Linux Device Drivers, Second Edition

Statistical Information

The last method a driver needs is get_stats. This method returns a pointer to the statistics for the device. Its implementation is pretty easy; the one shown works even when several interfaces are managed by the same driver, because the statistics are hosted within the device data structure.

struct net_device_stats *snull_stats(struct net_device *dev)
{
    struct snull_priv *priv = (struct snull_priv *) dev->priv;
    return &priv->stats;
}

The real work needed to return meaningful statistics is distributed throughout the driver, where the various fields are updated. The following list shows the most interesting fields in struct net_device_stats.

unsigned long rx_packets; , unsigned long tx_packets;

These fields hold the total number of incoming and outgoing packets successfully transferred by the interface.

unsigned long rx_bytes; , unsigned long tx_bytes;

The number of bytes received and transmitted by the interface. These fields were added in the 2.2 kernel.

unsigned long rx_errors; , unsigned long tx_errors;

The number of erroneous receptions and transmissions. There’s no end of things that can go wrong with packet transmission, and the net_device_stats structure includes six counters for specific receive errors and five for transmit errors. See <linux/netdevice.h> for the full list. If possible, your driver should maintain detailed error statistics, because they can be most helpful to system administrators trying to track down a problem.

unsigned long rx_dropped; , unsigned long tx_dropped;

The number of packets dropped during reception and transmission. Packets are dropped when there’s no memory available for packet data. tx_dropped is rarely used.

unsigned long collisions;

The number of collisions due to congestion on the medium.

unsigned long multicast;

The number of multicast packets received.

It is worth repeating that the get_stats method can be called at any time—even when the interface is down—so the driver should not release statistic information when running the stop method.