net: cdc_ncm: inform usbnet when rx buffers are reduced

It doesn't matter whether the buffer size goes up or down.  We have to
keep usbnet and device syncronized to be able to split transfers at the
correct boundaries. The spec allow skipping short packets when using
max sized transfers.  If we don't tell usbnet about our new expected rx
buffer size, then it will merge and/or split NTBs.  The driver does not
support this, and the result will be lots of framing errors.

Fix by always reallocating usbnet rx buffers when the rx_max value
changes.

Fixes: 68864abf ("net: cdc_ncm: support rx_max/tx_max updates when running")
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/cdc_ncm.c

@@ -215,19 +215,12 @@ static void cdc_ncm_update_rxtx_max(struct usbnet *dev, u32 new_rx, u32 new_tx)
 			min, max, val);
 	}
 
-	/* usbnet use these values for sizing rx queues */
-	dev->rx_urb_size = val;
-
 	/* inform device about NTB input size changes */
 	if (val != ctx->rx_max) {
 		__le32 dwNtbInMaxSize = cpu_to_le32(val);
 
 		dev_info(&dev->intf->dev, "setting rx_max = %u\n", val);
 
-		/* need to unlink rx urbs before increasing buffer size */
-		if (netif_running(dev->net) && dev->rx_urb_size > ctx->rx_max)
-			usbnet_unlink_rx_urbs(dev);
-
 		/* tell device to use new size */
 		if (usbnet_write_cmd(dev, USB_CDC_SET_NTB_INPUT_SIZE,
 				     USB_TYPE_CLASS | USB_DIR_OUT
@@ -238,6 +231,13 @@ static void cdc_ncm_update_rxtx_max(struct usbnet *dev, u32 new_rx, u32 new_tx)
 			ctx->rx_max = val;
 	}
 
+	/* usbnet use these values for sizing rx queues */
+	if (dev->rx_urb_size != ctx->rx_max) {
+		dev->rx_urb_size = ctx->rx_max;
+		if (netif_running(dev->net))
+			usbnet_unlink_rx_urbs(dev);
+	}
+
 	/* clamp new_tx to sane values */
 	min = ctx->max_datagram_size + ctx->max_ndp_size + sizeof(struct usb_cdc_ncm_nth16);
 	max = min_t(u32, CDC_NCM_NTB_MAX_SIZE_TX, le32_to_cpu(ctx->ncm_parm.dwNtbOutMaxSize));