLA_LINKMODE */
+	       + nla_total_size(4) /* IFLA_CARRIER_CHANGES */
 	       + nla_total_size(ext_filter_mask
 			        & RTEXT_FILTER_VF ? 4 : 0) /* IFLA_NUM_VF */
 	       + rtnl_vfinfo_size(dev, ext_filter_mask) /* IFLA_VFINFO_LIST */
···
 	    (dev->qdisc &&
 	     nla_put_string(skb, IFLA_QDISC, dev->qdisc->ops->id)) ||
 	    (dev->ifalias &&
-	     nla_put_string(skb, IFLA_IFALIAS, dev->ifalias)))
+	     nla_put_string(skb, IFLA_IFALIAS, dev->ifalias)) ||
+	    nla_put_u32(skb, IFLA_CARRIER_CHANGES,
+			atomic_read(&dev->carrier_changes)))
 		goto nla_put_failure;
 
 	if (1) {
···
 	[IFLA_NUM_TX_QUEUES]	= { .type = NLA_U32 },
 	[IFLA_NUM_RX_QUEUES]	= { .type = NLA_U32 },
 	[IFLA_PHYS_PORT_ID]	= { .type = NLA_BINARY, .len = MAX_PHYS_PORT_ID_LEN },
+	[IFLA_CARRIER_CHANGES]	= { .type = NLA_U32 },  /* ignored */
 };
 
 static const struct nla_policy ifla_info_policy[IFLA_INFO_MAX+1] = {

net/sched/sch_generic.c

···
 	if (test_and_clear_bit(__LINK_STATE_NOCARRIER, &dev->state)) {
 		if (dev->reg_state == NETREG_UNINITIALIZED)
 			return;
+		atomic_inc(&dev->carrier_changes);
 		linkwatch_fire_event(dev);
 		if (netif_running(dev))
 			__netdev_watchdog_up(dev);
···
 	if (!test_and_set_bit(__LINK_STATE_NOCARRIER, &dev->state)) {
 		if (dev->reg_state == NETREG_UNINITIALIZED)
 			return;
+		atomic_inc(&dev->carrier_changes);
 		linkwatch_fire_event(dev);
 	}
 }