Mittwoch, 15. Dezember 2010

itrackutil 0.1.3 available

After a recent update in Ubuntu 10.10 and 10.04 exits with the the error message “USBError: Resource Busy” before the user is able to download GPS data from the device.
This version fixes the problem.

Developer information:

A previously unrelated kernel module (cdc_acm) suddenly started to grab the USB device.  cdc_acm  which usually supports USB modems now creates the device /dev/ttyACM0 as soon as the GPS mouse is plugged in.

If you read /dev/ttyACM0 you get the real-time GPS data in NEMA format.  This is an unusual use case. The normal data connection is over Bluetooth.

However, the creation of this device file blocks all other USB access to the GPS mouse; in this case the direct USB communication which uses to access the data stored within the unit's memory.

Fortunately there is an USB API call for this situation: detachKernelDriver.
This function does, what it says it does.  You have to call it after opening the device. Root privileges are not necessary.

The call will fail, if there is no kernel driver to detach. You have to catch this exceptions:

... determine device, interface, and configuration ...

    handle =
except usb.USBError, err:
    if str(err).find('could not detach kernel driver from interface') >= 0:
        raise usb.USBError,err       # any other USB error


The companion API call attachKernelDriver is not available via PyUSB 0.1. But this is only a minor problem, because as soon as you unplug the unit and reconnect it, a new USB device file is created (with the kernel driver attached).