[PATCH net v4 3/3] net: ethernet: cortina: Bypass checksumming engine of alien ethertypes

[Linus Walleij <linus.walleij@linaro.org>]

We had workarounds were the ethernet checksumming engine would be bypassed
for larger frames, this fixed devices using DSA, but regressed devices
where the ethernet was connected directly to a PHY.

The devices with a PHY connected directly can't handle large frames
either way, with or without bypass. Looking at the size of the frame
is probably just wrong.

Rework the workaround such that we don't activate the checksumming engine if
the ethertype inside the actual frame is something else than 0x0800
(IPv4) or 0x86dd (IPv6). These are the only frames the checksumming engine
can actually handle. VLAN framing (0x8100) also works fine.

We can't inspect skb->protocol because DSA frames will sometimes have a
custom ethertype despite skb->protocol is e.g. 0x0800.

If the frame is ALSO over the size of an ordinary ethernet frame,
we will actively bypass the checksumming engine. (Always doing this
makes the hardware unstable.)

After this both devices with direct ethernet attached such as D-Link
DNS-313 and devices with a DSA switch with a custom ethertype such as
D-Link DIR-685 work fine.

Fixes: d4d0c5b4d279 ("net: ethernet: cortina: Handle large frames")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 drivers/net/ethernet/cortina/gemini.c | 33 +++++++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 8 deletions(-)

diff --git a/drivers/net/ethernet/cortina/gemini.c b/drivers/net/ethernet/cortina/gemini.c
index 5e399c6e095b..db828e4f258f 100644
--- a/drivers/net/ethernet/cortina/gemini.c
+++ b/drivers/net/ethernet/cortina/gemini.c
@@ -29,6 +29,7 @@
 #include <linux/of_net.h>
 #include <linux/of_platform.h>
 #include <linux/etherdevice.h>
+#include <linux/if_ether.h>
 #include <linux/if_vlan.h>
 #include <linux/skbuff.h>
 #include <linux/phy.h>
@@ -1142,22 +1143,38 @@ static int gmac_map_tx_bufs(struct net_device *netdev, struct sk_buff *skb,
 	struct gmac_txdesc *txd;
 	skb_frag_t *skb_frag;
 	dma_addr_t mapping;
+	u16 ethertype;
 	void *buffer;
 
 	/* TODO: implement proper TSO using MTU in word3 */
 	word1 = skb->len;
 	word3 = SOF_BIT | skb->len;
 
-	if (skb->ip_summed == CHECKSUM_PARTIAL) {
+	/* Dig out the the ethertype actually in the buffer and not what the
+	 * protocol claims to be. This is the raw data that the checksumming
+	 * offload engine will have to deal with.
+	 */
+	ethertype = ntohs(skb_eth_raw_ethertype(skb));
+	/* This is the only VLAN type supported by this hardware so check for
+	 * that: the checksumming engine can handle IP and IPv6 inside 802.1Q.
+	 */
+	if (ethertype == ETH_P_8021Q)
+		ethertype = ntohs(__vlan_get_protocol(skb, htons(ethertype), NULL));
+
+	if (ethertype != ETH_P_IP && ethertype != ETH_P_IPV6) {
+		/* Hardware offloaded checksumming isn't working on non-IP frames.
+		 * This happens for example on some DSA switches using a custom
+		 * ethertype. When a frame gets bigger than a standard ethernet
+		 * frame, it also needs to actively bypass the checksumming engine.
+		 * There is no clear explanation to why it is like this, the
+		 * reference manual has left the TSS completely undocumented.
+		 */
+		if (skb->len > ETH_FRAME_LEN)
+			word1 |= TSS_BYPASS_BIT;
+	} else if (skb->ip_summed == CHECKSUM_PARTIAL) {
 		int tcp = 0;
 
-		/* We do not switch off the checksumming on non TCP/UDP
-		 * frames: as is shown from tests, the checksumming engine
-		 * is smart enough to see that a frame is not actually TCP
-		 * or UDP and then just pass it through without any changes
-		 * to the frame.
-		 */
-		if (skb->protocol == htons(ETH_P_IP)) {
+		if (ethertype == ETH_P_IP) {
 			word1 |= TSS_IP_CHKSUM_BIT;
 			tcp = ip_hdr(skb)->protocol == IPPROTO_TCP;
 		} else { /* IPv6 */

-- 
2.34.1