[PATCH 3.16 323/370] cdc_ncm: workaround for EM7455 "silent" data interface

[Ben Hutchings]

3.16.42-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: BjÃrn Mork <bjorn@xxxxxxx>

[ 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 <ml001@xxxxxxxxxxxxxxxx>
Reported-by: Ralph Plawetzki <ralph@xxxxxxxxxxxx>
Reported-by: Andreas Fett <andreas.fett@xxxxxxxxxxx>
Reported-by: Rasmus Lerdorf <rasmus@xxxxxxxxxxx>
Reported-by: Samo Ratnik <samo.ratnik@xxxxxxxxx>
Reported-and-tested-by: Aleksander Morgado <aleksander@xxxxxxxxxxxxx>
Signed-off-by: BjÃrn Mork <bjorn@xxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/net/usb/cdc_ncm.c | 7 +++++++
 1 file changed, 7 insertions(+)

--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -830,6 +830,13 @@ advance:
 	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) {