Subject: [PATCH 3/4] cdc_ncm: workaround for EM7455 "silent" data interface

From: Bjørn Mork <[email-masked]>
Date: Sun, 3 Jul 2016 22:24:50 +0200

[ Upstream commit c086e7096170390594c425114d98172bc9aceb8a ]

Several Lenovo users have reported problems with their Sierra
Wireless EM7455 modem. The driver has loaded successfully and
the MBIM management channel has appeared to work, including
establishing a connection to the mobile network. But no frames
have been received over the data interface.

The problem affects all EM7455 and MC7455, and is assumed to
affect other modems based on the same Qualcomm chipset and
baseband firmware.

Testing narrowed the problem down to what seems to be a
firmware timing bug during initialization. Adding a short sleep
while probing is sufficient to make the problem disappear.
Experiments have shown that 1-2 ms is too little to have any
effect, while 10-20 ms is enough to reliably succeed.

Reported-by: Stefan Armbruster <[email-masked]>
Reported-by: Ralph Plawetzki <[email-masked]>
Reported-by: Andreas Fett <[email-masked]>
Reported-by: Rasmus Lerdorf <[email-masked]>
Reported-by: Samo Ratnik <[email-masked]>
Reported-and-tested-by: Aleksander Morgado <[email-masked]>
Signed-off-by: Bjørn Mork <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 drivers/net/usb/cdc_ncm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
index 8c2bb77..a790d5f 100644
--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -809,6 +809,13 @@ int cdc_ncm_bind_common(struct usbnet *dev, struct usb_interface *intf, u8 data_
 	if (cdc_ncm_init(dev))
 		goto error2;
 
+	/* Some firmwares need a pause here or they will silently fail
+	 * to set up the interface properly.  This value was decided
+	 * empirically on a Sierra Wireless MC7455 running 02.08.02.00
+	 * firmware.
+	 */
+	usleep_range(10000, 20000);
+
 	/* configure data interface */
 	temp = usb_set_interface(dev->udev, iface_no, data_altsetting);
 	if (temp) {
-- 
2.1.0