smsc95xx: fix transmission where ZLP is expected

Usbnet framework assumes USB hardware doesn't handle zero length
packets, but SMSC LAN95xx requires these to be sent for correct
operation.

This patch fixes an easily reproducible tx lockup when sending a frame
that results in exactly 512 bytes in a USB transmission (e.g. a UDP
frame with 458 data bytes, due to IP headers and our USB headers).  It
adds an extra flag to usbnet for the hardware driver to indicate that
it can handle and requires the zero length packets.

This patch should not affect other usbnet users, please also consider
for -stable.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 2 insertions, 2 deletions

diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
index 3aafebdbe7b5..c6c922247d05 100644
--- a/drivers/net/usb/smsc95xx.c
+++ b/drivers/net/usb/smsc95xx.c
@@ -1227,7 +1227,7 @@ static const struct driver_info smsc95xx_info = {
         .rx_fixup       = smsc95xx_rx_fixup,
         .tx_fixup       = smsc95xx_tx_fixup,
         .status         = smsc95xx_status,
-        .flags          = FLAG_ETHER,
+        .flags          = FLAG_ETHER | FLAG_SEND_ZLP,
 };
 
 static const struct usb_device_id products[] = {
diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 24b36f795151..ca5ca5ae061d 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -1049,7 +1049,7 @@ netdev_tx_t usbnet_start_xmit (struct sk_buff *skb,
          * NOTE:  strictly conforming cdc-ether devices should expect
          * the ZLP here, but ignore the one-byte packet.
          */
-        if ((length % dev->maxpacket) == 0) {
+        if (!(info->flags & FLAG_SEND_ZLP) && (length % dev->maxpacket) == 0) {
                 urb->transfer_buffer_length++;
                 if (skb_tailroom(skb)) {
                         skb->data[skb->len] = 0;