net: add a comment on netdev->last_rx

As some driver authors seem to reintroduce dev->last_rx use,
add a comment to strongly discourage this.

Since commit 6cf3f41e6c0 (bonding, net: Move last_rx update into bonding
recv logic), network drivers dont need to update last_rx themselves,
unless they use this field to implement a timeout.

Not updating last_rx helps not dirtying a cache line, improving
performance in SMP.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 0cf9448..c82220a 100644 (file)
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -953,7 +953,14 @@ struct net_device {
 /*
  * Cache line mostly used on receive path (including eth_type_trans())
  */
-       unsigned long           last_rx;        /* Time of last Rx      */
+       unsigned long           last_rx;        /* Time of last Rx
+                                                * This should not be set in
+                                                * drivers, unless really needed,
+                                                * because network stack (bonding)
+                                                * use it if/when necessary, to
+                                                * avoid dirtying this cache line.
+                                                */
+
        /* Interface address info used in eth_type_trans() */
        unsigned char           *dev_addr;      /* hw address, (before bcast
                                                   because most packets are