[PATCH 10/10] net: usb: smsc95xx: disable HW checksumming in driver

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

We currently enable hardware checksumming, which means the NIC will add
two bytes of checksum after each packet with the computed checksum. We
don't use those two bytes however and will recompute the checksum
anyway.

Because we also don't adjust the size reported to the network stack to
be 2 bytes shorter, we trigger the newly introduced WARN_ON_ONCE when
DEBUG is #define'd that checks this condition, so let's just disable
the checksumming altogether and remove left-over dead code associated
with it.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 drivers/net/usb/smsc95xx.c | 41 +++++++++++---------------------------
 1 file changed, 12 insertions(+), 29 deletions(-)

diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
index 291e3c2f80f7..a90041731a66 100644
--- a/drivers/net/usb/smsc95xx.c
+++ b/drivers/net/usb/smsc95xx.c
@@ -24,8 +24,6 @@
 #define MAX_SINGLE_PACKET_SIZE		(2048)
 #define LAN95XX_EEPROM_MAGIC		(0x9500)
 #define EEPROM_MAC_OFFSET		(0x01)
-#define DEFAULT_TX_CSUM_ENABLE		(1)
-#define DEFAULT_RX_CSUM_ENABLE		(1)
 #define SMSC95XX_INTERNAL_PHY_ID	(1)
 #define SMSC95XX_TX_OVERHEAD		(8)
 #define SMSC95XX_TX_OVERHEAD_CSUM	(12)
@@ -46,8 +44,6 @@
 
 struct smsc95xx_priv {
 	u32 mac_cr;
-	int use_tx_csum;
-	int use_rx_csum;
 };
 
 static int turbo_mode = 0;
@@ -320,10 +316,9 @@ static void smsc95xx_set_multicast(struct usbnet *dev)
 	smsc95xx_write_reg(dev, MAC_CR, pdata->mac_cr);
 }
 
-/* Enable or disable Tx & Rx checksum offload engines */
-static int smsc95xx_set_csums(struct usbnet *dev)
+/* Disable Tx & Rx IP checksum offload engines */
+static int smsc95xx_disable_csums(struct usbnet *dev)
 {
-	struct smsc95xx_priv *pdata = (struct smsc95xx_priv *)(dev->data[0]);
 	u32 read_buf;
 	int ret = smsc95xx_read_reg(dev, COE_CR, &read_buf);
 	if (ret < 0) {
@@ -331,15 +326,8 @@ static int smsc95xx_set_csums(struct usbnet *dev)
 		return ret;
 	}
 
-	if (pdata->use_tx_csum)
-		read_buf |= Tx_COE_EN_;
-	else
-		read_buf &= ~Tx_COE_EN_;
-
-	if (pdata->use_rx_csum)
-		read_buf |= Rx_COE_EN_;
-	else
-		read_buf &= ~Rx_COE_EN_;
+	read_buf &= ~Tx_COE_EN_;
+	read_buf &= ~Rx_COE_EN_;
 
 	ret = smsc95xx_write_reg(dev, COE_CR, read_buf);
 	if (ret < 0) {
@@ -670,7 +658,13 @@ static int smsc95xx_reset(struct usbnet *dev)
 		return ret;
 	}
 
-	ret = smsc95xx_set_csums(dev);
+	/*
+	 * barebox network stack doesn't care for hardware checksum offloading,
+	 * so this enabling them doesn't help and indeed introduces breakage:
+	 * The driver will be unaware of the two byte COE trailer and thus packet
+	 * sizes reported will be 2 bytes more than what was actually transmitted.
+	 */
+	ret = smsc95xx_disable_csums(dev);
 	if (ret < 0) {
 		netdev_warn(dev->net, "Failed to set csum offload: %d\n", ret);
 		return ret;
@@ -724,9 +718,6 @@ static int smsc95xx_bind(struct usbnet *dev)
 		return -ENOMEM;
 	}
 
-	pdata->use_tx_csum = DEFAULT_TX_CSUM_ENABLE;
-	pdata->use_rx_csum = DEFAULT_RX_CSUM_ENABLE;
-
 	/* Init all registers */
 	ret = smsc95xx_reset(dev);
 
@@ -810,15 +801,7 @@ static int smsc95xx_rx_fixup(struct usbnet *dev, void *buf, int len)
 
 	return 1;
 }
-#if 0
-static u32 smsc95xx_calc_csum_preamble(struct sk_buff *skb)
-{
-	int len = skb->data - skb->head;
-	u16 high_16 = (u16)(skb->csum_offset + skb->csum_start - len);
-	u16 low_16 = (u16)(skb->csum_start - len);
-	return (high_16 << 16) | low_16;
-}
-#endif
+
 static int smsc95xx_tx_fixup(struct usbnet *dev,
                                 void *buf, int len,
                                 void *nbuf, int *nlen)
-- 
2.39.2