Fix comment about VLAN encapsulation and checksum offload

Document that some chips actually could do hardware checksum offload for
encapsulated packets, though that would need special handling in those
drivers.

discussions and ok naddy@

diff --git a/sys/net/if_vlan.c b/sys/net/if_vlan.c
index 37f60b8..8f279e2 100644 (file)
--- a/sys/net/if_vlan.c
+++ b/sys/net/if_vlan.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: if_vlan.c,v 1.177 2018/04/03 02:52:50 dlg Exp $       */
+/*     $OpenBSD: if_vlan.c,v 1.178 2018/07/11 14:20:18 sf Exp $        */
 
 /*
  * Copyright 1998 Massachusetts Institute of Technology
@@ -473,6 +473,11 @@ vlan_up(struct ifvlan *ifv)
        ifp->if_hardmtu = hardmtu;
        SET(ifp->if_flags, ifp0->if_flags & IFF_SIMPLEX);
 
+       /*
+        * Note: In cases like vio(4) and em(4) where the offsets of the
+        * csum can be freely defined, we could actually do csum offload
+        * for VLAN and QINQ packets.
+        */
        if (ifv->ifv_type != ETHERTYPE_VLAN) {
                /*
                 * Hardware offload only works with the default VLAN
@@ -481,12 +486,8 @@ vlan_up(struct ifvlan *ifv)
                ifp->if_capabilities = 0;
        } else if (ISSET(ifp0->if_capabilities, IFCAP_VLAN_HWTAGGING)) {
                /*
-                * If the parent interface can do hardware-assisted
-                * VLAN encapsulation, then propagate its hardware-
-                * assisted checksumming flags.
-                *
-                * If the card cannot handle hardware tagging, it cannot
-                * possibly compute the correct checksums for tagged packets.
+                * Chips that can do hardware-assisted VLAN encapsulation, can
+                * calculate the correct checksum for VLAN tagged packets.
                 */
                ifp->if_capabilities = ifp0->if_capabilities & IFCAP_CSUM_MASK;
        }