Some modems, such as the Telit UE910, are using an Infineon Flash Loader
utility. It has two interfaces, 2/2/0 (Abstract Modem) and 10/0/0 (CDC
Data). The latter can be used as a serial interface to upgrade the
firmware of the modem. However, that isn't possible when the cdc-acm
driver takes control of the device.

The following is an explanation of the behaviour by Daniele Palmas during
discussion on linux-usb.

"This is what happens when the device is turned on (without modifying
the drivers):

[155492.352031] usb 1-3: new high-speed USB device number 27 using ehci-pci
[155492.485429] usb 1-3: config 1 interface 0 altsetting 0 endpoint 0x81 has an invalid bInterval 255, changing to 11
[155492.485436] usb 1-3: New USB device found, idVendor=058b, idProduct=0041
[155492.485439] usb 1-3: New USB device strings: Mfr=0, Product=0, SerialNumber=0
[155492.485952] cdc_acm 1-3:1.0: ttyACM0: USB ACM device

This is the flashing device that is caught by the cdc-acm driver. Once
the ttyACM appears, the application starts sending a magic string
(simple write on the file descriptor) to keep the device in flashing
mode. If this magic string is not properly received in a certain time
interval, the modem goes on in normal operative mode:

[155493.748094] usb 1-3: USB disconnect, device number 27
[155494.916025] usb 1-3: new high-speed USB device number 28 using ehci-pci
[155495.059978] usb 1-3: New USB device found, idVendor=1bc7, idProduct=0021
[155495.059983] usb 1-3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[155495.059986] usb 1-3: Product: 6 CDC-ACM + 1 CDC-ECM
[155495.059989] usb 1-3: Manufacturer: Telit
[155495.059992] usb 1-3: SerialNumber: 359658044004697
[155495.138958] cdc_acm 1-3:1.0: ttyACM0: USB ACM device
[155495.140832] cdc_acm 1-3:1.2: ttyACM1: USB ACM device
[155495.142827] cdc_acm 1-3:1.4: ttyACM2: USB ACM device
[155495.144462] cdc_acm 1-3:1.6: ttyACM3: USB ACM device
[155495.145967] cdc_acm 1-3:1.8: ttyACM4: USB ACM device
[155495.147588] cdc_acm 1-3:1.10: ttyACM5: USB ACM device
[155495.154322] cdc_ether 1-3:1.12 wwan0: register 'cdc_ether' at usb-0000:00:1a.7-3, Mobile Broadband Network Device, 00:00:11:12:13:14

Using the cdc-acm driver, the string, though being sent in the same way
than using the usb-serial-simple driver (I can confirm that the data is
passing properly since I used an hw usb sniffer), does not make the
device to stay in flashing mode."
Signed-off-by: Jonas Jonsson <jonas@ludd.ltu.se>
Tested-by: Daniele Palmas <dnlplm@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>