[PATCH net v4 1/3] net: ethernet: cortina: Drop software checksum and TSO

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

The recent change to allow large frames without hardware checksumming
slotted in software checksumming in the driver if hardware could not
do it.

This will however upset TSO (TCP Segment Offloading). Typical
error dumps includes this:

skb len=2961 headroom=222 headlen=66 tailroom=0
(...)
WARNING: CPU: 0 PID: 956 at net/core/dev.c:3259 skb_warn_bad_offload+0x7c/0x108
gemini-ethernet-port: caps=(0x0000010000154813, 0x00002007ffdd7889)

And the packets do not go through.

After investigating I drilled it down to the introduction of the
software checksumming in the driver.

Since the segmenting of packets will be done by the hardware this
makes a bit of sense since in that case the hardware also needs to
be keeping track of the checksumming.

That begs the question why large TCP or UDP packets also have to
bypass the checksumming (like e.g. ICMP does). If the hardware is
splitting it into smaller packets per-MTU setting, and checksumming
them, why is this happening then? I don't know. I know it is needed,
from tests: the OpenWrt webserver uhttpd starts sending big skb:s (up
to 2047 bytes, the max MTU) and above 1514 bytes it starts to fail
and hang unless the bypass bit is set: the frames are not getting
through.

Drop the size check and the offloading features for now: this
needs to be fixed up properly.

Suggested-by: Eric Dumazet <edumazet@google.com>
Fixes: d4d0c5b4d279 ("net: ethernet: cortina: Handle large frames")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 drivers/net/ethernet/cortina/gemini.c | 35 ++++-------------------------------
 1 file changed, 4 insertions(+), 31 deletions(-)

diff --git a/drivers/net/ethernet/cortina/gemini.c b/drivers/net/ethernet/cortina/gemini.c
index 78287cfcbf63..5e399c6e095b 100644
--- a/drivers/net/ethernet/cortina/gemini.c
+++ b/drivers/net/ethernet/cortina/gemini.c
@@ -79,8 +79,7 @@ MODULE_PARM_DESC(debug, "Debug level (0=none,...,16=all)");
 #define GMAC0_IRQ4_8 (GMAC0_MIB_INT_BIT | GMAC0_RX_OVERRUN_INT_BIT)
 
 #define GMAC_OFFLOAD_FEATURES (NETIF_F_SG | NETIF_F_IP_CSUM | \
-		NETIF_F_IPV6_CSUM | NETIF_F_RXCSUM | \
-		NETIF_F_TSO | NETIF_F_TSO_ECN | NETIF_F_TSO6)
+			       NETIF_F_IPV6_CSUM | NETIF_F_RXCSUM)
 
 /**
  * struct gmac_queue_page - page buffer per-page info
@@ -1143,39 +1142,13 @@ 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;
-	unsigned short mtu;
 	void *buffer;
-	int ret;
-
-	mtu  = ETH_HLEN;
-	mtu += netdev->mtu;
-	if (skb->protocol == htons(ETH_P_8021Q))
-		mtu += VLAN_HLEN;
 
+	/* TODO: implement proper TSO using MTU in word3 */
 	word1 = skb->len;
-	word3 = SOF_BIT;
-
-	if (word1 > mtu) {
-		word1 |= TSS_MTU_ENABLE_BIT;
-		word3 |= mtu;
-	}
+	word3 = SOF_BIT | skb->len;
 
-	if (skb->len >= ETH_FRAME_LEN) {
-		/* Hardware offloaded checksumming isn't working on frames
-		 * bigger than 1514 bytes. A hypothesis about this is that the
-		 * checksum buffer is only 1518 bytes, so when the frames get
-		 * bigger they get truncated, or the last few bytes get
-		 * overwritten by the FCS.
-		 *
-		 * Just use software checksumming and bypass on bigger frames.
-		 */
-		if (skb->ip_summed == CHECKSUM_PARTIAL) {
-			ret = skb_checksum_help(skb);
-			if (ret)
-				return ret;
-		}
-		word1 |= TSS_BYPASS_BIT;
-	} else if (skb->ip_summed == CHECKSUM_PARTIAL) {
+	if (skb->ip_summed == CHECKSUM_PARTIAL) {
 		int tcp = 0;
 
 		/* We do not switch off the checksumming on non TCP/UDP

-- 
2.34.1