net: cdc_mbim: send ZLP only for the specific buggy device

Reverting 328d7b8 and instead adding an exception for the
Sierra Wireless MC7710.

commit 328d7b8 (net: cdc_mbim: send ZLP after max sized NTBs)
added a workaround for an issue observed on one specific device.
Concerns were raised that this workaround adds a performance
penalty to all devices based on questionable, if not buggy,
behaviour of a single device:

 "If you add ZLP for NTBs of dwNtbOutMaxSize, you are heavily affecting CPU
  load, increasing interrupt load by factor of 2 in high load traffic
  scenario and possibly decreasing throughput for all other devices
  which behaves correctly."

 "The idea of NCM was to avoid extra ZLPs. If your transfer is exactly
  dwNtbOutMaxSize, it's known, you can submit such request on the receiver
  side and you do not need any EOT indicatation, so the frametime can be
  used for useful data."

Adding a device specific exception to prevent the workaround from
affecting well behaved devices.

The assumption here is that needing a ZLP is truly an *exception*.
We do not yet have enough data to verify this.  The generic
workaround in commit 328d7b8 should be considered acceptable despite
the performance penalty if the exception list becomes a maintainance
hassle.

Cc: Alexey ORISHKO <alexey.orishko@stericsson.com>
Cc: Yauheni Kaliuta <y.kaliuta@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/cdc_mbim.c

@@ -365,6 +365,21 @@ err:
 }
 
 static const struct driver_info cdc_mbim_info = {
+	.description = "CDC MBIM",
+	.flags = FLAG_NO_SETINT | FLAG_MULTI_PACKET | FLAG_WWAN,
+	.bind = cdc_mbim_bind,
+	.unbind = cdc_mbim_unbind,
+	.manage_power = cdc_mbim_manage_power,
+	.rx_fixup = cdc_mbim_rx_fixup,
+	.tx_fixup = cdc_mbim_tx_fixup,
+};
+
+/* MBIM and NCM devices should not need a ZLP after NTBs with
+ * dwNtbOutMaxSize length. This driver_info is for the exceptional
+ * devices requiring it anyway, allowing them to be supported without
+ * forcing the performance penalty on all the sane devices.
+ */
+static const struct driver_info cdc_mbim_info_zlp = {
 	.description = "CDC MBIM",
 	.flags = FLAG_NO_SETINT | FLAG_MULTI_PACKET | FLAG_WWAN | FLAG_SEND_ZLP,
 	.bind = cdc_mbim_bind,
@@ -385,6 +400,10 @@ static const struct usb_device_id mbim_devs[] = {
 	{ USB_INTERFACE_INFO(USB_CLASS_COMM, USB_CDC_SUBCLASS_NCM, USB_CDC_PROTO_NONE),
 	  .driver_info = (unsigned long)&cdc_mbim_info,
 	},
+	/* Sierra Wireless MC7710 need ZLPs */
+	{ USB_DEVICE_AND_INTERFACE_INFO(0x1199, 0x68a2, USB_CLASS_COMM, USB_CDC_SUBCLASS_MBIM, USB_CDC_PROTO_NONE),
+	  .driver_info = (unsigned long)&cdc_mbim_info_zlp,
+	},
 	{ USB_INTERFACE_INFO(USB_CLASS_COMM, USB_CDC_SUBCLASS_MBIM, USB_CDC_PROTO_NONE),
 	  .driver_info = (unsigned long)&cdc_mbim_info,
 	},