usbnet: fix alignment for frames with no ethernet header

The qmi_wwan minidriver support a 'raw-ip' mode where frames are
received without any ethernet header. This causes alignment issues
because the skbs allocated by usbnet are "IP aligned".

Fix by allowing minidrivers to disable the additional alignment
offset. This is implemented using a per-device flag, since the same
minidriver also supports 'ethernet' mode.

Fixes: 32f7adf6 ("net: qmi_wwan: support "raw IP" mode")
Reported-and-tested-by: Jay Foster <jay@systech.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/qmi_wwan.c

@@ -261,9 +261,11 @@ static void qmi_wwan_netdev_setup(struct net_device *net)
 		net->hard_header_len = 0;
 		net->addr_len        = 0;
 		net->flags           = IFF_POINTOPOINT | IFF_NOARP | IFF_MULTICAST;
+		set_bit(EVENT_NO_IP_ALIGN, &dev->flags);
 		netdev_dbg(net, "mode: raw IP\n");
 	} else if (!net->header_ops) { /* don't bother if already set */
 		ether_setup(net);
+		clear_bit(EVENT_NO_IP_ALIGN, &dev->flags);
 		netdev_dbg(net, "mode: Ethernet\n");
 	}
 

drivers/net/usb/usbnet.c

@@ -484,7 +484,10 @@ static int rx_submit (struct usbnet *dev, struct urb *urb, gfp_t flags)
 		return -ENOLINK;
 	}
 
-	skb = __netdev_alloc_skb_ip_align(dev->net, size, flags);
+	if (test_bit(EVENT_NO_IP_ALIGN, &dev->flags))
+		skb = __netdev_alloc_skb(dev->net, size, flags);
+	else
+		skb = __netdev_alloc_skb_ip_align(dev->net, size, flags);
 	if (!skb) {
 		netif_dbg(dev, rx_err, dev->net, "no rx skb\n");
 		usbnet_defer_kevent (dev, EVENT_RX_MEMORY);

include/linux/usb/usbnet.h

@@ -82,6 +82,7 @@ struct usbnet {
 #		define EVENT_RX_KILL	10
 #		define EVENT_LINK_CHANGE	11
 #		define EVENT_SET_RX_MODE	12
+#		define EVENT_NO_IP_ALIGN	13
 };
 
 static inline struct usb_driver *driver_of(struct usb_interface *intf)