In the weekend I upgraded my Libretto U100 to Fedora 12 (from Fedora 9 previously). I was extremely surprised (and happy) to find that everything worked correctly first time with the exception of the docking station (which I shall blog about later). Considering the rarity and uniqueness of this particular machine, it’s an excellent result.

HUAWEI E220 IS A PITA

However I discovered that my Huawei E220 3G Modem (branded as “Vodem” here in NZ) was now failing to work – when trying to connect, NetworkManager would start, the connection would run for a few seconds and then suddenly disconnect. I would also receive a popup saying that sr0 was unable to be mounted.

The 3G modem would then fail to appear in Network Manager and the kernel log showed lots of weird USB errors.

The Huawei E220 is an interesting device, it has both a 3G modem and also a USB “SCSI CDROM” drive which contains drivers for when plugged into a Windows computer. However this dual-device operation has historically caused no end of different problems across various Linux releases.

In Fedora 12, it seems that the “cdrom” (usbstorage) and 3G Modem (usbserial) drivers fight each other – first the usbserial driver works as expected, connects to the network and Network Manager runs OK. However a second later the “cdrom” tries to get mounted and glitches, breaking both drivers and dropping the connection.

SOLUTION

You can’t work around it by trying some of the past workarounds with older Linux releases such as removing the usbstorage module or apply custom vendor & product options to the usbserial module, either workaround will break the newer version of NetworkManager/ModemManager.

Fortunately the fix is relatively simple – we just need to tell the system to ignore the “cdrom” – which we can do by using Udev. Simply create the file /etc/udev/rules.d/20-custom-huawei.rules with the contents of:

# work around dodgy Huawei modem
SUBSYSTEMS=="scsi" ATTRS{vendor}=="HUAWEI", OPTIONS+="ignore_device"

Then re-plug the Huawei and the system will detect both the 3G Modem and the “cdrom”, however the ignore_device option will cause udev to avoid trying to mount the CDROM and therefore permits the 3G modem to work uninterrupted. :-)