Investigating OBEX over USB

I’ve had a number of requests for USB support in gnome-vfs-obexftp. At first I didn’t have much luck talking to my phone via USB. Running the obex_test utility from OpenOBEX gave the following results:

$ obex_test -u
Using USB transport, querying available interfaces
Interface 0:   (null)
Interface 1:   (null)
Interface 2:   (null)
Use 'obex_test -u interface_number' to run interactive OBEX test client

Trying to talk via any of these interface numbers failed. After reading up a bit, it turned out that I needed to add a udev rule to give permissions on my phone. After doing so, I got a better result:

$ obex_test -u
Using USB transport, querying available interfaces
Interface 0: Nokia Nokia 6230 (null)
Interface 1: Nokia Nokia 6230 (null)
Interface 2: Nokia Nokia 6230 (null)
Use 'obex_test -u interface_number' to run interactive OBEX test client

With the change, I was also able to access the phone using the obexftp command line client. This seemed enough to start investigating a bit further. The OpenOBEX API for setting up USB connections goes something like this:

  1. The app calls OBEX_FindInterfaces(), which returns a list of obex_interface_t structures that represent the different discovered interfaces.
  2. The app picks one of the discovered interfaces (based on the manufacturer, product and serial number strings), then connects to it using OBEX_InterfaceConnect().

There are a number of issues with this interface though.

  • If the phone doesn’t provide a serial number via its USB interface (like my 6230 doesn’t), the obex_interface_t structure is not enough to identify a particular phone.
  • If the phone exposes multiple OBEX USB interfaces for some reason, OpenOBEX lists it multiple times. In the obex_test output shown above, there was a single phone attached – not three.
  • There is no way to tell when phones are connected or disconnected. While HAL can do that job for us, there is no way to map from the device information provided by HAL to one of the discovered interfaces provided by OpenOBEX.

To sum up, it shouldn’t be difficult to hack support for USB connections into gnome-vfs-obexftp with URLs like obex://usb-N/ (where N is the number of the discovered interface), but there are a number of features I’d need to provide a good user experience:

  1. The ability to ask OpenOBEX to connect to a particular USB device, rather than having to deal with its discovery interface.
  2. A good set of udev rules to grant the needed permissions on common phones so they don’t need to find out why things only work as root.

This Post Has 5 Comments

  1. davidz

    Someone posted a patch to HAL that I merged that exports the so-called “USB Interface Decscription” textual value;a=commit;h=8e00f386dd4af855116cb1082c0ae6fd8db7ce5e

    This patch is in hal 0.5.9. There’s some more info in this thread

    It might be worth having a bunch of .fdi files in hal-info tag the appropriate USB interface with something so you can discover them from gnome-vfs? I bet it varies what the textual description of the interface is given the mobile phone though. But it should be feasible.

    Hope this helps and keep on rocking on OBEX! Thanks!

  2. davidz

    Btw, forgot to mention that one point of having it in HAL is that with the ACL-patch (that is also in 0.5.9) we’d use this rather than udev rules to add ACL’s to the device file.

  3. James Henstridge

    Hi David,

    I just checked my own phone, none of the three OBEX interfaces provided an interface description (lsusb reports iInterface = 0). From a look at the code, the “(null)” bits in the obex_test output would have displayed these interface description strings were provided. As all three interfaces behaved identically, I didn’t realise that HAL needed any extra features 🙂

    Your suggestion of using FDI files to tag the usable interfaces seems sensible — I’d originally been planning to just look up the devices by the interface class/subclass like OpenOBEX currently does.

    That still leaves the question of how to get OpenOBEX to connect to a device I discover with HAL, but that is an OpenOBEX problem rather than a HAL problem.

  4. James Henstridge

    Actually, there is one area I wouldn’t mind seeing HAL improve is to add Bluetooth device support. If I am going to add USB support, it’d be nice if I could share as much discovery code between the Bluetooth and USB cases as possible.

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