netdev - [PATCH][RFC] TSO for VLAN devices without VLAN tag insertion

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Message-ID: <20080610195943.GO11300@solarflare.com>
Date:	Tue, 10 Jun 2008 20:59:44 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	netdev@...r.kernel.org
Cc:	linux-net-drivers@...arflare.com
Subject: [PATCH][RFC] TSO for VLAN devices without VLAN tag insertion

The SFC4000 controller can set IP header, TCP and UDP checksums in IPv4
packets, with or without VLAN encapsulation.  However it does not do VLAN
tag insertion.

When I try to make TSO work for VLAN devices via the new vlan_features,
dev_queue_xmit() finds skb->ip_summed == CHECKSUM_PARTIAL &&
skb->protocol == htons(ETH_P_8021Q) and calls skb_checksum_help(), which
sets skb->ip_summed = CHECKSUM_NONE.

This causes netif_needs_gso() to return true when called by
dev_hard_start_xmit(), and GSO then does its worst.

Below is a patch which worked for me, but I'm not sure whether it's the
best way to fix the problem (so I'm not submitting or signing it off yet).

Ben.

--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -119,6 +119,7 @@
 #include <linux/err.h>
 #include <linux/ctype.h>
 #include <linux/if_arp.h>
+#include <linux/if_vlan.h>
 
 #include "net-sysfs.h"
 
@@ -1638,14 +1639,24 @@ int dev_queue_xmit(struct sk_buff *skb)
 	 * checksumming for this protocol, complete checksumming here.
 	 */
 	if (skb->ip_summed == CHECKSUM_PARTIAL) {
+		unsigned long features = dev->features;
+		__be16 protocol = skb->protocol;
+
+		if (protocol == htons(ETH_P_8021Q)) {
+			struct vlan_ethhdr *veh =
+				(struct vlan_ethhdr *)skb->data;
+			features &= dev->vlan_features;
+			protocol = veh->h_vlan_encapsulated_proto;
+		}
+
 		skb_set_transport_header(skb, skb->csum_start -
 					      skb_headroom(skb));
 
-		if (!(dev->features & NETIF_F_GEN_CSUM) &&
-		    !((dev->features & NETIF_F_IP_CSUM) &&
-		      skb->protocol == htons(ETH_P_IP)) &&
-		    !((dev->features & NETIF_F_IPV6_CSUM) &&
-		      skb->protocol == htons(ETH_P_IPV6)))
+		if (!(features & NETIF_F_GEN_CSUM) &&
+		    !((features & NETIF_F_IP_CSUM) &&
+		      protocol == htons(ETH_P_IP)) &&
+		    !((features & NETIF_F_IPV6_CSUM) &&
+		      protocol == htons(ETH_P_IPV6)))
 			if (skb_checksum_help(skb))
 				goto out_kfree_skb;
 	}

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
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